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fseek(3)							      fseek(3)

NAME
       fseek,  fseeko, fseek_unlocked, rewind, ftell, ftello, fgetpos, fsetpos
       - Reposition the file pointer of a stream

SYNOPSIS
       #include <stdio.h>

       int fseek(
	       FILE *stream,
	       long int offset,
	       int whence ); int fseeko(
	       FILE *stream,
	       off_t offset,
	       int whence ); int fseek_unlocked(
	       FILE *stream,
	       long int offset,
	       int whence ); void rewind(
	       FILE *stream ); long int ftell(
	       FILE *stream ); off_t ftello(
	       FILE *stream ); int fsetpos(
	       FILE *stream,
	       const fpos_t *position ); int fgetpos(
	       FILE *stream,
	       fpos_t *position );

LIBRARY
       Standard C Library (libc)

STANDARDS
       Interfaces documented on this reference page conform to industry	 stan‐
       dards as follows:

       fseek(), fgetpos(), fsetpos(), ftell(), rewind():  XSH5.0, XNS5.0

       Refer  to  the  standards(5)  reference page for more information about
       industry standards and associated tags.

PARAMETERS
       Specifies the I/O stream.  Determines the position of the  next	opera‐
       tion.   Determines  the	value for the file pointer associated with the
       stream parameter.  Specifies the value of the file position indicator.

DESCRIPTION
       The fseek() function sets the position of  the  next  input  or	output
       operation  on  the  I/O	stream	specified by the stream parameter. The
       position of the next operation is determined by the  offset  parameter,
       which can be either positive or negative.

       The  fseek()  function sets the file pointer associated with the speci‐
       fied stream as follows: If the whence  parameter	 is  SEEK_SET(0),  the
       pointer	is  set	 to  the value of the offset parameter.	 If the whence
       parameter is SEEK_CUR(1), the pointer is set to	its  current  location
       plus  the  value	 of  the offset parameter.  If the whence parameter is
       SEEK_END(2), the pointer is set to the size of the file plus the	 value
       of the offset parameter.

       The  fseek()  function fails if attempted on a file that was not opened
       with the fopen() function. In particular, the fseek()  function	cannot
       be used on a terminal or on a file opened with the popen() function.

       A  successful call to the fseek() function clears the End-of-File indi‐
       cator for the stream and undoes any effects of the ungetc() function on
       the  same stream. After a call to the fseek() function, the next opera‐
       tion on an update stream may be either input or output.

       If the stream is writable, and buffered data was	 not  written  to  the
       underlying  file,  the fseek() function causes the unwritten data to be
       written to the file and marks the st_ctime and st_mtime fields  of  the
       file for update.

       If  the	most  recent  operation (ignoring any ftell() operations) on a
       given stream was fflush(), then the fseek() function  causes  the  file
       offset in the underlying open file descriptor to be adjusted to reflect
       the location specified by the fseek() function.

       The fseek() function allows  the	 file-position	indicator  to  be  set
       beyond  the  end of existing data in the file. If data is later written
       at this point, subsequent reads of data in the gap  will	 return	 bytes
       with the value 0 (zero) until data is actually written into the gap.

       The  rewind()  function	is  equivalent	to:  (void) fseek (stream, 0L,
       SEEK_SET) Except that rewind() also clears the error indicator.

       The ftell() function obtains the current value  of  the	file  position
       indicator for the specified stream.

       The  fgetpos()  and  fsetpos() functions are similar to the ftell() and
       fseek() functions, respectively. The fgetpos() function stores the cur‐
       rent  value of the file position indicator for the stream pointed to by
       the stream parameter in the object pointed to by the  position  parame‐
       ter. The fsetpos function sets the file position indicator according to
       the value of the position parameter that was returned by a  prior  call
       to the fgetpos() function.

       A  successful  call  to the fsetpos() function clears the EOF indicator
       and undoes any effects of the ungetc() function.

       [Tru64 UNIX]  The fseek_unlocked() function is  functionally  identical
       to  the	fseek()	 function,  except that fseek_unlocked() may be safely
       used only within a scope that is protected by the flockfile() and  fun‐
       lockfile()  functions  used  as a pair. The caller must ensure that the
       stream is locked before using these functions.

       The fseeko() and ftello() functions behave identically to  fseek()  and
       ftell().

RETURN VALUES
       Upon  successful completion, the fseek() and fseek_unlocked() functions
       return a value of 0 (zero). If the fseek() or fseek_unlocked() function
       fails,  a  value	 of  -1	 is  returned and errno is set to indicate the
       error.

       The rewind() function does not return a value. You can detect errors by
       first  clearing	errno and then calling the rewindfunction. If errno is
       then nonzero, assume an error has occurred.

       Upon successful completion, the ftell() function returns the offset  of
       the  current byte relative to the beginning of the file associated with
       the named stream. Otherwise, a value of -1 is returned and errno is set
       to indicate the error.

       Upon  successful	 completion,  the  fgetpos()  and  fsetpos() functions
       return a value of 0 (zero). If  the  fgetpos()  or  fsetpos()  function
       fails,  a  value	 of  -1	 is  returned and errno is set to indicate the
       error.

ERRORS
       The fseek() or fseek_unlocked() function fails if either the stream  is
       unbuffered  or the stream's buffer needed to be flushed and the call to
       fseek() or fseek_unlocked() caused an  underlying  lseek()  or  write()
       function to be invoked. In addition, if any of the following conditions
       occurs, the fseek() or fseek_unlocked()	function  sets	errno  to  the
       value  that corresponds to the condition.  The O_NONBLOCK option is set
       for the file descriptor underlying the stream parameter and the process
       would  be delayed in the write operation.  The file descriptor underly‐
       ing the stream parameter is not a valid file descriptor open for	 writ‐
       ing.  An attempt was made to write to a file that exceeds the process's
       file size limit or the maximum file size. (See the ulimit(3)  reference
       page.)

	      The  file	 is a regular file and an attempt was made to write at
	      or beyond the offset maximum associated with  the	 corresponding
	      stream.	The  write  operation  was terminated by a signal, and
	      either none, some, or all the data was transferred. If  buffered
	      I/O  is being used, it is recommended that you call the fflush()
	      function before the fseek() function to guarantee that the  buf‐
	      fer characters were written.  The whence parameter is an invalid
	      value or the resulting file offset would be invalid.  A physical
	      I/O  error  has  occurred or the following set of conditions has
	      occurred: the process is a member of a background process	 group
	      attempting to write to its controlling terminal, the TOSTOP sig‐
	      nal is set, the process is neither ignoring nor  blocking	 SIGT‐
	      TOU,  and the process group of the process is orphaned.  No free
	      space remained on the device  containing	the  file.   The  file
	      descriptor underlying stream is associated with a pipe or FIFO.

	      An  attempt was made to write to a pipe or FIFO that is not open
	      for reading by any process. A SIGPIPE signal will also  be  sent
	      to  the  process.	 A request was made of a nonexistent device or
	      the request was outside the capabilities of the device.

       The rewind() function fails under the same conditions  as  the  fseek()
       function, with the exception of [EINVAL], which does not apply.

       If  the following conditions occur, the fgetpos(), fsetpos() or ftell()
       function sets errno to the value that  corresponds  to  the  condition.
       The file descriptor underlying the stream parameter is not a valid file
       descriptor.  [Tru64 UNIX]  The stream parameter does  not  point	 to  a
       valid FILE structure or the position parameter is negative.  An illegal
       attempt was made to get or set the file position of a pipe or FIFO.

	      [XNS5.0]	The file descriptor  parameter	underlying  stream  is
	      associated with a socket.

SEE ALSO
       Functions: lseek(2), fopen(3)

       Standards: standards(5)

								      fseek(3)
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