fseek(3) Library Functions Manual fseek(3)
NAME
fgetpos, fseek, fsetpos, ftell, rewind - reposition a stream
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <stdio.h>
int fseek(FILE *stream, long offset, int whence);
long ftell(FILE *stream);
void rewind(FILE *stream);
int fgetpos(FILE *restrict stream, fpos_t *restrict pos);
int fsetpos(FILE *stream, const fpos_t *pos);
DESCRIPTION
The fseek() function sets the file position indicator for the stream
pointed to by stream. The new position, measured in bytes, is obtained
by adding offset bytes to the position specified by whence. If whence
is set to SEEK_SET, SEEK_CUR, or SEEK_END, the offset is relative to
the start of the file, the current position indicator, or end-of-file,
respectively. A successful call to the fseek() function clears the
end-of-file indicator for the stream and undoes any effects of the
ungetc(3) function on the same stream.
The ftell() function obtains the current value of the file position in-
dicator for the stream pointed to by stream.
The rewind() function sets the file position indicator for the stream
pointed to by stream to the beginning of the file. It is equivalent
to:
(void) fseek(stream, 0L, SEEK_SET)
except that the error indicator for the stream is also cleared (see
clearerr(3)).
The fgetpos() and fsetpos() functions are alternate interfaces equiva-
lent to ftell() and fseek() (with whence set to SEEK_SET), setting and
storing the current value of the file offset into or from the object
referenced by pos. On some non-UNIX systems, an fpos_t object may be a
complex object and these routines may be the only way to portably repo-
sition a text stream.
If the stream refers to a regular file and the resulting stream offset
is beyond the size of the file, subsequent writes will extend the file
with a hole, up to the offset, before committing any data. See
lseek(2) for details on file seeking semantics.
RETURN VALUE
The rewind() function returns no value. Upon successful completion,
fgetpos(), fseek(), fsetpos() return 0, and ftell() returns the current
offset. Otherwise, -1 is returned and errno is set to indicate the er-
ror.
ERRORS
EINVAL The whence argument to fseek() was not SEEK_SET, SEEK_END, or
SEEK_CUR. Or: the resulting file offset would be negative.
ESPIPE The file descriptor underlying stream is not seekable (e.g., it
refers to a pipe, FIFO, or socket).
The functions fgetpos(), fseek(), fsetpos(), and ftell() may also fail
and set errno for any of the errors specified for the routines
fflush(3), fstat(2), lseek(2), and malloc(3).
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ fseek(), ftell(), rewind(), fgetpos(), │ Thread safety │ MT-Safe │
│ fsetpos() │ │ │
└───────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
C11, POSIX.1-2008.
HISTORY
POSIX.1-2001, C89.
SEE ALSO
lseek(2), fseeko(3)
Linux man-pages 6.7 2023-10-31 fseek(3)
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