fseek(3) - Linux manual page (original) (raw)

fseek(3) Library Functions Manual fseek(3)

NAME top

   fgetpos, fseek, fsetpos, ftell, rewind - reposition a stream

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#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 top

   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)](../man3/ungetc.3.html) function on the same
   stream.

   The **ftell**() function obtains the current value of the file
   position indicator 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)](../man3/clearerr.3.html)).

   The **fgetpos**() and **fsetpos**() functions are alternate interfaces
   equivalent 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
   _fpost_ object may be a complex object and these routines may be
   the only way to portably reposition 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)](../man2/lseek.2.html) for details on file seeking semantics.

RETURN VALUE top

   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](../man3/errno.3.html)_
   is set to indicate the error.

ERRORS top

   **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](../man3/errno.3.html)_ for any of the errors specified for the
   routines [fflush(3)](../man3/fflush.3.html), [fstat(2)](../man2/fstat.2.html), [lseek(2)](../man2/lseek.2.html), and [malloc(3)](../man3/malloc.3.html).

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├──────────────────────────────────────┼───────────────┼─────────┤
   │ **fseek**(), **ftell**(), **rewind**(),          │ Thread safety │ MT-Safe │
   │ **fgetpos**(), **fsetpos**()                 │               │         │
   └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

   C11, POSIX.1-2008.

HISTORY top

   POSIX.1-2001, C89.

SEE ALSO top

   [lseek(2)](../man2/lseek.2.html), [fseeko(3)](../man3/fseeko.3.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 fseek(3)

Pages that refer to this page:lseek(2), fgetc(3), fmemopen(3), fopen(3), fopencookie(3), fseeko(3), gets(3), open_memstream(3), puts(3), stdio(3), feature_test_macros(7)