On Linux, why does this print 0 and hang rather than printing 0 up to 999?

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

int main() {
    int n = 1000;
    int fromparent[2];
    int fromchild[2];
    int *x = 0;

    pipe(fromparent);
    pipe(fromchild);
    if (fork() == 0) {
        /* child */
        while (1) {
            read(fromparent[0], x, 1);
            write(fromchild[1], x, 1);
        }
    } else {
        /* parent */
        for (int i = 0; i < n; i  ) {
            printf("%d\n",  i);
            write(fromparent[1], x, 1);
            read(fromchild[0], x, 1);
        }
        exit(0);
    }
}

CodePudding user response：

The problem is simple: you pass a null pointer to read and write.

Change the definition of x from int *x = 0; to:

char x[1] = { 0 };

As posted, the behavior is somewhat counterintuitive:

• passing a null pointer to write causes it to return -1 immediately, setting errno to EINVAL.

• conversely, passing a null pointer to read causes it to wait for input and only return -1 and set errno to EINVAL once input is available. This is causing both processes to block on the read calls.

Here is a simple test to illustrate this behavior:

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

int main() {
    int n;
    errno = 0;
    n = read(0, NULL, 1);
    printf("n=%d, errno=%d\n", n, errno);
    return 0;
}

This program waits for input if run from the console and prints n=-1, errno=14 after the user hits enter.

Conversely, it prints n=0, errno=0 when standard input is redirected from /dev/null.