fork子进程执行cmd，用管道交互 From https://stackoverflow.com/questions/15058876/how-to-capture-the-exit-code-and-stderr-of-the-command-that-is-run-in-c

childproc.c

#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");
        }
    }
}

输入：write(PARENT_WRITE_FD, "2^32\n", 5); 要带回车!!!