This is fileio/seek_io.c (Listing 4-3, page 84), 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 fileio/seek_io.c

  Cover of The Linux Programming Interface

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

/* seek_io.c

   Demonstrate the use of lseek() and file I/O system calls.

   Usage: seek_io file {r<length>|R<length>|w<string>|s<offset>}...

   This program opens the file named on its command line, and then performs
   the file I/O operations specified by its remaining command-line arguments:

           r<length>    Read 'length' bytes from the file at current
                        file offset, displaying them as text.

           R<length>    Read 'length' bytes from the file at current
                        file offset, displaying them in hex.

           w<string>    Write 'string' at current file offset.

           s<offset>    Set the file offset to 'offset'.


        seek_io myfile wxyz s1 r2
#include <sys/stat.h>
#include <fcntl.h>
#include <ctype.h>
#include "tlpi_hdr.h"
main(int argc, char *argv[])
    size_t len;
    off_t offset;
    int fd, ap, j;
    unsigned char *buf;
    ssize_t numRead, numWritten;

    if (argc < 3 || strcmp(argv[1], "--help") == 0)
        usageErr("%s file {r<length>|R<length>|w<string>|s<offset>}...\n",

    fd = open(argv[1], O_RDWR | O_CREAT,
                S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP |
                S_IROTH | S_IWOTH);                     /* rw-rw-rw- */
    if (fd == -1)

    for (ap = 2; ap < argc; ap++) {
        switch (argv[ap][0]) {
        case 'r':   /* Display bytes at current offset, as text */
        case 'R':   /* Display bytes at current offset, in hex */
            len = getLong(&argv[ap][1], GN_ANY_BASE, argv[ap]);

            buf = malloc(len);
            if (buf == NULL)

            numRead = read(fd, buf, len);
            if (numRead == -1)

            if (numRead == 0) {
                printf("%s: end-of-file\n", argv[ap]);
            } else {
                printf("%s: ", argv[ap]);
                for (j = 0; j < numRead; j++) {
                    if (argv[ap][0] == 'r')
                        printf("%c", isprint(buf[j]) ?  buf[j] : '?');
                        printf("%02x ", buf[j]);


        case 'w':   /* Write string at current offset */
            numWritten = write(fd, &argv[ap][1], strlen(&argv[ap][1]));
            if (numWritten == -1)
            /*FIXME: should use %zd here, and remove (long) cast */
            printf("%s: wrote %ld bytes\n", argv[ap], (long) numWritten);

        case 's':   /* Change file offset */
            offset = getLong(&argv[ap][1], GN_ANY_BASE, argv[ap]);
            if (lseek(fd, offset, SEEK_SET) == -1)
            printf("%s: seek succeeded\n", argv[ap]);

            cmdLineErr("Argument must start with [rRws]: %s\n", argv[ap]);

    if (close(fd) == -1)



Download fileio/seek_io.c

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