bug.c

// Build with gcc -o bug bug.c
//
// Prints non-NULL address of y when executed outside of valgrind, and a NULL
// (rendered as "(nil)" by printf) when executed inside valgrind.
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
        int shmid = shmget(IPC_PRIVATE, 1024 * 1024 * 2, IPC_CREAT | 0666);
        if (shmid < 0) {
                fprintf(stderr, "Failed to create SHM segment\n");
                return 1;
        }

        char *x = shmat(shmid, NULL, 0);
        if (x == (void *) -1) {
                fprintf(stderr, "Failed to attach to SHM segment\n");
                return 1;
        }

        char *y = malloc(1);
        printf("y = %p\n", y);
        free(y);

        shmdt(x);

        shmctl(shmid, IPC_RMID, NULL);
}