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FSEEK(3) Library Functions Manual FSEEK(3)

fgetpos, fseek, fseeko, fsetpos, ftell, ftello, rewind
reposition a stream

#include <stdio.h>
int
fgetpos(FILE *stream, fpos_t *pos);
int
fseek(FILE *stream, long offset, int whence);
int
fseeko(FILE *stream, off_t offset, int whence);
int
fsetpos(FILE *stream, const fpos_t *pos);
long
ftell(FILE *stream);
off_t
ftello(FILE *stream);
void
rewind(FILE *stream);

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 fseeko() function is identical to fseek() except that it takes an off_t as its offset.
The ftell() function obtains the current value of the file position indicator for the stream pointed to by stream.
The ftello() function is identical to ftell() except that its return value is of type off_t.
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 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 systems an “fpos_t” object may be a complex object and these routines may be the only way to portably reposition a text stream.

The rewind() function returns no value. Prefer fseek(), which is just as portable, and does not hide errors. Upon successful completion, fgetpos(), fseek(), fseeko(), and fsetpos() return 0 and ftell() and ftello() return the current offset. Otherwise, fseek(), fseeko(), ftell(), and ftello() return -1 and fgetpos() and fsetpos() return a non-zero value and the global variable errno is set to indicate the error.

The fgetpos(), fseek(), fseeko(), fsetpos(), ftell(), ftello(), and rewind() functions will fail if:
 
 
[]
The stream specified is not a seekable stream.
Additionally, the fseek() and fseeko() functions will fail if:
 
 
[]
The whence argument was not SEEK_SET, SEEK_END, or SEEK_CUR.
Additionally, the ftell() function will fail if:
 
 
[]
The value of the file position indicator is too large to be represented by a long.
Finally, the functions fgetpos(), fseek(), fseeko(), fsetpos(), ftell(), and ftello() may also fail and set errno for any of the errors specified for the routines fflush(3), fstat(2), lseek(2), and malloc(3).

lseek(2)

The fgetpos(), fsetpos(), fseek(), ftell(), and rewind() functions conform to ANSI X3.159-1989 (“ANSI C89”) and X/Open Portability Guide Issue 4 (“XPG4”).
The fseeko() and ftello() functions conform to X/Open Portability Guide Issue 4 (“XPG4”).

The functions fseek(), ftell(), and rewind() first appeared in Version 7 AT&T UNIX.
March 26, 2016 OpenBSD-current