madvise_fail.c

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

/* Demonstration of using madvise.
 *
 * 1. Shows how using madvise on non page-aligned memory will fail.
 * 2. Shows how using madvise on page-aligned memory will succeed but
 *    reserve the alignment amount after the requested size making
 *    subsequent allocations start at the next page.
 * 3. Shows where subsequent allocations would start if not using
 *    page-alignment or madvise.
 *
 * gcc madvise_fail.c -o madvise_fail
 */

static void print_usage(const char *name)
{
    printf("usage: %s option [print ptr address]\n", name);
    printf("options:\n");
    printf("\t1 = no page-alignment, madvise:\n");
    printf("\t2 = page-alignment, madvise:\n");
    printf("\t1 = no pate-alignment, no madvise:\n");
    printf("print ptr address: Will print the pointer address after allocation (and madvise if in use\n");
}

int main(int argc, char **argv)
{
    char         *tmp;
    size_t        i;
    int           flag      = 0;
    int           print_ptr = 0;
    const size_t  size      = 8;
    size_t        page_size;

    if (argc < 2) {
        print_usage(argv[0]);
        return 1;
    }

    if (strcasecmp(argv[1], "1") == 0) {
        flag = 1;
    } else if (strcasecmp(argv[1], "2") == 0) {
        flag = 2;
    } else if (strcasecmp(argv[1], "3") == 0) {
        flag = 3;
    } else {
        print_usage(argv[0]);
        return 1;
    }

    if (argc >= 3)
        print_ptr = 1;

    page_size = sysconf(_SC_PAGESIZE);
    printf("page_size=%zu\n", page_size);
    for (i=0; i<10000; i++) {
        if (flag == 2) {
            posix_memalign((void **)&tmp, page_size, size);
        } else {
            tmp = malloc(size);
        }

        if (tmp == NULL) {
            printf("%zu: allocation failed\n", i);
            return 2;
        }

        if (flag == 1 || flag == 2) {
            if (madvise(tmp, size, MADV_DONTDUMP) != 0) {
                printf("%zu: madvise failed: %s\n", i, strerror(errno));
                return 3;
            }
        }

        if (print_ptr) {
            printf("%p\n", tmp);
        }
    }

    return 0;
}