Portable Positioning (The GNU C Library)

### 12.19 Portable File-Position Functions ¶

On GNU systems, 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 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. See 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 C Library, `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 C Library, `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)` ¶

  Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe lock corrupt | See POSIX Safety Concepts.

  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)` ¶

  Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe lock corrupt | See POSIX Safety Concepts.

  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)` ¶

  Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe lock corrupt | See POSIX Safety Concepts.

  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)` ¶

  Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe lock corrupt | See POSIX Safety Concepts.

  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.