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?

Answer 1

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

see freopen and setbuf for their definitions

Answer 2

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);

Answer 3

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).

Answer 4

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

Answer 5

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);
}