This is sockets/sendfile.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 61-3 (page 1287).

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 sockets/sendfile.c

  Cover of The Linux Programming Interface

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

/* sendfile.c

   Implement sendfile() in terms of read(), write(), and lseek().
#include "tlpi_hdr.h"
#define BUF_SIZE 8192
sendfile(int out_fd, int in_fd, off_t *offset, size_t count)
    off_t orig;

    if (offset != NULL) {

        /* Save current file offset and set offset to value in '*offset' */

        orig = lseek(in_fd, 0, SEEK_CUR);
        if (orig == -1)
            return -1;
        if (lseek(in_fd, *offset, SEEK_SET) == -1)
            return -1;

    size_t totSent = 0;

    while (count > 0) {
        size_t toRead = min(BUF_SIZE, count);

        char buf[BUF_SIZE];
        ssize_t numRead = read(in_fd, buf, toRead);
        if (numRead == -1)
            return -1;
        if (numRead == 0)
            break;                      /* EOF */

        ssize_t numSent = write(out_fd, buf, numRead);
        if (numSent == -1)
            return -1;
        if (numSent == 0)               /* Should never happen */
            fatal("sendfile: write() transferred 0 bytes");

        count -= numSent;
        totSent += numSent;

    if (offset != NULL) {

        /* Return updated file offset in '*offset', and reset the file offset
           to the value it had when we were called. */

        *offset = lseek(in_fd, 0, SEEK_CUR);
        if (*offset == -1)
            return -1;
        if (lseek(in_fd, orig, SEEK_SET) == -1)
            return -1;

    return totSent;


Download sockets/sendfile.c

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