This is vmem/memlock.c (Listing 50-2, page 1052), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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Download vmem/memlock.c

  Cover of The Linux Programming Interface

Function list (Bold in this list means a function is not static)

/* memlock.c

   Demonstrate the use of mlock(), to place memory locks, and mincore(), to
   retrieve memory-residence information about the calling processes virtual
   memory pages.

   Note: some UNIX implementations do not provide mincore().

   The madvise() system call is supported on Linux since kernel 2.4.
#define _BSD_SOURCE             /* Get mincore() declaration and MAP_ANONYMOUS
                                   definition from <sys/mman.h> */
#include <sys/mman.h>
#include "tlpi_hdr.h"
/* Display residency of pages in range [addr .. (addr + length - 1)] */

static void
displayMincore(char *addr, size_t length)
    unsigned char *vec;
    long pageSize, numPages, j;

#ifndef _SC_PAGESIZE
    pageSize = getpagesize();   /* Some systems don't have _SC_PAGESIZE */
    pageSize = sysconf(_SC_PAGESIZE);

    numPages = (length + pageSize - 1) / pageSize;
    vec = malloc(numPages);
    if (vec == NULL)

    if (mincore(addr, length, vec) == -1)

    for (j = 0; j < numPages; j++) {
        if (j % 64 == 0)
            printf("%s%10p: ", (j == 0) ? "" : "\n", addr + (j * pageSize));
        printf("%c", (vec[j] & 1) ? '*' : '.');

main(int argc, char *argv[])
    char *addr;
    size_t len, lockLen;
    long pageSize, stepSize, j;

    if (argc != 4 || strcmp(argv[1], "--help") == 0)
        usageErr("%s num-pages lock-page-step lock-page-len\n", argv[0]);

#ifndef _SC_PAGESIZE
    pageSize = getpagesize();
    if (pageSize == -1)
    pageSize = sysconf(_SC_PAGESIZE);
    if (pageSize == -1)

    len =      getInt(argv[1], GN_GT_0, "num-pages") * pageSize;
    stepSize = getInt(argv[2], GN_GT_0, "lock-page-step") * pageSize;
    lockLen =  getInt(argv[3], GN_GT_0, "lock-page-len") * pageSize;

    addr = mmap(NULL, len, PROT_READ, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
    if (addr == MAP_FAILED)

    printf("Allocated %ld (%#lx) bytes starting at %p\n",
            (long) len, (unsigned long) len, addr);
    /*FIXME: above: should use %zu here, and remove (long) cast */

    printf("Before mlock:\n");
    displayMincore(addr, len);

    /* Lock pages specified by command-line arguments into memory */

    for (j = 0; j + lockLen <= len; j += stepSize)
        if (mlock(addr + j, lockLen) == -1)

    printf("After mlock:\n");
    displayMincore(addr, len);



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