Redirecting stdout to pipe in C

Question

Here's a program I'm trying to make:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>



int main(int argc, char* argv[])
{
    char* arguments[] = {"superabundantes.py", NULL};

    int my_pipe[2];
    if(pipe(my_pipe) == -1)
    {
        fprintf(stderr, "Error creating pipe\n");
    }

    pid_t child_id;
    child_id = fork();
    if(child_id == -1)
    {
        fprintf(stderr, "Fork error\n");
    }
    if(child_id == 0) // child process
    {
        close(my_pipe[0]); // child doesn't read
        dup2(my_pipe[1], 1); // redirect stdout

        execvp("cat", arguments);

        fprintf(stderr, "Exec failed\n");
    }
    else
    {
        close(my_pipe[1]); // parent doesn't write

        char reading_buf[1];
        while(read(my_pipe[0], reading_buf, 1) > 0)
        {
            write(1, reading_buf, 1); // 1 -> stdout
        }
        close(my_pipe[0]);
        wait();
    }
}

I want to execute the exec in the child redirecting the stdout of the child to the parent (through the pipe). I think the problem might be related to dup2, but I haven't used it before.

Please specify what "the problem" is instead of just dumping your code to let us find out. If you don't know what the problem is, then add error reporting to your program. — Fred Foo, May 22 '12 at 11:48
Don't resolve your problem, but you must specify a int *sts for wait() function (can be NULL). — André A. G. Scotá, May 22 '12 at 12:17
thanks for this! works perfect after change char* arguments[] = {"cat', "superabundantes.py", NULL}; — user1932637, Aug 21 '19 at 22:08

score 3 · Accepted Answer · answered May 22 '12 at 11:48

3

You need to also provide argv[0] when you call exec. So your arguments should read:

char* arguments[] = {"cat", "superabundantes.py", NULL};

answered May 22 '12 at 11:48

kmkaplan

18,655
4
51
65

Yes! You're right. char* arguments[] = {"cat", "superabundantes.py", NULL}; execvp(argv[0], arguments); It works perfectly, thank you. – XavierusWolf May 22 '12 at 12:12

score -2 · Answer 2 · answered Aug 21 '19 at 22:34

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>



int main(int argc, char* argv[])
{
    //char* arguments[] = {"cat","tricky.txt", NULL};
    char* arguments[] = {"./son1", NULL};
    int my_pipe[2];
    if(pipe(my_pipe) == -1)
    {
       fprintf(stderr, "Error creating pipe\n");
    }

    pid_t child_id;
    child_id = fork();
    if(child_id == -1)
    {
        fprintf(stderr, "Fork error\n");
    }
    if(child_id == 0) // child process
    {
        close(my_pipe[0]); // child doesn't read
        dup2(my_pipe[1], 1); // redirect stdout

        execvp(arguments[0], arguments);

        fprintf(stderr, "Exec failed\n");
    }
    else
    {
        close(my_pipe[1]); // parent doesn't write

        char reading_buf[1];

        while(read(my_pipe[0], reading_buf, 1) > 0)
        {
           write(1, reading_buf, 1); // 1 -> stdout
        }

        close(my_pipe[0]);
        wait();
   }

}

/* if ./son1 returns printf() in son1 will be output by write(1 ..) in parent if son1 is in dead loop then printf() in son1 will NOT be output by write(1 ..) in parent

void main()
{
    printf( "***** son run *****\n\r" );
    return;
    while(1);
}

any idea?
*/

Redirecting stdout to pipe in C

2 Answers2

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