FSEEK(3C) Standard C Library Functions FSEEK(3C)


fseek, fseeko - reposition a file-position indicator in a stream


#include <stdio.h>

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

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


The fseek() function sets the file-position indicator for the stream
pointed to by stream. The fseeko() function is identical to fseek()
except for the type of offset.

The new position, measured in bytes from the beginning of the file, is
obtained by adding offset to the position specified by whence, whose
values are defined in <stdio.h> as follows:

Set position equal to offset bytes.

Set position to current location plus offset.

Set position to EOF plus offset.

If the stream is to be used with wide character input/output functions,
offset must either be 0 or a value returned by an earlier call to
ftell(3C) on the same stream and whence must be SEEK_SET. This constraint
does not hold for streams created by open_wmemstream(3C).

A successful call to fseek() clears the end-of-file indicator for the
stream and undoes any effects of ungetc(3C) and ungetwc(3C) on the same
stream. After an fseek() call, the next operation on an update stream
may be either input or output.

If the most recent operation, other than ftell(3C), on a given stream is
fflush(3C), the file offset in the underlying open file description will
be adjusted to reflect the location specified by fseek().

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 until data is actually written into the gap.

The value of the file offset returned by fseek() on devices which are
incapable of seeking is undefined.

If the stream is writable and buffered data had not been written to the
underlying file, fseek() will cause the unwritten data to be written to
the file and mark the st_ctime and st_mtime fields of the file for

When using a stream based on open_wmemstream(3C), the fseek() and
fseeko() functions no longer operate in terms of bytes. Instead, like the
stream itself, the values used in offset are measured in units of wide
characters, the underlying data unit of the stream. The values returned
by ftell(3C) or ftello(3C) are also in these same units, allowing them to
be used in the same way. These streams do not have the constraints of
other wide character streams and may the full range of values in whence
and offset, the same as would be done for a normal byte-oriented stream.


The fseek() and fseeko() functions return 0 on success; otherwise, they
returned -1 and set errno to indicate the error.


The fseek() and fseeko() functions will fail if, either the stream is
unbuffered or the stream's buffer needed to be flushed, and the call to
fseek() or fseeko() causes an underlying lseek(2) or write(2) to be

The O_NONBLOCK flag is set for the file descriptor and the
process would be delayed in the write operation.

The file descriptor underlying the stream file is not open for
writing or the stream's buffer needed to be flushed and the
file is not open.

An attempt was made to write a file that exceeds the maximum
file size or the process's file size limit, or 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 due to the receipt of a
signal, and no data was transferred.

The whence argument is invalid. The resulting file-position
indicator would be set to a negative value.

A physical I/O error has occurred; or the process is a member
of a background process group attempting to perform a write(2)
operation to its controlling terminal, TOSTOP is set, the
process is neither ignoring nor blocking SIGTTOU, and the
process group of the process is orphaned.

There was no free space remaining on the device containing the

A request was made of a non-existent device, or the request was
outside the capabilities of the device.

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 calling thread.

The fseek() function will fail if:

The resulting file offset would be a value which cannot be
represented correctly in an object of type long.

The fseeko() function will fail if:

The resulting file offset would be a value which cannot be
represented correctly in an object of type off_t.


Although on the UNIX system an offset returned by ftell() or ftello()
(see ftell(3C)) is measured in bytes, and it is permissible to seek to
positions relative to that offset, portability to non-UNIX systems
requires that an offset be used by fseek() directly. Arithmetic may not
meaningfully be performed on such an offset, which is not necessarily
measured in bytes.

The fseeko() function has a transitional interface for 64-bit file
offsets. See lf64(7).


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Standard |
|MT-Level | MT-Safe |


getrlimit(2), ulimit(2), ftell(3C), rewind(3C), ungetc(3C), ungetwc(3C),
attributes(7), lf64(7), standards(7)

illumos March 25, 2020 FSEEK(3C)