fseek() -- STDIO Function (libc)

Seek on file stream
#include <stdio.h>
int fseek(fp, where, how)
FILE *fp; long where; int how;

fseek() changes  where the next  read or write operation  will occur within
the file stream fp. It handles  any effects the seek routine might have had
on the  internal buffering strategies  of the system.   The arguments where
and how  specify the desired  seek position.  where indicates  the new seek
position in  the file.  It  is measured from  the start of the  file if how
equals  SEEK_SET  (zero), from  the  current seek  position  if how  equals
SEEK_CUR (one),  and from the  end of the  file if how  equals two SEEK_END
(two).

fseek()  differs from  its cousin  lseek()  in that  lseek() is  a COHERENT
system call  and takes a file  number, whereas fseek() is  a STDIO function
and takes a FILE pointer.

Example

This  example opens  file argv[1]  and prints  its last  argv[2] characters
(default,  100).   It  demonstrates  the  functions fseek(),  ftell(),  and
fclose().

#include <stdio.h>
extern long atol();

void fatal(message)
char *message;
{
    fprintf(stderr, "tail: %s\n", message);
    exit(1);
}

main(argc, argv)
int argc; char *argv[];
{
    register FILE *ifp;
    register int c;
    long nchars, size;

    if (argc < 2 || argc > 3)
        fatal("Usage: tail file [ nchars ]");
    nchars = (argc == 3) ? atol(argv[2]) : 100L;

    if ((ifp = fopen(argv[1], "r")) == NULL)
        fatal("cannot open input file");
    /* Seek to end */
    if (fseek(ifp, 0L, 2) == -1)
        fatal("seek error");

    /* Find current size */
    size = ftell(ifp);
    size = (size < nchars) ? 0L : size - nchars;

    /* Seek to point */
    if (fseek(ifp, size, 0) == -1)
        fatal("seek error");
    while ((c = getc(ifp)) != EOF)
        /* Copy rest to stdout */
        putchar(c);
    if (fclose(ifp) == EOF)
        fatal("cannot close");
    exit(0);
}

See Also

fsetpos(),
ftell(),
libc,
lseek()
ANSI Standard, §7.9.9.2
POSIX Standard, §8.1

Diagnostics

For any  diagnostic error, fseek() returns -1;  otherwise, it returns zero.
If fseek()  goes beyond the  end of the  file, it will not  return an error
message until the corresponding read or write is performed.

© 2012 by Stephen A. Ness