redirect output from stdout to a string?

Question

in C i want to redirect the output of a process from stdout to write to a "shared memory segment" which can be thought of as a char array or a string with a pointer
i know that there is dup2 but it takes file discriptors as argument not a pointer to an array. is there any way to redirect it to a string?

sr01853 · Accepted Answer · 2013-01-20T19:46:31.813

7

char string[SIZE];
freopen("/dev/null", "a", stdout);
setbuf(stdout, string);

see freopen and setbuf for their definitions

edited Jan 20 '13 at 19:46

answered Jan 20 '13 at 19:39

sr01853

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Note that the buffer size MUST be exactly `BUFSIZ` in bytes! If it's smaller, you'll have a buffer overflow vulnerability, and if it's larger, it will never be used. This solution will not work if you want to buffer more than BUFSIZ bytes. – Vladimir Panteleev Apr 24 '14 at 13:23
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how to undo this ? Is it need to undo it ? – kangear Jan 18 '15 at 07:12
Windows equivalent please? – Unknown123 Oct 16 '22 at 17:56
@Unknown123 Replace on line 2 `freopen("/dev/null", "a", stdout);` with `freopen("nul", "a", stdout);` – FFmpegEnthusiast Aug 24 '23 at 06:50

score 3 · Answer 2 · answered Aug 21 '17 at 18:02

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This should work on UNIX systems:

// set buffer size, SIZE
SIZE = 255;

char buffer[SIZE];
freopen("/dev/null", "a", stdout);
setbuf(stdout, buffer);
printf("This will be stored in the buffer");
freopen ("/dev/tty", "a", stdout);

answered Aug 21 '17 at 18:02

setnoset

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Thanks, searched for ages trying to find how to do this. A few points !) requires c99 or later, 2) I had to set buffer[SIZE]={0} to avoid trailing garbage and 3) also works on Windows if you replace "/dev/tty" with "CON". – NoComprende Jan 17 '22 at 16:11

score 0 · Answer 3 · answered Jan 20 '13 at 19:34

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You could write to a pipe, and read from it into shared memory (that is, if you can't use the pipe instead of the string in shared memory).

answered Jan 20 '13 at 19:34

NPE

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i have two different processes (not a child and a parent) so i cant do pipe between them. right? – CSawy Jan 20 '13 at 21:04

score 0 · Answer 4 · answered Jan 23 '16 at 07:22

with shm_open you can have a file descriptor pointing to a shared memory and pass it to dup2 function as below:

int fd = shm_open("shm-name", O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
dup2(fd, STDOUT_FILENO);
fprintf(stdout, "This is string is gonna be printed on shared memory");

And after all seek the shared memory to the beginning (with lseek read it and save it to a string; But be careful

Also you can find an example of buffering into pipe in Here

score 0 · Answer 5 · edited Mar 30 '17 at 05:29

In order to do a simple redirection of stdout to a memory string, just do this:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define PATH_MAX 1000

int main()
{
    FILE *fp;
    int status;
    char path[PATH_MAX];

    fp = popen("ls ", "r");

    if (fp == NULL)  return 0;


    while (fgets(path, PATH_MAX, fp) != NULL)
        printf("\nTest=> %s", path);

    status = pclose(fp);
}

redirect output from stdout to a string?

5 Answers5

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