Question

I’m writing a coprocess program using pipe. It works fine when the child read some data, handle it and output it. But when I read all the data and handle it, it just pending. Any body have some idea? Thank you.

Here is the source code:

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>


int main()
{
    #define MAXSIZE 1024

    char workload[MAXSIZE];
    char result[MAXSIZE];
    workload[strlen(workload)] = EOF;
    int workload_size = strlen(workload);

    int fd1[2], fd2[2];
    int n;
    pid_t pid;
    if (pipe(fd1) < 0 || pipe(fd2) < 0) {
        fprintf(stderr, "pipe error: %s\n", strerror(errno));
        exit(1);
    }

    if ((pid = fork()) < 0) {
        fprintf(stderr, "fork error: %s\n", strerror(errno));
        exit(1);
    } else if (pid > 0) {
        close(fd1[0]);
        close(fd2[1]);
        while(fgets(workload, MAXSIZE, stdin) != NULL)
        {
            workload_size = strlen(workload);
            if (write(fd1[1], workload, workload_size) != workload_size) {
                fprintf(stderr, "write to pipe error: %s\n", strerror(errno));
                exit(1);
            }

            if ((n = read(fd2[0], result, MAXSIZE)) < 0) {
                fprintf(stderr, "read from pipe error: %s\n", strerror(errno));
                exit(1);
            }

            if (n == 0) {
                fprintf(stderr, "child closed the pipe\n");
                exit(1);
            }

            result[n] = 0;

            if (puts(result) == EOF) {
                fprintf(stderr, "fputs error\n");
                exit(1);
            }
        }
    } else {
        close(fd1[1]);
        close(fd2[0]);
        if (fd1[0] != STDIN_FILENO) {
            if (dup2(fd1[0] ,STDIN_FILENO) != STDIN_FILENO) {
                fprintf(stderr, "dup2 error to stdin.\n");
                exit(1);
            }
            close(fd1[0]);
        }
        if (fd2[1] != STDOUT_FILENO) {
            if (dup2(fd2[1] ,STDOUT_FILENO) != STDOUT_FILENO) {
                fprintf(stderr, "dup2 error to stdout.\n");
                exit(1);
            }
            close(fd2[1]);
        }

        if (execl("./a.out", "a.out", NULL) < 0) {
            fprintf(stderr, "execl error: %s\n", strerror(errno));
            exit(1);
        }
        exit(0);
    }

    return 0;
}

Here is the source code of a.out, it works well with this:

#include <stdio.h>
#include <string.h>

int main()
{
    #define MAXSIZE 1024
    char x[MAXSIZE];
    int n;
    while(scanf("%s", x) != EOF)
    {
        printf("len:%d %s", strlen(x), x);
        fflush(stdout);
    }
    return 0;
}

But it seems just pending when I write the code like this:

#include <stdio.h>
#include <string.h>

int main()
{
    #define MAXSIZE 1024
    char x[MAXSIZE];
    int n;
    while(scanf("%s", x) != EOF);
    printf("Ok\n");
    fflush(stdout);

    return 0;
}

Answer 1

The way you are calling scanf with %s may overflow the x buffer. You should at least modify the scanf with a width modifier.

#include <stdio.h>
#include <string.h>

int main()
{
    #define MAXSIZE 1024
    char x[MAXSIZE];
    int n;
    while(scanf("%1024s", x) != EOF)
    {
        printf("len:%d %s", strlen(x), x);
        fflush(stdout);
    }
    return 0;
}

And similarly for your other program.

The reason your program is getting blocked is because your second a.out program is looped doing another scanf, when at the same time the parent program is trying to read a response back into result.