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the6p4c/bug.c

Created September 27, 2019 11:28
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  1. the6p4c created this gist Sep 27, 2019.
    33 changes: 33 additions & 0 deletions bug.c
    View file
    Original file line number Diff line number Diff line change
    @@ -0,0 +1,33 @@
    // 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);
    }