How to capture the exit_code and stderr of the command that is run in C++?

Question

I\'m writing a c++ program that executes and outputs (in real-time) a shell script, makefile or just another program. However I would like to have my program return differen

Answer 1

The returnvalue of the child process is in the top 16 8 bits. You have to divide the returned value of pclose by 256, then you get the searched return value of the child process.

Gotten from http://bytes.com/topic/c/answers/131694-pclose-returning-termination-status-command

My answer would be pclose(in)/256 is exit code.

I still don't know how to capture stderr or sdtout differently but until there's an answer for that I will accept this as my answer.

Answer 2

Thanks for the reply about exit code Logan.

I believe a round-trip to get stderr would be to redirect it to a temporary file:

FILE* f = popen("cmd 2>/tmp/tmpfile.txt", "r");

Answer 3

If you are interested in the error code, this is a more portable way of getting it rather than dividing by 256:

printf("Exit code: %i\n", WEXITSTATUS(pclose(fp)));

However, popen is one way, so you are either creating further workarounds by the usual redirection style in shell, or you follow this untested code to do it right:

#include <unistd.h>
#include <stdio.h>

/* since pipes are unidirectional, we need two pipes.
   one for data to flow from parent's stdout to child's
   stdin and the other for child's stdout to flow to
   parent's stdin */

#define NUM_PIPES          2

#define PARENT_WRITE_PIPE  0
#define PARENT_READ_PIPE   1

int pipes[NUM_PIPES][2];

/* always in a pipe[], pipe[0] is for read and 
   pipe[1] is for write */
#define READ_FD  0
#define WRITE_FD 1

#define PARENT_READ_FD  ( pipes[PARENT_READ_PIPE][READ_FD]   )
#define PARENT_WRITE_FD ( pipes[PARENT_WRITE_PIPE][WRITE_FD] )

#define CHILD_READ_FD   ( pipes[PARENT_WRITE_PIPE][READ_FD]  )
#define CHILD_WRITE_FD  ( pipes[PARENT_READ_PIPE][WRITE_FD]  )

void
main()
{
    int outfd[2];
    int infd[2];

    // pipes for parent to write and read
    pipe(pipes[PARENT_READ_PIPE]);
    pipe(pipes[PARENT_WRITE_PIPE]);

    if(!fork()) {
        char *argv[]={ "/usr/bin/bc", "-q", 0};

        dup2(CHILD_READ_FD, STDIN_FILENO);
        dup2(CHILD_WRITE_FD, STDOUT_FILENO);

        /* Close fds not required by child. Also, we don't
           want the exec'ed program to know these existed */
        close(CHILD_READ_FD);
        close(CHILD_WRITE_FD);
        close(PARENT_READ_FD);
        close(PARENT_WRITE_FD);

        execv(argv[0], argv);
    } else {
        char buffer[100];
        int count;

        /* close fds not required by parent */       
        close(CHILD_READ_FD);
        close(CHILD_WRITE_FD);

        // Write to child’s stdin
        write(PARENT_WRITE_FD, "2^32\n", 5);

        // Read from child’s stdout
        count = read(PARENT_READ_FD, buffer, sizeof(buffer)-1);
        if (count >= 0) {
            buffer[count] = 0;
            printf("%s", buffer);
        } else {
            printf("IO Error\n");
        }
    }
}

The code is from here:

http://jineshkj.wordpress.com/2006/12/22/how-to-capture-stdin-stdout-and-stderr-of-child-program/