Question

Tried to create shared memory block, and what I got is strange behaviour.

#include <sys/shm.h>
#include "stdio.h"
#include <sys/ipc.h>

int main() {
printf("starting\n");

int mid = -1;
mid = shmget((key_t)1234, 4096, IPC_CREAT|0666);
if(mid == -1) {
    printf("cant get mid\n");
    return 1;
} else {
    printf("got mid");
}

int* maddr = 0;
maddr = shmat(mid, NULL ,0);
if(maddr == (int*)-1) {
    printf("cant attach memory\n");
    return 1;
} else {
    printf("got maddr");
}

while(1) {
    int cval = __sync_add_and_fetch(maddr, 1);
    if(cval % 2) { // odd values
            printf("messager 1");
            sleep(1000);
    }
}
}

If I try to execute that code, it prints starting and hangs, nothing more happens, but some why it accepts input from stdin, so I can print just like if scanf is running

Answer 1

stdout is line-buffered by default, which means that it is not flushed until a newline is printed. This means that you need to put \n at the end of your "got mid", "got maddr" and "messager 1" strings, or fflush(); after those printf()s.

By the way, SYSV shared memory is outdated. The POSIX mechanisms are significantly better designed – see shm_open() and related calls.