Tribblix: manual page: fseek.3c

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

NAME

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

SYNOPSIS

#include <stdio.h>

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

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

DESCRIPTION

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:

SEEK_SET
Set position equal to offset bytes.

SEEK_CUR
Set position to current location plus offset.

SEEK_END
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 update.

open_wmemstream(3C)
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.

RETURN VALUES

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

ERRORS

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 invoked:

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

EBADF
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.

EFBIG
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.

EINTR
The write operation was terminated due to the receipt of a
signal, and no data was transferred.

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

EIO
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.

ENOSPC
There was no free space remaining on the device containing
the file.

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

EPIPE
The file descriptor underlying stream is associated with a
pipe or FIFO.

EPIPE
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:

EOVERFLOW
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:

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

USAGE

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).

ATTRIBUTES