fseek(3S) SDK R4.11 fseek(3S)
NAME
fseek, rewind, ftell - reposition a file pointer in a stream
SYNOPSIS
#include <stdio.h>
int fseek (FILE *stream, long offset, int ptrname);
void rewind (FILE *stream);
long ftell (FILE *stream);
DESCRIPTION
fseek sets the position of the next input or output operation on the
stream [see intro(3)]. The new position is at the signed distance
offset bytes from the beginning, from the current position, or from
the end of the file, according to a ptrname value of SEEK_SET,
SEEK_CUR, or SEEK_END (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.
fseek allows the file position indicator to be set beyond the end of
the existing data in the file. If data is later written at this
point, subsequent reads of data in the gap will return zero until
data is actually written into the gap. fseek, by itself, does not
extend the size of the file.
rewind (stream) is equivalent to:
(void) fseek (stream, 0L, SEEK_SET);
except that rewind also clears the error indicator on stream.
fseek and rewind clear the EOF indicator and undo any effects of
ungetc on stream. After fseek or rewind, the next operation on a
file opened for update may be either input or output.
If stream is writable and buffered data has not been written to the
underlying file, fseek and rewind cause the unwritten data to be
written to the file.
ftell returns the offset of the current byte relative to the
beginning of the file associated with the named stream.
Errors
fseek returns -1 for improper seeks, otherwise zero. An improper
seek can be, for example, an fseek done on a file that has not been
opened via fopen; in particular, fseek may not be used on a terminal
or on a file opened via popen. After a stream is closed, no further
operations are defined on that stream.
Considerations for Threads Programming
+---------+-----------------------------+
| | async- |
|function | reentrant cancel cancel |
| | point safe |
+---------+-----------------------------+
|fseek | Y Y N |
|ftell | Y Y N |
|rewind | Y Y N |
+---------+-----------------------------+
REFERENCES
lseek(2), reentrant(3), fopen(3S), popen(3S), setvbuf(3S), stdio(3S),
ungetc(3S), write(2)
NOTICES
Although on the UNIX system an offset returned by ftell 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 stdio routines in libc automatically set their buffer size to an
optimal value for sequential I/O. If your application is doing alot
of random I/O and reading small amounts of data, it would be wise to
use the setvbuf() function (see the setvbuf(3S) man page). Use of
setvbuf() to buffer data at a size closer to the average amount
read/written will probably increase performance.
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