The file position of a stream describes where in the file the stream is currently reading or writing. I/O on the stream advances the file position through the file. In the GNU system, the file position is represented as an integer, which counts the number of bytes from the beginning of the file. See section File Position.
During I/O to an ordinary disk file, you can change the file position whenever you wish, so as to read or write any portion of the file. Some other kinds of files may also permit this. Files which support changing the file position are sometimes referred to as random-access files.
You can use the functions in this section to examine or modify the file position indicator associated with a stream. The symbols listed below are declared in the header file `stdio.h'.
This function can fail if the stream doesn't support file positioning,
or if the file position can't be represented in a
long int, and
possibly for other reasons as well. If a failure occurs, a value of
-1 is returned.
fseekfunction is used to change the file position of the stream stream. The value of whence must be one of the constants
SEEK_END, to indicate whether the offset is relative to the beginning of the file, the current file position, or the end of the file, respectively.
This function returns a value of zero if the operation was successful,
and a nonzero value to indicate failure. A successful call also clears
the end-of-file indicator of stream and discards any characters
that were "pushed back" by the use of
fseek either flushes any buffered output before setting the file
position or else remembers it so it will be written later in its proper
place in the file.
Portability Note: In non-POSIX systems,
fseek might work reliably only on binary streams. See section Text and Binary Streams.
The following symbolic constants are defined for use as the whence
fseek. They are also used with the
function (see section Input and Output Primitives) and to specify offsets for file locks
(see section Control Operations on Files).
fseekfunction, specifies that the offset provided is relative to the beginning of the file.
fseekfunction, specifies that the offset provided is relative to the current file position.
fseekfunction, specifies that the offset provided is relative to the end of the file.
rewindfunction positions the stream stream at the begining of the file. It is equivalent to calling
fseekon the stream with an offset argument of
0Land a whence argument of
SEEK_SET, except that the return value is discarded and the error indicator for the stream is reset.
These three aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'.
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