Test RLIMIT_MEMLOCK and mlock behavior.

rlimit_memlock.c

#include <sys/time.h>
#include <sys/resource.h>
#include <sys/mman.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>

int main() {
    long page_sz = sysconf(_SC_PAGESIZE);

    printf("page size: %lu\n", page_sz);

    const struct rlimit mlock_rlimit = {
        .rlim_cur = page_sz,
        .rlim_max = page_sz,
    };

    if ((setrlimit(RLIMIT_MEMLOCK, &mlock_rlimit))) {
        printf("setrlimit error: %s\n", strerror(errno));
    }

    const long mem_sz = page_sz * 2;

    void *ptr;
    if ((posix_memalign(&ptr, page_sz, mem_sz))) {
        printf("posix_memalign error: %s\n", strerror(errno));
    }

    if (mlock(ptr, mem_sz)) {
        printf("mlock error: %s\n", strerror(errno));
    }

    return 0;
}

expected result:

page size: 4096
mlock error: Resource temporarily unavailable

---

explaination: mlock will fail to lock a block of memory space larger than the resouce limit.

   ENOMEM (Linux 2.6.9 and later) the caller had a nonzero
       RLIMIT_MEMLOCK soft resource limit, but tried to lock more
       memory than the limit permitted.  This limit is not
       enforced if the process is privileged (CAP_IPC_LOCK).