Program to dump/cat shm segment

shmcat.c

/* gcc -Wall -pedantic -g -std=c99 shmcat.c -o shmcat */

#define _XOPEN_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/shm.h>

int main(int argc, char **argv) {
    if (argc < 2) {
        fprintf(stderr, "Usage: shmcat <shmid>\n");
        return 1;
    }

    int shmid = atoi(argv[1]);

    struct shmid_ds buf = {{0}};
    if (shmctl(shmid, IPC_STAT, &buf) < 0) {
        perror("shmctl");
        return 1;
    }

    int nbytes = buf.shm_segsz;

    if (nbytes > 0) {
        void *shm = shmat(shmid, NULL, SHM_RDONLY);
        if (shm == (void *)-1) {
            perror("shmat");
            return 1;
        }
        ssize_t rv = write(STDOUT_FILENO, shm, nbytes);
        if (rv < 0) {
            perror("write");
            return 1;
        } else if (rv != nbytes) {
            fprintf(stderr, "shmcat: Partial write of %zd/%d byte(s) to stdout\n", rv, nbytes);
            return 1;
        }
        shmdt(shm);
    }

    return 0;
}