question stringlengths 25 894 | answer stringlengths 4 863 |
|---|---|
How do I write nothing inifpart if the condition is true?
For example:
```
if(true)
// I want to keep this space blank or want compiler to do nothing here
else
break;
```
How do I do this?
|
Typically, this is written by negating the condition:
```
if (not true)
break;
```
|
By now, I found only this way: get a pointer to the global variable via a function.
LLVM:
```
@a = global i8 0
define i8* @ggvp_a () {
ret i8* @a
}
```
C:
```
char* ggvp_a (void);
```
usage:
```
char* gvp_a = ggvp_a ();
*gvp_a = 8;
return *gvp_a;
```
(“ggvp” — “get global variable pointer”)
Is there a more... |
It's an external declaration like any other - just add
```
extern signed char a;
```
to your.cfile.
|
I'm having a hard time understanding my issue:
```
uint8_t nal_type=6;
for(i=7;i!=0;i--){
printf("%d",(nal_type>>i)&0x01U);}
printf("\n");
```
I would expect the following code to display the binary value from MSB to LSB. But it displays the following:
```
0000011
```
Can someone enlighten me ?
|
It's just a simple mistake in your loop, so you're not seeing the LS bit (bit 0) - make it:
```
for (i = 7; i >= 0; i--)
{ ^^^^^^
printf("%d", (nal_type >> i) & 0x01U);
}
```
This will then give the output as:
```
00000110
```
which I think is what you are looking for (MSB to LSB).
|
I have a socket where I set a timeout forrecv().
I have two steps forrecv(), first I check content of received data if complete usingMSG_PEEK | MSG_DONTWAIT.
```
recvTimeout.tv_sec = mRecvTimeoutSecs;
recvTimeout.tv_usec = mRecvTimeoutUSecs;
sendTimeout.tv_sec = mSendTimeoutSecs;
sendTimeout.tv_usec = mSendTimeoutUS... |
No, the socket will not timeout, asMSG_DONTWAITwill causerecv()to return immediately. Note that if you set like 1 msec timeout, then it might timeout - that would depend on the implementation (on which OS your code runs on).
|
How can I write three different matrices of different sizes say A, B and C using C?
The doubt being that once a particular row is printed the cursor is in the bottom most position, now how do we bring it back up?
|
You don't. you plan in advanced the rows and then print each row (one you go down - you can't go up....)
If you can't handle the logic - use a 2d array to store everything and then print it.
|
This question already has answers here:Removing trailing newline character from fgets() input(15 answers)Closed8 years ago.
```
while (fgets(line, LINELEN, inputFile)){
printf("%s$", line)
}
```
I am trying to read a file and print it out to the console as a CAT command. In this case I wanted to put a dollar s... |
fgetsincludes the'\n'at the end of the line.
Use something like:
```
char line[hopefullybigenough];
while (fgets(line, sizeof(line), inputFile)) {
size_t len = strlen(line);
if (line[len - 1] == '\n') line[len - 1] = '\0';
printf("%s$\n", line);
}
```
|
Closed.This question does not meetStack Overflow guidelines. It is not currently accepting answers.This question does not appear to be about programming within the scope defined in thehelp center.Closed8 years ago.Improve this question
How to find coefficient of
```
x^m (m<=n)
```
in the polynomial of type
```
(a1... |
```
(a1+b1x)(a2+b2x)...(an+bnx)=b1*b2*...*bn*(a1/b1+x)*(a2/b2+x)...(a/bn+x)
```
Right part is polynomial with roots (-a1/b1,-a2/b2...-an/bn)
There is O(N^2) algorithm to find coefficients of this polynomial, implementedhere
(Don't forget to multiply coefficients by product of b[i])
|
So, i have the following code:
```
#include <stdio.h>
int main() {
FILE* f = fopen("test.txt", "r");
FILE* p = fopen("test2.txt", "w+");
double i;
char j;
while (!feof(f)){
fscanf(f, " %c", &j);
if ((j == '(')||(j == ')'))
fprintf(p, "%c ", j);
else {
ungetc(j,f);
fscanf(f, "%lf ", &i);
... |
You shouldn't check forfeof()like that. Instead, simply check the result offscanf():
```
while (fscanf(f, " %c", &j) == 1) {
```
|
This question already has answers here:Floating point does not print accurately [duplicate](4 answers)Closed8 years ago.
I want to convert a char like "12345678901234567890.123 " to double,
so I use sscanf(str, "%lf", &d),
and I print it use printf("%20.3lf", d);
But I get the result is "12345678901234567178.000",
... |
You probably can't, since you're trying to store a number with more digits of precision than will fit in your machine'sdoubledata type.
You need to use a big-number library, that "manually" computes with arbitrary number of digits and thus can go beyond the limits of your machine's basic data types.
Oh, and"12345678... |
I'm wondering something like this is possible:
```
// declaration
void func();
int main()
{
int ar[] = { 1, 2, 3 };
func(ar); // call with parameter
return 1;
}
void func() // no parameters
{
// do something
}
```
Can someone explain me this and especially how can I accessarinfunc()?
|
In C (not C++), a function declared asfunc()is treated as having an unspecified number of untyped parameters. A function with no parameters should be explicitly declared asfunc(void).
|
I have this C code:
```
int main()
{
int *p=(int *)malloc(100); //100 bytes
for(int i=0;i<10;i++)
{
p++;
}
free(p);
return 0;
}
```
Now my question is will free(p) free all the 100 bytes or only 90 bytes. How does free() know how many bytes to free..?
|
It's illegal.
The argument offreemust be a pointer that is returned bymallocor its cousins, or a null pointer. In your example,phas changed its value byp++.
|
I am trying to read a file of the form
```
A number,number
A number,number
[space] B number,number
etc
```
where I want to read only the lines that do start with a space (ie, in the example above, the last line only). I wrote this loop:
```
while ((g = fscanf(fp," %c %x,%d\n",&a,&b,&c) != EOF){
printf("%c %... |
OP: it possible to specify it in thefscanf?A: Yes, but its ugly. Use the best tool for the job.
Usefgets(), then scan withsscanf().
```
char buf[100];
char a;
unsigned b;
int c;
while (fgets(buf, sizeof buf, fp) != NULL) {
// Thank-you @Chris Dodd
if ((buf[0] == ' ') && (sscanf(buf, " %c%x,%d", &a,&b,&c) == 3))... |
I have an char array like this:
```
char res[80]="00001111001010100100001010101100000010011100001000110010000101000100001010010001"
```
want to change into
```
int res[80]=00001111001010100100001010101100000010011100001000110010000101000100001010010001
```
have done something like this:
```
char res[80]="00001111... |
ASCII '0' is actually 48 in scalar form, so subtract it out:
```
resInt[i] = res[i] - '0';
```
|
What is the unary-& doing here?
```
int * a = 1990;
int result = &5[a];
```
If you were to printresultyou would get the value 2010.
You have to compile it with-fpermissiveor it will stop due to errors.
|
In C, x [y] and y [x] are identical. So &5[a] is the same as &a[5].
|
Is it possible to intercept primitive operations (+,-,% etc.) like this one, with LD_PRELOAD?
```
int64_t endWhenNsec = startWhenNsec + gTimeLimitSec;
```
If not, any way to achieve something similar?
|
No. Primitive operators are compiled directly to machine code. There is no library involved, so preloading libraries cannot alter their behavior. What a mess if it could!
|
I am a little confused.
I write in pure C. The following code doesn't work:
```
char arr[] = "0x123";
printf(arr[0]);
```
It is just an example.
IDE(Code::Blocks) generates this:Program recieved signal SIGSEGV, Sigmentation fault.
Why does this code not work?
|
If you use llvm/clang, the compiler will issue a rather self-explanatory warning:
```
test.c:3:9: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
printf(a[0]);
```
It means you should provide a formatting string as the first argument e.g.printf("%c", arr[0]);
|
My assignment is to write a version ofcatcommand in Linux using C. unlike thecatcommand, my version ofcatwon't work with standard input. My question is what doest it mean by saying "catwon't work with standard input". Please include examples. Thank you in advance guys.
|
It simply means you don't have to work with stdin. Meaning you don't have to handle the case where someone can just type "mycat" then type data followed by ^D to close the file and save it. Also should mean you don't have to handle data being piped to it. Looks like all you have to do is read the file passed as a p... |
This question already has answers here:Floating point does not print accurately [duplicate](4 answers)Closed8 years ago.
I want to convert a char like "12345678901234567890.123 " to double,
so I use sscanf(str, "%lf", &d),
and I print it use printf("%20.3lf", d);
But I get the result is "12345678901234567178.000",
... |
You probably can't, since you're trying to store a number with more digits of precision than will fit in your machine'sdoubledata type.
You need to use a big-number library, that "manually" computes with arbitrary number of digits and thus can go beyond the limits of your machine's basic data types.
Oh, and"12345678... |
I'm wondering something like this is possible:
```
// declaration
void func();
int main()
{
int ar[] = { 1, 2, 3 };
func(ar); // call with parameter
return 1;
}
void func() // no parameters
{
// do something
}
```
Can someone explain me this and especially how can I accessarinfunc()?
|
In C (not C++), a function declared asfunc()is treated as having an unspecified number of untyped parameters. A function with no parameters should be explicitly declared asfunc(void).
|
I have this C code:
```
int main()
{
int *p=(int *)malloc(100); //100 bytes
for(int i=0;i<10;i++)
{
p++;
}
free(p);
return 0;
}
```
Now my question is will free(p) free all the 100 bytes or only 90 bytes. How does free() know how many bytes to free..?
|
It's illegal.
The argument offreemust be a pointer that is returned bymallocor its cousins, or a null pointer. In your example,phas changed its value byp++.
|
I am trying to read a file of the form
```
A number,number
A number,number
[space] B number,number
etc
```
where I want to read only the lines that do start with a space (ie, in the example above, the last line only). I wrote this loop:
```
while ((g = fscanf(fp," %c %x,%d\n",&a,&b,&c) != EOF){
printf("%c %... |
OP: it possible to specify it in thefscanf?A: Yes, but its ugly. Use the best tool for the job.
Usefgets(), then scan withsscanf().
```
char buf[100];
char a;
unsigned b;
int c;
while (fgets(buf, sizeof buf, fp) != NULL) {
// Thank-you @Chris Dodd
if ((buf[0] == ' ') && (sscanf(buf, " %c%x,%d", &a,&b,&c) == 3))... |
I'm trying to read this .txt file:
( 1 2 ( 3 4 ( 5
with this code:
```
#include <stdio.h>
int main() {
FILE* f = fopen("teste.txt", "r");
int i;
char j;
while (feof(f) == 0){
fscanf(f, "%c", &j);
switch (j) {
case '(':
printf("%c ", j);
break;
default:
ungetc... |
1) Useint j;fgets()returns anunsigned charorEOF257 different values. Using acharloses information.
2) Do not usefeof()
```
// while (feof(f) == 0){
// fscanf(f, "%c", &j);
while (fscanf(f, " %c", &j) == 1) { // note space before %c
```
3) Testfscanf()return value
```
// fscanf(f, "%d", &i);
if (fscanf(f, "... |
I'm just doing practice problems out of my C book and am stuck on this one:
Write a program that creates an array with 26 elements and stores the 26
lowercase letters in it. Also have it show the array contents.
I'm getting stuck here
```
char abc[26];
char index;
for(index="a", abc[0]; index<="z"; index++, abc[0... |
No need to append as mentioned in the comments, also be careful withchar i= "a"andchar i = 'a'
here is a code, that does create array of 26 lower case alphabet
```
#include <stdio.h>
int main()
{
char arr[27];
int i;
for(i=0; i<26; i++)
{
arr[i] = 'a'+i;
}
arr[i]='\0'; //null term... |
This question already has answers here:Unsigned int in C behaves negative(8 answers)Closed8 years ago.
I wrote simple C program to find the maximum positive number I can reach using unsigned integer as given below. Size of integer on my machine is 4 bytes.
```
#include <stdio.h>
#include <math.h>
main()
{
unsig... |
You want to use %u for unsigned int:
```
printf("%u\n", x);
printf("%u\n", y);
```
|
I am trying to assign a C array to a C++ std::array.
How do I do that, the cleanest way and without making unneeded copies etc?
When doing
```
int X[8];
std::array<int,8> Y = X;
```
I get an compiler error: "no suitable constructor exists".
|
There is no conversion from plain array tostd::array, but you can copy the elements from one to the other:
```
std::copy(std::begin(X), std::end(X), std::begin(Y));
```
Here's a working example:
```
#include <iostream>
#include <array>
#include <algorithm> // std::copy
int main() {
int X[8] = {0,1,2,3,4,5,6,7... |
It was a header that was available in old Borland compilers. Used
strictly for graphics in MS-Dos apps. You are at least 7 operating
systems, 16 bits, 20 years and untold compiler versions removed from
using it.
Yes, I know that<graphics.h>is gone. I am new to C/C++ and my knowledge is confined to the computer ... |
The similar level of abstraction in C can be easily reached withSDLYou will wantSDL_Surfaceto put points of Bresenham to.
I'd also recommendClanLibfor use with C++, as more convenient to start with.
Both libraries are cross-platform (and can use OpenGL as engine)
|
```
char c;
c = '2';
printf("%d\n",c);
```
So this question is from my quiz, it is asking what is the display. The answer is 50, i tried by using program, but why it is 50? not 2 or anything?
|
The apostrophes mean "character literal", i.e.'2'is not the integer 2, but instead the character 2, i.e. the glyph used to represent the single digit 2.
You print this value using%dinprintf(), which means "signed integer", so you get the integer value of the character, often called the "code point" (or, classically, ... |
Let's say I have
```
char *names[] = { "Tom", "Jerry" };
```
and I want to print the "e" in "Jerry" usingprintf. My first instinct was
```
printf("%c\n", *names[5]);
```
but when I applied what I've been learning about pointers, I realized this is total junk code because the 5 refers to the nonexistent fifth point... |
I think what you're looking for isprintf("%c\n", names[1][1]);.
|
Input:9 notation
```
printf("Input N value\n");
scanf("%d",&N);
char X[N];
int i=0,N=0;
for (i=0;i<N-1;i++)
{
scanf("%c",&X[i]);
}
X[N-1]='\0';
for (i=0;i<N-1;i++)
{
printf("%c",X[i]);
}
```
Expected output:notation
Output:notatio
Why is this so?
|
When you read the length with your first scanf call you leave a newline in your input buffer.
At the second scanf call the first character read is the newline and then the rest. Adding a space before the %c in your second scanf string parameter will consume any left newlines.
```
scanf(" %c",&X[i]);
```
You also do... |
I don't really understand why gcc has subtract 12 to esp before calling the function.
```
pushl %ebp
movl %esp,%ebp
sub $12,%esp
socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
movl $AF_INET,(%esp)
```
|
The current*x86 ABI requires the stack pointer to be aligned mod 16 at the time of function call. This is the typical reason for otherwise-unexplained adjustments of the stack pointer.
* I saycurrentbecause GCC actually unilaterally changed the ABI and introduced this requirement somewhere back in the 3.x series. I d... |
I have this code:
```
/* ... headers ... */
struct my_callback_struct {
int data;
int (*callback_func)(struct my_callback_struct *p, int data);
};
int reg(struct my_callback_struct *p, int data)
{
return 1;
}
void init(struct my_callback_struct *p)
{
p->callback_func = ®
}
int main ()
{
... |
You simply haven't allocated memory for yourstructinstance. Try something like
```
struct my_callback_struct s;
init(&s);
printf("%d", s.callback_func(p,12));
```
Or callmallocfor yourpbefore callinginit.
|
This question already has answers here:Asymptotic analysis(2 answers)Closed8 years ago.
I dont get the part where T(n) of the second for loop is log(n). Both loops are connected by
i and it is confusing.How is T(n) of the code O(nlog(n)) using fundamental product rule?
```
for(i = 1; i <= n; i++)
{
for(j = 1; j ... |
Fori=1inner loop executesntimes. Fori=2inner loop executesn/2times and so on. The running time isT(n) = n + n/2 + n/3 + ... + n/n. This is equal ton(1 + 1/2 + 1/3 + ...+ 1/n) = nH(n)whereH(n)is the nth harmonic number.H(n) ~ lg nhence the running time ofO(n lg n).
|
Yesterday I tried to inject compilation error in my working tree. Usually I use #error to generate compilation error. But I tried #1 but it doesn't work. What could be the reason for this?
```
t1.c
#include <stdio.h>
int main (){
#error
}
**compilation error**
t1.c:3:3: error:
#e... |
Variations on the theme of# 1etc are generated by the C preprocessor as a shorthand for#line 1.
It is non-standard, but common.
You can probably see it in the output fromgcc -Eor similar options that only run the preprocessor.
|
I Have a certain C program Now I want to Port it into a Java Program. I got most of the things but I have a code like
```
tsum=(float *) malloc(cr_nos*sizeof(float));
rsum=(float *) malloc(cr_nos*sizeof(float));
ex_sum=(float *) malloc(cr_nos*sizeof(float));
bsum=(float *) malloc(cr_nos*sizeof(float));
``... |
The Java equivalent of the code above is:
```
float[] tSum = new float[crCount];
float[] rSum = new float[crCount];
float[] exSum = new float[crCount];
float[] bSum = new float[crCount];
```
... although, in Java, "t", "r", "ex", "b", and "cr" would typically be unabbreviated.
|
In the C language, when printing something on the screen, we usually useprintf,putsand so on. Which are all defined in the or other header documents.
Is there any way to print something on screen without using such functions? That is to say, how isprintfrealised?
|
Eventually the C functionprintfwill result in asys_writesystem call, directly or by going throughwrite(seeman 2 write). The actual implementation depends on the compiler and the standard libraries.
|
I'm confused about this example program from my computer architecture textbook.
Here's the C code...
And here's the generated Y86 code...
My question is with 0x046
```
mrmovl 8(%ebp), %ecx
```
Why exactly is it setting Start to 8 bytes in front of the stack pointer? I think I'm mostly confused as to where eve... |
The code pushes things onto the stack in the following order:
Count(4)Start(array)Return%eip(implicitly pushed bycall);%ebp.
The code then sets%ebpto%esp, and the stack looks like this:
(You are mainly interested in the part marked%EBPand above.)
Hope this clarifies things. You can read morehere.
|
we know that c inserts the NULL(\0) automatically at the end of an character array.But why it does not insert the NULL character at the end of an Array of pointers to strings.
For Eample: If i write
```
char name[]={40,20,30,20,22,22,'c','a','b'}
```
The NULL character will be automatically added. But if i create a... |
Your understanding is not correct. You said,
For Eample: If i writechar name[]={40,20,30,20,22,22,'c','a','b'}The NULL character will be automatically added.
That is not true.
If you use:
```
char name[] = "abc";
```
a terminating null character is added but not in the form you are using.
|
Let's say I have
```
char *greeting = "Hello";
char *names[] = { "Tom", "Jerry" };
```
I can declare a pointer to a pointer to char fornames, like this
```
char **names2 = names;
```
but I can NOT do so forgreeting.
I getwarning: initialization from incompatible pointer type.
Why?greetingis a pointer tochar, jus... |
You only have to use 1x *!
Because char * and char ** are different types!
So you tried to assign a variable with the type char * to a variable with the type char **
And that gives you a warning
So this should work:
```
char *greeting = "Hello";
char *names[] = { "Tom", "Jerry" };
char *greeti... |
I initially had a problem with my arguments, but that was fixed. However now I get a seg fault error and am not sure how to fix it.
here's my code
```
int hash_code(const char* str){
int len = strlen(str);
char dst[len-1];
if(len == 0 )
return 0;
else{
strncpy(dst, str, (len - 1));
return (hash_code(dst) * 2... |
" error: passing argument 1 of 'strncpy' discards 'const' qualifier
from pointer target type"
This error results from the fact that first argument taken by strncpy is of typechar*but you are passingconst char*. Strncpy prototype is
```
char * strncpy ( char * destination, const char * source, size_t num );
... |
I try to print the maximum value ofintin a program.
Using the following code::
```
#include <stdio.h>
#include <limits.h>
int main(void) {
printf("%d",INT_MAX);
return 0;
}
```
The output I get is::2147483647
But, when I change myprintfstatement toprintf("%lld",INT_MAX);my output becomes577732524332023807.I... |
printfis a variadic function, it doesn't know its argument types, it relies on recieving the correct hints in the format string.
You invoked undefined behaviour with"%lld", because you haven't passed along long int.
To fix it, you need to cast - then you'll see the correct result:
```
printf("%lld", (long long int)... |
Closed. This question needsdetails or clarity. It is not currently accepting answers.Want to improve this question?Add details and clarify the problem byediting this post.Closed8 years ago.Improve this question
I have a question about the two folders indicated in the image ( External dependencies and Ressource files ... |
On the top of my head, external dependencies may include DLLs having an entry point, whereas resource file may include Dlls without entry point(resource DLL) or an image.
|
I don't really understand why gcc has subtract 12 to esp before calling the function.
```
pushl %ebp
movl %esp,%ebp
sub $12,%esp
socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
movl $AF_INET,(%esp)
```
|
The current*x86 ABI requires the stack pointer to be aligned mod 16 at the time of function call. This is the typical reason for otherwise-unexplained adjustments of the stack pointer.
* I saycurrentbecause GCC actually unilaterally changed the ABI and introduced this requirement somewhere back in the 3.x series. I d... |
I have this code:
```
/* ... headers ... */
struct my_callback_struct {
int data;
int (*callback_func)(struct my_callback_struct *p, int data);
};
int reg(struct my_callback_struct *p, int data)
{
return 1;
}
void init(struct my_callback_struct *p)
{
p->callback_func = ®
}
int main ()
{
... |
You simply haven't allocated memory for yourstructinstance. Try something like
```
struct my_callback_struct s;
init(&s);
printf("%d", s.callback_func(p,12));
```
Or callmallocfor yourpbefore callinginit.
|
This question already has answers here:Asymptotic analysis(2 answers)Closed8 years ago.
I dont get the part where T(n) of the second for loop is log(n). Both loops are connected by
i and it is confusing.How is T(n) of the code O(nlog(n)) using fundamental product rule?
```
for(i = 1; i <= n; i++)
{
for(j = 1; j ... |
Fori=1inner loop executesntimes. Fori=2inner loop executesn/2times and so on. The running time isT(n) = n + n/2 + n/3 + ... + n/n. This is equal ton(1 + 1/2 + 1/3 + ...+ 1/n) = nH(n)whereH(n)is the nth harmonic number.H(n) ~ lg nhence the running time ofO(n lg n).
|
Yesterday I tried to inject compilation error in my working tree. Usually I use #error to generate compilation error. But I tried #1 but it doesn't work. What could be the reason for this?
```
t1.c
#include <stdio.h>
int main (){
#error
}
**compilation error**
t1.c:3:3: error:
#e... |
Variations on the theme of# 1etc are generated by the C preprocessor as a shorthand for#line 1.
It is non-standard, but common.
You can probably see it in the output fromgcc -Eor similar options that only run the preprocessor.
|
I Have a certain C program Now I want to Port it into a Java Program. I got most of the things but I have a code like
```
tsum=(float *) malloc(cr_nos*sizeof(float));
rsum=(float *) malloc(cr_nos*sizeof(float));
ex_sum=(float *) malloc(cr_nos*sizeof(float));
bsum=(float *) malloc(cr_nos*sizeof(float));
``... |
The Java equivalent of the code above is:
```
float[] tSum = new float[crCount];
float[] rSum = new float[crCount];
float[] exSum = new float[crCount];
float[] bSum = new float[crCount];
```
... although, in Java, "t", "r", "ex", "b", and "cr" would typically be unabbreviated.
|
In the C language, when printing something on the screen, we usually useprintf,putsand so on. Which are all defined in the or other header documents.
Is there any way to print something on screen without using such functions? That is to say, how isprintfrealised?
|
Eventually the C functionprintfwill result in asys_writesystem call, directly or by going throughwrite(seeman 2 write). The actual implementation depends on the compiler and the standard libraries.
|
I'm confused about this example program from my computer architecture textbook.
Here's the C code...
And here's the generated Y86 code...
My question is with 0x046
```
mrmovl 8(%ebp), %ecx
```
Why exactly is it setting Start to 8 bytes in front of the stack pointer? I think I'm mostly confused as to where eve... |
The code pushes things onto the stack in the following order:
Count(4)Start(array)Return%eip(implicitly pushed bycall);%ebp.
The code then sets%ebpto%esp, and the stack looks like this:
(You are mainly interested in the part marked%EBPand above.)
Hope this clarifies things. You can read morehere.
|
we know that c inserts the NULL(\0) automatically at the end of an character array.But why it does not insert the NULL character at the end of an Array of pointers to strings.
For Eample: If i write
```
char name[]={40,20,30,20,22,22,'c','a','b'}
```
The NULL character will be automatically added. But if i create a... |
Your understanding is not correct. You said,
For Eample: If i writechar name[]={40,20,30,20,22,22,'c','a','b'}The NULL character will be automatically added.
That is not true.
If you use:
```
char name[] = "abc";
```
a terminating null character is added but not in the form you are using.
|
I have an empty string (char *str) and I want to concatenate characters to this string. I have tried this
char *str;char arr[10]; //where as arr contains some characters
And when I do this:
```
str=str + arr[0];
```
It does nothing tostr.
|
In C you cannot concatenate two strings using+, as strings are not inbuilt datatype, usestrcat()which is defined instring.h.
A small example:
```
/* strcat example */
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main ()
{
char *str;
char arr[10];
puts("Enter some string:");
scanf(... |
Is there any way to do this withstrtokfunction? or any suggestions ?
Example :
```
Insert "hello world" to dbms
```
Result:
```
Insert
"hello world"
to
dbms
```
|
strtokor any other function in the standard C library can't do this for you. To get it, you have to write code for it yourself, or you have to find some existing code in some external library.
|
I use glib's lists (linked-lists) for storing a list of strings in my program. I'm already able to append, prepend or insert new items to the list but I can't edit an existing item in the list. I cannot find the appropriate function for editing items in glib's documentation. Is there any functions implemented in glib ... |
I think you're supposed to use the structure directly, as the internal structure isexposed:
```
struct GList {
gpointer data;
GList *next;
GList *prev;
};
```
So, for example:
```
GList *bar = g_list_last(foo);
bar->data = NULL; //for instance
```
|
How can I convert alongvariable to achar[]variable without using library functions?
|
Working example (1) - thread-safe, requires min. buffsize = 40.
```
static const char *
xllitoa(long long int x, char *buff)
{
char *p = buff + 40;
int sign = 0;
*(p--) = 0;
if (x < 0) sign = 1;
else x = -x;
do { *(p--) = -(x % 10) + '0'; x /= 10; } while(x);
if... |
Is usingwhile(*p++)for checking if an array has more elements dangerous?
Could there be problems if on the next memory location there is some value and this value is not a part of an array.
This simple code:
```
#include <stdio.h>
void f(float *p) {
printf("%p : %.f\n", p, *p);
while(*p++){
printf(... |
As soon aspgoes outside of the bounds the array, dereferencing it invokes undefined behavior. So yes, doing this is dangerous.
|
I want to programmatically enable and disable the mouse in Linux using C/C++. There is the input extension for X11 that allows you to do it using the terminal command:
```
xinput set-int-prop "<device name>" "Device Enabled" 8 0
```
This works perfectly, but I'm looking to do this directly using a library. I can't f... |
You do realize thatxinputis open source, right? :)Here's the source tree.
It seems to boil down to a call toXIChangeProperty().
You can probably read the code a bit more closely than I did, and the manual page of course, to figure out the required arguments.
|
I'm new to C and would like some help with an issue I have. I'm reading and writing to pipes as follows:
```
f = fdopen(fdH2P[WRITE], "w"); // writing to pipe, returns a file pointer
```
and
```
r = fdopen(fdP2H[READ], "r"); // reading from pipe
```
I want to return both these file pointers from my function. What'... |
You can either put the two file pointers into a structure and return that, or you can pass pointers to the function, like this
```
void GetPipes( FILE **wptr, FILE **rptr )
{
*wptr = fdopen(fdH2P[WRITE], "w");
*rptr = fdopen(fdP2H[READ], "r");
}
void SomeOtherFunction( void )
{
FILE *wptr, *rptr;
GetPipe... |
```
#include <stdio.h>
int main(void)
{
double a = 1234.5f;
int b = 71;
int c = 68;
int d;
printf("%d %d %d %d\n", a,b,c,d);
return 0;
... |
"%d"in format specifier expectsint, butahas a type ofdouble, so it's undefined behaviour.
One possibility of what can happen is, the compiler puts the variables one by one on the stack. If, on your platform, the size ofdoublewere 8 bytes and twice the size ofintthe compiler makes wrong assumption of where to read the... |
Is usingwhile(*p++)for checking if an array has more elements dangerous?
Could there be problems if on the next memory location there is some value and this value is not a part of an array.
This simple code:
```
#include <stdio.h>
void f(float *p) {
printf("%p : %.f\n", p, *p);
while(*p++){
printf(... |
As soon aspgoes outside of the bounds the array, dereferencing it invokes undefined behavior. So yes, doing this is dangerous.
|
I want to programmatically enable and disable the mouse in Linux using C/C++. There is the input extension for X11 that allows you to do it using the terminal command:
```
xinput set-int-prop "<device name>" "Device Enabled" 8 0
```
This works perfectly, but I'm looking to do this directly using a library. I can't f... |
You do realize thatxinputis open source, right? :)Here's the source tree.
It seems to boil down to a call toXIChangeProperty().
You can probably read the code a bit more closely than I did, and the manual page of course, to figure out the required arguments.
|
I'm new to C and would like some help with an issue I have. I'm reading and writing to pipes as follows:
```
f = fdopen(fdH2P[WRITE], "w"); // writing to pipe, returns a file pointer
```
and
```
r = fdopen(fdP2H[READ], "r"); // reading from pipe
```
I want to return both these file pointers from my function. What'... |
You can either put the two file pointers into a structure and return that, or you can pass pointers to the function, like this
```
void GetPipes( FILE **wptr, FILE **rptr )
{
*wptr = fdopen(fdH2P[WRITE], "w");
*rptr = fdopen(fdP2H[READ], "r");
}
void SomeOtherFunction( void )
{
FILE *wptr, *rptr;
GetPipe... |
```
#include <stdio.h>
int main(void)
{
double a = 1234.5f;
int b = 71;
int c = 68;
int d;
printf("%d %d %d %d\n", a,b,c,d);
return 0;
... |
"%d"in format specifier expectsint, butahas a type ofdouble, so it's undefined behaviour.
One possibility of what can happen is, the compiler puts the variables one by one on the stack. If, on your platform, the size ofdoublewere 8 bytes and twice the size ofintthe compiler makes wrong assumption of where to read the... |
I must be missing something but I'm trying to use the Leptonica C library in Go using a C wrapper that was made for that purpose:
https://github.com/GeertJohan/go.leptonica/blob/master/leptonica.go
But I don't understand... it allows me to import files but then how do I actually invoke the Leptonica functions? (Spec... |
go.leptonicasimply hasn't implemented that. You'll have to call the C functions yourself.
Useleptonica.goas a starting point, and write your own wrapper.
|
According to the man page for pause, it returns -1 and sets errno to EINTR.
Why does it do this? I understood -1 to mean a failure, not something operating as normal.
|
The only timepause()stops pausing is when it is interrupted, and that is a return on failure (to continue pausing). So, correctly, it returns -1 to indicate that it failed to continue pausing.
|
I want to send a http message with a signature header. Some thing like that:
```
POST / HTTP/1.1
Host: server.com
Content-Type: text/xml; charset=utf-8
Content-Length: 8
Signature: 42A424FB0C4938BBF08259514C64ABFC1E74C61F
any data
```
The signature header contains the 40-character hexadecimal representation (case
... |
No it doesn't support that header specifically, but it supports thatyouinsert that header in the request, usingCURLOPT_HTTPHEADERas shown in thehttpcustomheaderexample.
|
I wrote the following C code:
```
#include<stdio.h>
int main(){
printf("A");
if(fork() == 0){
printf("B");
}
else{
printf("C");
}
}
```
The output I got is:
```
ACAB
```
I expected this code to print A only once.Can anyone explain this output?
|
Your error is not flushing the buffers beforefork-ing, thus both processes will write it.
Add this beforefork():
```
fflush(0); // Flush all output-streams
```
|
Libcurl is putting Accept on the http header with the default valueAccept: */*.
I want to avoid libcurl putting Accept in the http header. How I can do that?
|
You set a header with no "contents" on the right of the colon to remove it from libcurl's request (see theCURLOPT_HTTPHEADERdocs):
```
struct curl_slist *chunk = NULL;
chunk = curl_slist_append(chunk, "Accept:");
res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, chunk);
res = curl_easy_perform(curl);
```
For a sligh... |
Are both \ and 0 characters stored in the same location or in different locations in the end of the string?
```
main()
{
char x[]="Hello\0";
char y[]="Hello12";
char z[]="Hello\012";
char w[]="Hello1234";
printf("%d %d %d %d", sizeof(x), sizeof(y), sizeof(z), sizeof(w));
}
```
Output:
```
7 8 7... |
\0in a C string is a single character, ASCII value0. All C string literals also include an implicit terminating\0character, regardless of what else is included in the string (even another\0).
\012is the octal character ASCII 10 (Line Feed)
So:
```
char x[]="Hello\0"; // 5 letters + your \0 + implicit \0 == 7
c... |
Suppose I have a function
```
int myfunction(char *s)
```
In this function, the string pointed to bysgets manipulated, and thereturnvalue is an error code.
Suppose the pointer passed already pointed tomalloc()memory. Is it my responsibility (developing a library) to free this memory at the top of the function, or i... |
The usual contract is the person doing themallocis the person doing thefree.
Also consider that the data pointed to may not have beenmalloc'ed in the first place. Could be on the stack
|
I have the following code in which I want to modify printf and write in a file. I have used macros for the same.
```
#include<stdio.h>
#define printf(A) {FILE *fp;\
fp=fopen("oup.txt","wb");\
fprintf(fp,A);\
fclose(fp);}
int main()
{
int i;
for(i=0;i<1... |
@interjay, NPE - simple but brilliant answer. I will add an example withfreopen:
```
#include <stdio.h>
int main ()
{
FILE *fp;
printf("This text is redirected to stdout\n");
fp = freopen("file.txt", "w+", stdout);
printf("This text is redirected to file.txt\n");
fclose(fp);
return(0);
}
```
|
Can I do this?
```
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headerlist);
curl_slist_free_all(headerlist);
```
That is, can I free the header list before I send the request? Does curl make a copy?
|
No. In the source the setopt function just saves your pointer.
```
case CURLOPT_HTTPHEADER:
/*
* Set a list with HTTP headers to use (or replace internals with)
*/
data->set.headers = va_arg(param, struct curl_slist *);
break;
```
|
i want to split a 29 bit value into bytes ? how can i accomplish this in C?
```
unsigned long value = 0x18FEF512;
```
I need the output like this val=0x18, val1=FE, val2=F5 , val3=12.
How to do this using bit twidlling?
|
try this:
```
unsigned long value = 0x18FEF512;
unsigned char values[4];
values[0] = value;
values[1] = value >> 8;
values[2] = value >> 16;
values[3] = value >> 24;
printf("%02X %02X %02X %02X\n",values[0],values[1],values[2],values[3]);
```
|
I am working on polynom over Z/2Z and we represent it by an integer in hexadecimal :
for example, 0xD4 is X^7+x^6+X^4+X^2 (0xD4 is 11010100 in binary)
And I just want to implement a function that prints the polynom just like I did with 0xD4.
So I can convert my hexadecimal number into binary and then print X^n when th... |
You don't need to convert anything. Just test
```
if (poly & (1 << k)) {...}
```
to see if thek-th power is present (i.e. the corresponding coefficient is 1 rather than 0).
Wrap this in a loop and you're done.
(Here,polyis the0xD4from your example.)
|
Is it possible to clear the output at the command prompt using C on windows?
For example, on linux I could do
```
printf("\033[2J");
```
But as far as I know windows doesn't recognise the ANSI escape codes
Thanks.
EDIT: I guess I'll also need to get the cursor back to 0,0 fo r the next output after the clear...
|
There are many way to do that on windows.
You includeconio.hand call_clrscr();
Or you can callsystem("cls");
|
I must be missing something but I'm trying to use the Leptonica C library in Go using a C wrapper that was made for that purpose:
https://github.com/GeertJohan/go.leptonica/blob/master/leptonica.go
But I don't understand... it allows me to import files but then how do I actually invoke the Leptonica functions? (Spec... |
go.leptonicasimply hasn't implemented that. You'll have to call the C functions yourself.
Useleptonica.goas a starting point, and write your own wrapper.
|
I have 4 integer variables,a,b,c,dthat vary between 0 andA,B,C,Drespectively, i.e.
```
0 <= a < A
0 <= b < B
0 <= c < C
0 <= d < D
```
I encode them into an index as tightly as possible so that each unique (a,b,c,d) combo produces a unique index:
```
int index(int a, int b, int c, int d) {
return a + A*b + A*B... |
```
a = index % A;
index /= A;
b = index % B;
index /= B;
c = index % C;
index /= C;
d = index;
```
Or
```
a = index % A;
b = (index / A) % B;
c = (index / (A*B)) % C;
d = (index / (A*B*C));
```
Assuming that you correct your conditions to
```
0 <= a < A
0 <= b < B
0 <= c < C
0 <= d < D
```
With<=instead of<you d... |
Given the following struct:
```
typedef struct {
char a;
int b[10];
} elem;
elem s[100];
```
and knowing that s, i, and j are located in %ebx, %esi, and %edi respectively, how do I determine the memory address of
```
s[i].b[j]
```
?
Thank you!
|
That's pretty easy:
The address ofs[i]is offset from the address ofs[0]byi * sizeof(elem)bytes.The address of the memberb[j]is offset from the memberb[0]byj * sizeof(int)bytes.The address ofb[0]inside an objectelem xis offset from the address ofxbyoffsetof(elem, b)bytes. You need to#include <stddef.h>for this macro.
... |
I have the following function in assembly, but I need it in C. The code is:
```
push ebp
mov ebp, esp
mov eax, DWORD PTR [ebp+8]
add eax, 1667940388
pop ebp
ret
```
Could anybody translate this into C for me. Thanks.
|
Sorry for the silly mistake on theeaxreturn value ... its' fixed now ... my assembly is a little rusty :)
```
int function(int value)
{
return value + 1667940388;
}
```
|
I am new in C programming and I am currently learning aboutwhileloops. The problem I have is, that thewhileloop must continue until the user wishes to terminate thewhileloop. But when i run my code it seems thatscanf()only once scans for input and thewhileloops terminates afterwards and I don't know why.
```
int main... |
You need to consume the newline character.
Change
```
scanf("%c", &answer);
```
to
```
scanf(" %c", &answer);
```
|
All is in the title.
In time.h :
The C library function clock_t clock(void)returns the number of clock
ticks elapsed since the program was launched
But for the current process or all process of the OS ?
Thx.
|
Thx isedev.
This is actually the approximated clock ticks of the current process.
```
clock_t t;
t = clock();
printf ("Calculating...\n");
float u = 0;
for(int i = 0 ; i < 1000000 ; i++)
u += sqrt(i);
printf("%f", u);
t = clock() - t;
printf ("%d clock ticks elapsed\n%f ms elapsed.\n", t, ((float) t)/CLOCKS_PER_S... |
I am facing a small problem that I haven't seen before. I am trying to debug a software package and running through GDB (on Ubuntu Linux).
After few seconds/minutes, I get this message:
```
Program terminated with signal SIGALRM, Alarm clock.
The program no longer exists
```
Then, I quit GDB, restart and continue de... |
Tell gdb toignorethe signal:
```
(gdb) handle SIGALRM ignore
```
|
I came across a warning in my compiler: "address will never be NULL"The code looks like:
```
struct mydata * var = NULL;
/* some function which may modify var*/
if(NULL != &var) {
// do something
}
```
The actual warning (-Werror flagged):
error: the comparison will always evaluate as 'true' for the address of ... |
If you use it yes, it will always have a valid address.
If you don't use it, it will probably be optimized or ignored by the compiler. So this address will have no value, but you will never know it.
|
I have 4 integer variables,a,b,c,dthat vary between 0 andA,B,C,Drespectively, i.e.
```
0 <= a < A
0 <= b < B
0 <= c < C
0 <= d < D
```
I encode them into an index as tightly as possible so that each unique (a,b,c,d) combo produces a unique index:
```
int index(int a, int b, int c, int d) {
return a + A*b + A*B... |
```
a = index % A;
index /= A;
b = index % B;
index /= B;
c = index % C;
index /= C;
d = index;
```
Or
```
a = index % A;
b = (index / A) % B;
c = (index / (A*B)) % C;
d = (index / (A*B*C));
```
Assuming that you correct your conditions to
```
0 <= a < A
0 <= b < B
0 <= c < C
0 <= d < D
```
With<=instead of<you d... |
Given the following struct:
```
typedef struct {
char a;
int b[10];
} elem;
elem s[100];
```
and knowing that s, i, and j are located in %ebx, %esi, and %edi respectively, how do I determine the memory address of
```
s[i].b[j]
```
?
Thank you!
|
That's pretty easy:
The address ofs[i]is offset from the address ofs[0]byi * sizeof(elem)bytes.The address of the memberb[j]is offset from the memberb[0]byj * sizeof(int)bytes.The address ofb[0]inside an objectelem xis offset from the address ofxbyoffsetof(elem, b)bytes. You need to#include <stddef.h>for this macro.
... |
I have the following function in assembly, but I need it in C. The code is:
```
push ebp
mov ebp, esp
mov eax, DWORD PTR [ebp+8]
add eax, 1667940388
pop ebp
ret
```
Could anybody translate this into C for me. Thanks.
|
Sorry for the silly mistake on theeaxreturn value ... its' fixed now ... my assembly is a little rusty :)
```
int function(int value)
{
return value + 1667940388;
}
```
|
I am new in C programming and I am currently learning aboutwhileloops. The problem I have is, that thewhileloop must continue until the user wishes to terminate thewhileloop. But when i run my code it seems thatscanf()only once scans for input and thewhileloops terminates afterwards and I don't know why.
```
int main... |
You need to consume the newline character.
Change
```
scanf("%c", &answer);
```
to
```
scanf(" %c", &answer);
```
|
All is in the title.
In time.h :
The C library function clock_t clock(void)returns the number of clock
ticks elapsed since the program was launched
But for the current process or all process of the OS ?
Thx.
|
Thx isedev.
This is actually the approximated clock ticks of the current process.
```
clock_t t;
t = clock();
printf ("Calculating...\n");
float u = 0;
for(int i = 0 ; i < 1000000 ; i++)
u += sqrt(i);
printf("%f", u);
t = clock() - t;
printf ("%d clock ticks elapsed\n%f ms elapsed.\n", t, ((float) t)/CLOCKS_PER_S... |
I am facing a small problem that I haven't seen before. I am trying to debug a software package and running through GDB (on Ubuntu Linux).
After few seconds/minutes, I get this message:
```
Program terminated with signal SIGALRM, Alarm clock.
The program no longer exists
```
Then, I quit GDB, restart and continue de... |
Tell gdb toignorethe signal:
```
(gdb) handle SIGALRM ignore
```
|
I came across a warning in my compiler: "address will never be NULL"The code looks like:
```
struct mydata * var = NULL;
/* some function which may modify var*/
if(NULL != &var) {
// do something
}
```
The actual warning (-Werror flagged):
error: the comparison will always evaluate as 'true' for the address of ... |
If you use it yes, it will always have a valid address.
If you don't use it, it will probably be optimized or ignored by the compiler. So this address will have no value, but you will never know it.
|
Given the following struct:
```
typedef struct {
char a;
int b[10];
} elem;
elem s[100];
```
and knowing that s, i, and j are located in %ebx, %esi, and %edi respectively, how do I determine the memory address of
```
s[i].b[j]
```
?
Thank you!
|
That's pretty easy:
The address ofs[i]is offset from the address ofs[0]byi * sizeof(elem)bytes.The address of the memberb[j]is offset from the memberb[0]byj * sizeof(int)bytes.The address ofb[0]inside an objectelem xis offset from the address ofxbyoffsetof(elem, b)bytes. You need to#include <stddef.h>for this macro.
... |
I have the following function in assembly, but I need it in C. The code is:
```
push ebp
mov ebp, esp
mov eax, DWORD PTR [ebp+8]
add eax, 1667940388
pop ebp
ret
```
Could anybody translate this into C for me. Thanks.
|
Sorry for the silly mistake on theeaxreturn value ... its' fixed now ... my assembly is a little rusty :)
```
int function(int value)
{
return value + 1667940388;
}
```
|
I am new in C programming and I am currently learning aboutwhileloops. The problem I have is, that thewhileloop must continue until the user wishes to terminate thewhileloop. But when i run my code it seems thatscanf()only once scans for input and thewhileloops terminates afterwards and I don't know why.
```
int main... |
You need to consume the newline character.
Change
```
scanf("%c", &answer);
```
to
```
scanf(" %c", &answer);
```
|
All is in the title.
In time.h :
The C library function clock_t clock(void)returns the number of clock
ticks elapsed since the program was launched
But for the current process or all process of the OS ?
Thx.
|
Thx isedev.
This is actually the approximated clock ticks of the current process.
```
clock_t t;
t = clock();
printf ("Calculating...\n");
float u = 0;
for(int i = 0 ; i < 1000000 ; i++)
u += sqrt(i);
printf("%f", u);
t = clock() - t;
printf ("%d clock ticks elapsed\n%f ms elapsed.\n", t, ((float) t)/CLOCKS_PER_S... |
I am facing a small problem that I haven't seen before. I am trying to debug a software package and running through GDB (on Ubuntu Linux).
After few seconds/minutes, I get this message:
```
Program terminated with signal SIGALRM, Alarm clock.
The program no longer exists
```
Then, I quit GDB, restart and continue de... |
Tell gdb toignorethe signal:
```
(gdb) handle SIGALRM ignore
```
|
I came across a warning in my compiler: "address will never be NULL"The code looks like:
```
struct mydata * var = NULL;
/* some function which may modify var*/
if(NULL != &var) {
// do something
}
```
The actual warning (-Werror flagged):
error: the comparison will always evaluate as 'true' for the address of ... |
If you use it yes, it will always have a valid address.
If you don't use it, it will probably be optimized or ignored by the compiler. So this address will have no value, but you will never know it.
|
All the examples I've found in the net about the usage ofgetopt_long(for examplethis here) declare thestruct optionwith the long options asstatic.
I don't understand why all these examples declare a static variable, I don't see why this should be done and the man pages don't say anything about this either.
Should I ... |
The reason is efficiency. If you declare thestruct option long_options[]not static, it will be constructed at run time upon entry into the main function, but if it is declared as static, it will be constructed at compile time.
But it will work either way.
|
I'm reading a 2 MB word list with about 200 000 words into memory. To be able to index them I use achar *array. Since the words won't grow or shrink, is it better to read the whole file into one memory block and let the pointers point into that, rather than doing amalloc()for every word?
|
Contrary to some of the comments posted so far, if you are tight on memory, you should allocate a single large block. This is because eachmalloc()you do has some overhead which is more or less fixed. This overhead will be a few bytes per allocation, so many small allocations could have you losing half your memory to... |
The following code in C accepts integers as long as data is fed into it.
```
#include<stdio.h>
#include<conio.h>
void main()
{
int x;
while(scanf("%d",&x)!=EOF)
{
printf("%d\n",x);
}
getche ();
}
```
It works fine for integer inputs but on entering any character/float input it becomes an infinite loop.Why is it so... |
Ifscanffailes to parse any input, it will leave the input in the buffer and return0. As the erroneous input will never leave the buffer and you don't check for anything butEOFbeing returned, the loop will continue for ever.
|
In a function if I am returning an address of some primitive type of object. Can I be assured that it will never be aNULLvalue?
Do I need to check forNULLwith a statement like
```
if (r_value) { //do something }
else { //ERROR }
```
?
|
The address of an object will never be null.
Be careful you're not returning a pointer to a non-static local variable. That will be destroyed when the function returns, so the pointer will be invalid; but it won't be null, and there'll be no way to tell that it's no longer valid.
In C++, I'd return a reference rathe... |
How do I create a soft link programmatically in C/C++? link() system call in freebsd will create a hard link.
|
The system call you want issymlink(2).
```
#include <unistd.h>
int symlink(const char *name1, const char *name2);
```
A symbolic linkname2is created toname1
|
```
hCom = CreateFile(
"COM1",
GENERIC_READ | GENERIC_WRITE,
0,
NULL,
OPEN_EXISTING,
0,
NULL
);
```
I have just one COM port on my computer, but this function returns for COM1, COM3, COM4, COM5, COM6, COM7 a value unless "INVALID_HANDLE_VALUE". Why is this so?
|
You are calling the function correctly. Unless it returns INVALID_HANDLE_VALUE, there should be an available port. Note that this might include USB ports!
Also please note that for COM port 10 and above, you need to open them with the command\\.\\COMn, which corresponds to the C string\\\\.\\COMn(where n is the 1 or ... |
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