fileio/seek_io.c

This is fileio/seek_io.c (Listing 4-3, page 84), an example program file from the book, The Linux Programming Interface.

The source code file is copyright 2010, Michael Kerrisk, and is licensed under the GNU Affero General Public License, version 3.

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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.

  Cover of The Linux Programming Interface

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

#include <sys/stat.h>
#include <fcntl.h>
#include <ctype.h>
#include "tlpi_hdr.h"
int
main(int argc, char *argv[])
{
    size_t len;
    off_t offset;
    int fd, ap, j;
    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",
                 argv[0]);

    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)
        errExit("open");

    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)
                errExit("malloc");

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

            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((unsigned char) buf[j]) ?
                                                buf[j] : '?');
                    else
                        printf("%02x ", (unsigned int) buf[j]);
                }
                printf("\n");
            }

            free(buf);
            break;

        case 'w':   /* Write string at current offset */
            numWritten = write(fd, &argv[ap][1], strlen(&argv[ap][1]));
            if (numWritten == -1)
                errExit("write");
            printf("%s: wrote %ld bytes\n", argv[ap], (long) numWritten);
            break;

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

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

    exit(EXIT_SUCCESS);
}

 

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