FSEEK(3) Library Functions Manual FSEEK(3)

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

Standard C Library (libc, -lc)

#include <stdio.h>

int
fgetpos(FILE *restrict stream, fpos_t *restrict 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 () 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) and ungetwc(3) functions on the same stream.

The () function obtains the current value of the file position indicator for the stream pointed to by stream.

The () 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)).

Since () does not return a value, an application wishing to detect errors should clear errno, then call rewind(), and if errno is non-zero, assume an error has occurred.

The () function is identical to fseek(), except it takes an off_t argument instead of a long. Likewise, the () function is identical to ftell(), except it returns an off_t.

The () and () functions are alternate interfaces for retrieving and setting the current position in the file, similar to ftell() and fseek(), except that the current position is stored in an opaque object of type fpos_t pointed to by pos. These functions provide a portable way to seek to offsets larger than those that can be represented by a long int. They may also store additional state information in the fpos_t object to facilitate seeking within files containing multibyte characters with state-dependent encodings. Although fpos_t has traditionally been an integral type, applications cannot assume that it is; in particular, they must not perform arithmetic on objects of this type.

If the stream is a wide character stream (see fwide(3)), the position specified by the combination of offset and whence must contain the first byte of a multibyte sequence.

The rewind() function returns no value.


The fgetpos(), fseek(), fseeko(), and fsetpos() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

Upon successful completion, ftell() and ftello() return the current offset. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

[]
The stream argument is not a seekable stream.
[]
The whence argument is invalid or the resulting file-position indicator would be set to a negative value.
[]
The resulting file offset would be a value which cannot be represented correctly in an object of type off_t for fseeko() and ftello() or long for fseek() and ftell().
[]
The file descriptor underlying stream is associated with a pipe or FIFO or file-position indicator value is unspecified (see ungetc(3)).

The functions 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).

#include <stdio.h> #include <sys/types.h>

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

The include file <sys/types.h> supplies the definition for off_t.

lseek(2), clearerr(3), fwide(3), ungetc(3), ungetwc(3), compat(5)

The fgetpos(), fsetpos(), fseek(), ftell(), and rewind() functions conform to ISO/IEC 9899:1990 (“ISO C90”).

The fseeko() and ftello() functions conform to IEEE Std 1003.1-2001 (“POSIX.1”).

March 19, 2004 macOS 15.2