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.
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.
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 per‐
form 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 con‐
taining 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 necessar‐
ily measured in bytes.
The fseeko() function has a transitional interface for 64-bit file off‐
sets. See lf64(5).
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Standard │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │MT-Safe │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOgetrlimit(2), ulimit(2), fopen(3UCB), ftell(3C), rewind(3C),
ungetc(3C), ungetwc(3C), attributes(5), lf64(5), standards(5)SunOS 5.10 1 Nov 2003 fseek(3C)