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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() ex‐
cept for the type of offset.
The new position, measured in bytes from the beginning of the file, ex‐
cept in the case of streams opened with open_wmemstream() function for
which the position is measured in wide characters, is obtained by
adding offset to the position specified by whence, whose values are de‐
fined 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 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 be‐
yond 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 up‐
date.
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 in‐
voked:
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.
ESPIPE The file descriptor underlying stream is associated with a
pipe, a FIFO, or a STREAMS device.
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 re‐
quires 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 off‐
sets. See lf64(7).
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelMT-Safe _ Stan‐
dardSee standards(7).
SEE ALSO
getrlimit(2), ulimit(2), ftell(3C), rewind(3C), ungetc(3C),
ungetwc(3C), attributes(7), lf64(7), standards(7)
Oracle Solaris 11.4 09 May 2016 fseek(3C)