This is mmap/mmcopy.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it is the solution to Exercise 49-1 (page 1014).

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

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.


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  Cover of The Linux Programming Interface

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

/* mmcopy.c

   Copy the contents of one file to another file, using memory mappings.

   Usage mmcopy source-file dest-file
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "tlpi_hdr.h"
main(int argc, char *argv[])
    if (argc != 3)
        usageErr("%s source-file dest-file\n", argv[0]);

    int fdSrc = open(argv[1], O_RDONLY);
    if (fdSrc == -1)

    /* Use fstat() to obtain size of file: we use this to specify the
       size of the two mappings */

    struct stat sb;
    if (fstat(fdSrc, &sb) == -1)

    /* Handle zero-length file specially, since specifying a size of
       zero to mmap() will fail with the error EINVAL */

    if (sb.st_size == 0)

    char *src = mmap(NULL, sb.st_size, PROT_READ, MAP_PRIVATE, fdSrc, 0);
    if (src == MAP_FAILED)

    int fdDst = open(argv[2], O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
    if (fdDst == -1)

    if (ftruncate(fdDst, sb.st_size) == -1)

    char *dst = mmap(NULL, sb.st_size, PROT_READ | PROT_WRITE,
                     MAP_SHARED, fdDst, 0);
    if (dst == MAP_FAILED)

    memcpy(dst, src, sb.st_size);       /* Copy bytes between mappings */

    if (msync(dst, sb.st_size, MS_SYNC) == -1)



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