Manual browser: fseek(3)

FSEEK(3) Library Functions Manual FSEEK(3)


fgetpos, fseek, fseeko, fsetpos, ftell, ftello, rewindreposition a stream


Standard C Library (libc, -lc)


#include <stdio.h>

fseek(FILE *stream, long int offset, int whence);

fseeko(FILE *stream, off_t offset, int whence);

long int
ftell(FILE *stream);

ftello(FILE *stream);

rewind(FILE *stream);

fgetpos(FILE * restrict stream, fpos_t * restrict pos);

fsetpos(FILE * restrict stream, const fpos_t * restrict pos);


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 the fseek() function except that the offset argument is of type off_t.

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 the ftell() function except that the 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)).

In this implementations, an “fpos_t” object is a complex object that represents both the position and the parse state of the stream making these routines are the only way to portably reposition a text stream. The pos argument of fsetpos() must always be initialized by a call to fgetpos().


The rewind() function returns no value. Upon successful completion, fgetpos(), fseek(), fseeko(), and fsetpos() return 0. The functions ftell() and ftello() return the current offset. Otherwise, fseek(), fseeko(), ftell(), and ftello() return -1 while fgetpos() and fsetpos() return a nonzero value. On error all functions the global variable errno is set to indicate the error. Since the rewind() function does not return an error code, applications need to clear errno before calling it in order to detect errors.


The stream specified is not a seekable stream.
The whence argument to fseek() was not SEEK_SET, SEEK_END, or SEEK_CUR.
For ftell(), the current file offset cannot be represented correctly in an object of type long.

The function fgetpos(), fseek(), fseeko(), fsetpos(), ftell(), ftello(), and rewind() may also fail and set errno for any of the errors specified for the routines fflush(3), fstat(2), lseek(2), and malloc(3).




The fgetpos(), fsetpos(), fseek(), ftell(), and rewind() functions conform to ANSI X3.159-1989 (“ANSI C89”). The fseeko() and ftello() functions conform to X/Open System Interfaces and Headers Issue 5 (“XSH5”).


The fgetpos() and fsetpos() functions don't store/set shift states of the stream in this implementation.
January 21, 2012 NetBSD 7.0