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

This file is not printed in the book; it is a supplementary file for Chapter 13.

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.


Download filebuff/write_bytes.c

  Cover of The Linux Programming Interface

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

/* write_bytes.c

   Write bytes to a file. (A simple program for file I/O benchmarking.)

   Usage: write_bytes file num-bytes buf-size

   Writes 'num-bytes' bytes to 'file', using a buffer size of 'buf-size'
   for each write().

   If compiled with -DUSE_O_SYNC, open the file with the O_SYNC flag,
   so that all data and metadata changes are flushed to the disk.

   If compiled with -DUSE_FDATASYNC, perform an fdatasync() after each write,
   so that data--and possibly metadata--changes are flushed to the disk.

   If compiled with -DUSE_FSYNC, perform an fsync() after each write, so that
   data and metadata are flushed to the disk.
#include <sys/stat.h>
#include <fcntl.h>
#include "tlpi_hdr.h"
main(int argc, char *argv[])
    if (argc != 4 || strcmp(argv[1], "--help") == 0)
        usageErr("%s file num-bytes buf-size\n", argv[0]);

    size_t numBytes = getLong(argv[2], GN_GT_0, "num-bytes");
    size_t bufSize = getLong(argv[3], GN_GT_0, "buf-size");

    char *buf = malloc(bufSize);
    if (buf == NULL)

    int openFlags = O_CREAT | O_WRONLY;

#if defined(USE_O_SYNC) && defined(O_SYNC)
    openFlags |= O_SYNC;

    int fd = open(argv[1], openFlags, S_IRUSR | S_IWUSR);
    if (fd == -1)

    size_t thisWrite, totWritten;
    for (totWritten = 0; totWritten < numBytes;
            totWritten += thisWrite) {
        thisWrite = min(bufSize, numBytes - totWritten);

        if (write(fd, buf, thisWrite) != thisWrite)
            fatal("partial/failed write");

#ifdef USE_FSYNC
        if (fsync(fd))
        if (fdatasync(fd))

    if (close(fd) == -1)


Download filebuff/write_bytes.c

Note that, in most cases, the programs rendered in these web pages are not free standing: you'll typically also need a few other source files (mostly in the lib/ subdirectory) as well. Generally, it's easier to just download the entire source tarball and build the programs with make(1). By hovering your mouse over the various hyperlinked include files and function calls above, you can see which other source files this file depends on.

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