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.
A successful call to fseek() clears the end-of-file indica-
tor 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 underly-
ing 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.
RETURN VALUES
The fseek() and fseeko() functions return 0 on success; oth-
erwise, 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 termi-
nal, 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 con-
taining the file.
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 process.
ENXIO A request was made of a non-existent device, or the
request was outside the capabilities of the device.
The fseek() function will fail if:
EOVERFLOW
The resulting file offset would be a value which can-
not 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 can-
not 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(5).
When a UFS file system is mounted with logging enabled, file
system transactions that free blocks from files might not
actually add those freed blocks to the file system's free
list until some unspecified time in the future. This
behavior improves file system performance but does not con-
form to the POSIX, Single UNIX Specification, SPARC Confor-
mance Definition, System V Application Binary Interface,
System V Interface Definition, and X/Open Portability Guide
Standards, which require that freed space be available
immediately. To enable standards conformance regarding file
deletions or to address the problem of not being able to
grow files on a relatively full UFS file system even after
files have been deleted, disable UFS logging (see
mount_ufs(1M).
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| MT-Level | MT-Safe |
|_____________________________|_____________________________|
SEE ALSO
mount_ufs(1M), getrlimit(2), ulimit(2), fopen(3UCB),
ftell(3C), rewind(3C), ungetc(3C), ungetwc(3C), attri-
butes(5), lf64(5)
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