Standard C Library Functions fseek(3C)
NAME
fseek, fseeko - reposition a file-position indicator in a
stream
SYNOPSIS
#include
int fseek(FILE *stream, long offset, int whence);
int fseeko(FILE *stream, offt 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
as follows:
SEKSET Set position equal to offset bytes.
SEKCUR Set position to current location plus offset.
SEKEND 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 SEKSET.
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().
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Standard C Library Functions fseek(3C)
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
stctime and stmtime 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.
ERORS
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 ONONBLOCK 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.
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Standard C Library Functions fseek(3C)
EIO A physical I/O error has occurred; or the process
is a member of a background process group attempt-
ing to perform a write(2) operation to its con-
trolling terminal, TOSTOP is set, the process is
neither ignoring nor blocking SIGTOU, 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 associ-
ated 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 offt.
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.
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Standard C Library Functions fseek(3C)
The fseeko() function has a transitional interface for 64-
bit file offsets. See lf64(5).
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Interface Stability Standard
MT-Level MT-Safe
SEE ALSO
getrlimit(2), ulimit(2), fopen(3UCB), ftell(3C), rewind(3C),
ungetc(3C), ungetwc(3C), attributes(5), lf64(5), stan-
dards(5)
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