GNU Info
(libc.info)Portable Positioning
Portable File-Position Functions
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On the GNU system, the file position is truly a character count. You
can specify any character count value as an argument to `fseek' or
`fseeko' and get reliable results for any random access file. However,
some ISO C systems do not represent file positions in this way.
On some systems where text streams truly differ from binary streams,
it is impossible to represent the file position of a text stream as a
count of characters from the beginning of the file. For example, the
file position on some systems must encode both a record offset within
the file, and a character offset within the record.
As a consequence, if you want your programs to be portable to these
systems, you must observe certain rules:
* The value returned from `ftell' on a text stream has no predictable
relationship to the number of characters you have read so far.
The only thing you can rely on is that you can use it subsequently
as the OFFSET argument to `fseek' or `fseeko' to move back to the
same file position.
* In a call to `fseek' or `fseeko' on a text stream, either the
OFFSET must be zero, or WHENCE must be `SEEK_SET' and and the
OFFSET must be the result of an earlier call to `ftell' on the
same stream.
* The value of the file position indicator of a text stream is
undefined while there are characters that have been pushed back
with `ungetc' that haven't been read or discarded. Note:
Unreading.
But even if you observe these rules, you may still have trouble for
long files, because `ftell' and `fseek' use a `long int' value to
represent the file position. This type may not have room to encode all
the file positions in a large file. Using the `ftello' and `fseeko'
functions might help here since the `off_t' type is expected to be able
to hold all file position values but this still does not help to handle
additional information which must be associated with a file position.
So if you do want to support systems with peculiar encodings for the
file positions, it is better to use the functions `fgetpos' and
`fsetpos' instead. These functions represent the file position using
the data type `fpos_t', whose internal representation varies from
system to system.
These symbols are declared in the header file `stdio.h'.
- Data Type: fpos_t
This is the type of an object that can encode information about the
file position of a stream, for use by the functions `fgetpos' and
`fsetpos'.
In the GNU system, `fpos_t' is an opaque data structure that
contains internal data to represent file offset and conversion
state information. In other systems, it might have a different
internal representation.
When compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine
this type is in fact equivalent to `fpos64_t' since the LFS
interface transparently replaces the old interface.
- Data Type: fpos64_t
This is the type of an object that can encode information about the
file position of a stream, for use by the functions `fgetpos64' and
`fsetpos64'.
In the GNU system, `fpos64_t' is an opaque data structure that
contains internal data to represent file offset and conversion
state information. In other systems, it might have a different
internal representation.
- Function: int fgetpos (FILE *STREAM, fpos_t *POSITION)
This function stores the value of the file position indicator for
the stream STREAM in the `fpos_t' object pointed to by POSITION.
If successful, `fgetpos' returns zero; otherwise it returns a
nonzero value and stores an implementation-defined positive value
in `errno'.
When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
32 bit system the function is in fact `fgetpos64'. I.e., the LFS
interface transparently replaces the old interface.
- Function: int fgetpos64 (FILE *STREAM, fpos64_t *POSITION)
This function is similar to `fgetpos' but the file position is
returned in a variable of type `fpos64_t' to which POSITION points.
If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
bits machine this function is available under the name `fgetpos'
and so transparently replaces the old interface.
- Function: int fsetpos (FILE *STREAM, const fpos_t *POSITION)
This function sets the file position indicator for the stream
STREAM to the position POSITION, which must have been set by a
previous call to `fgetpos' on the same stream. If successful,
`fsetpos' clears the end-of-file indicator on the stream, discards
any characters that were "pushed back" by the use of `ungetc', and
returns a value of zero. Otherwise, `fsetpos' returns a nonzero
value and stores an implementation-defined positive value in
`errno'.
When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
32 bit system the function is in fact `fsetpos64'. I.e., the LFS
interface transparently replaces the old interface.
- Function: int fsetpos64 (FILE *STREAM, const fpos64_t *POSITION)
This function is similar to `fsetpos' but the file position used
for positioning is provided in a variable of type `fpos64_t' to
which POSITION points.
If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
bits machine this function is available under the name `fsetpos'
and so transparently replaces the old interface.
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