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.TH FSEEK 3C "March 25, 2020"
.SH NAME
fseek, fseeko \- reposition a file-position indicator in a stream
.SH SYNOPSIS
.nf
#include <stdio.h>

\fBint\fR \fBfseek\fR(\fBFILE *\fR\fIstream\fR, \fBlong\fR \fIoffset\fR, \fBint\fR \fIwhence\fR);
.fi

.LP
.nf
\fBint\fR \fBfseeko\fR(\fBFILE *\fR\fIstream\fR, \fBoff_t\fR \fIoffset\fR, \fBint\fR \fIwhence\fR);
.fi

.SH DESCRIPTION
The \fBfseek()\fR function sets the file-position indicator for the stream
pointed to by \fIstream\fR. The \fBfseeko()\fR function is identical to
\fBfseek()\fR except for the type of \fIoffset\fR.
.sp
.LP
The new position, measured in bytes from the beginning of the file, is obtained
by adding \fIoffset\fR to the position specified by \fIwhence\fR, whose values
are defined in <\fBstdio.h\fR> as follows:
.sp
.ne 2
.na
\fB\fBSEEK_SET\fR\fR
.ad
.RS 12n
Set position equal to \fIoffset\fR bytes.
.RE

.sp
.ne 2
.na
\fB\fBSEEK_CUR\fR\fR
.ad
.RS 12n
Set position to current location plus \fIoffset\fR.
.RE

.sp
.ne 2
.na
\fB\fBSEEK_END\fR\fR
.ad
.RS 12n
Set position to \fBEOF\fR plus \fIoffset\fR.
.RE

.sp
.LP
If the stream is to be used with wide character input/output functions,
\fIoffset\fR must either be 0 or a value returned by an earlier call to
\fBftell\fR(3C) on the same stream and \fIwhence\fR must be \fBSEEK_SET\fR. This
constraint does not hold for streams created by \fBopen_wmemstream\fR(3C).
.sp
.LP
A successful call to \fBfseek()\fR clears the end-of-file indicator for the
stream and undoes any effects of \fBungetc\fR(3C) and \fBungetwc\fR(3C) on the
same stream.  After an \fBfseek()\fR call, the next operation on an update
stream may be either input or output.
.sp
.LP
If the most recent operation, other than \fBftell\fR(3C), on a given stream is
\fBfflush\fR(3C), the file offset in the underlying open file description will
be adjusted to reflect the location specified by \fBfseek()\fR.
.sp
.LP
The \fBfseek()\fR 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.
.sp
.LP
The value of the file offset returned by \fBfseek()\fR on devices which are
incapable of seeking is undefined.
.sp
.LP
If the stream is writable and buffered data had not been written to the
underlying file, \fBfseek()\fR will cause the unwritten data to be written to
the file and mark the \fBst_ctime\fR and \fBst_mtime\fR fields of the file for
update.
.SS \fBopen_wmemstream\fR(3C)
When using a stream based on
.B open_wmemstream\fR(3C),
the
.B fseek()
and
.B fseeko()
functions no longer operate in terms of bytes. Instead, like the stream itself,
the values used in
.I offset
are measured in units of wide characters, the underlying data unit of the
stream. The values returned by
.B ftell(3C)
or
.B 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
.I whence
and
.I offset,
the same as would be done for a normal byte-oriented stream.
.SH RETURN VALUES
The \fBfseek()\fR and \fBfseeko()\fR functions return \fB0\fR on success;
otherwise, they returned \fB\(mi1\fR and set \fBerrno\fR to indicate the error.
.SH ERRORS
The \fBfseek()\fR and \fBfseeko()\fR functions will fail if, either the
\fIstream\fR is unbuffered or the \fIstream\fR's buffer needed to be flushed,
and the call to \fBfseek()\fR or \fBfseeko()\fR causes an underlying
\fBlseek\fR(2) or \fBwrite\fR(2) to be invoked:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 10n
The \fBO_NONBLOCK\fR flag is set for the file descriptor and the process would
be delayed in the write operation.
.RE

.sp
.ne 2
.na
\fB\fBEBADF\fR\fR
.ad
.RS 10n
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.
.RE

.sp
.ne 2
.na
\fB\fBEFBIG\fR\fR
.ad
.RS 10n
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.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR\fR
.ad
.RS 10n
The write operation was terminated due to the receipt of a signal, and no data
was transferred.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fIwhence\fR argument is invalid. The resulting file-position indicator
would be set to a negative value.
.RE

.sp
.ne 2
.na
\fB\fBEIO\fR\fR
.ad
.RS 10n
A physical I/O error has occurred; or the process is a member of a background
process group attempting to perform a \fBwrite\fR(2) operation to its
controlling terminal, \fBTOSTOP\fR is set, the process is neither ignoring nor
blocking \fBSIGTTOU\fR, and the process group of the process is orphaned.
.RE

.sp
.ne 2
.na
\fB\fBENOSPC\fR\fR
.ad
.RS 10n
There was no free space remaining on the device containing the file.
.RE

.sp
.ne 2
.na
\fB\fBENXIO\fR\fR
.ad
.RS 10n
A request was made of a non-existent device, or the request was outside the
capabilities of the device.
.RE

.sp
.ne 2
.na
\fB\fBEPIPE\fR\fR
.ad
.RS 10n
The file descriptor underlying \fIstream\fR is associated with a pipe or FIFO.
.RE

.sp
.ne 2
.na
\fB\fBEPIPE\fR\fR
.ad
.RS 10n
An attempt was made to write to a pipe or \fBFIFO\fR that is not open for
reading by any process. A \fBSIGPIPE\fR signal will also be sent to the calling
thread.
.RE

.sp
.LP
The \fBfseek()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEOVERFLOW\fR\fR
.ad
.RS 13n
The resulting file offset would be a value which cannot be represented
correctly in an object of type \fBlong\fR.
.RE

.sp
.LP
The \fBfseeko()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEOVERFLOW\fR\fR
.ad
.RS 13n
The resulting file offset would be a value which cannot be represented
correctly in an object of type \fBoff_t\fR.
.RE

.SH USAGE
Although on the UNIX system an offset returned by \fBftell()\fR or
\fBftello()\fR (see \fBftell\fR(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 \fBfseek()\fR directly.
Arithmetic may not meaningfully be performed on such an offset, which is not
necessarily measured in bytes.
.sp
.LP
The \fBfseeko()\fR function has a transitional interface for 64-bit file
offsets.  See \fBlf64\fR(5).
.SH ATTRIBUTES
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
\fBgetrlimit\fR(2), \fBulimit\fR(2), \fBftell\fR(3C),
\fBrewind\fR(3C), \fBungetc\fR(3C), \fBungetwc\fR(3C), \fBattributes\fR(5),
\fBlf64\fR(5), \fBstandards\fR(5)