### 13.3 Setting the File Position of a Descriptor

Just as you can set the file position of a stream with `fseek`, you can set the file position of a descriptor with `lseek`. This specifies the position in the file for the next `read` or `write` operation. See File Positioning, for more information on the file position and what it means.

To read the current file position value from a descriptor, use `lseek (`desc`, 0, SEEK_CUR)`.

— Function: off_t lseek (int filedes, off_t offset, int whence)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> The `lseek` function is used to change the file position of the file with descriptor filedes.

> The whence argument specifies how the offset should be interpreted, in the same way as for the `fseek` function, and it must be one of the symbolic constants `SEEK_SET`, `SEEK_CUR`, or `SEEK_END`.

> `SEEK_SET`
>   Specifies that offset is a count of characters from the beginning of the file.
> `SEEK_CUR`
>   Specifies that offset is a count of characters from the current file position. This count may be positive or negative.
> `SEEK_END`
>   Specifies that offset is a count of characters from the end of the file. A negative count specifies a position within the current extent of the file; a positive count specifies a position past the current end. If you set the position past the current end, and actually write data, you will extend the file with zeros up to that position.

> The return value from `lseek` is normally the resulting file position, measured in bytes from the beginning of the file. You can use this feature together with `SEEK_CUR` to read the current file position.

> If you want to append to the file, setting the file position to the current end of file with `SEEK_END` is not sufficient. Another process may write more data after you seek but before you write, extending the file so the position you write onto clobbers their data. Instead, use the `O_APPEND` operating mode; see Operating Modes.

> You can set the file position past the current end of the file. This does not by itself make the file longer; `lseek` never changes the file. But subsequent output at that position will extend the file. Characters between the previous end of file and the new position are filled with zeros. Extending the file in this way can create a “hole”: the blocks of zeros are not actually allocated on disk, so the file takes up less space than it appears to; it is then called a “sparse file”. If the file position cannot be changed, or the operation is in some way invalid, `lseek` returns a value of -1. The following `errno` error conditions are defined for this function:

> `EBADF`
>   The filedes is not a valid file descriptor.
> `EINVAL`
>   The whence argument value is not valid, or the resulting file offset is not valid. A file offset is invalid.
> `ESPIPE`
>   The filedes corresponds to an object that cannot be positioned, such as a pipe, FIFO or terminal device. (POSIX.1 specifies this error only for pipes and FIFOs, but on GNU systems, you always get `ESPIPE` if the object is not seekable.)

> When the source file is compiled with `_FILE_OFFSET_BITS == 64` the `lseek` function is in fact `lseek64` and the type `off_t` has 64 bits which makes it possible to handle files up to 2^63 bytes in length.

> This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `lseek` is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `lseek` should be protected using cancellation handlers.

> The `lseek` function is the underlying primitive for the `fseek`, `fseeko`, `ftell`, `ftello` and `rewind` functions, which operate on streams instead of file descriptors.

— Function: off64_t lseek64 (int filedes, off64_t offset, int whence)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> This function is similar to the `lseek` function. The difference is that the offset parameter is of type `off64_t` instead of `off_t` which makes it possible on 32 bit machines to address files larger than 2^31 bytes and up to 2^63 bytes. The file descriptor `filedes` must be opened using `open64` since otherwise the large offsets possible with `off64_t` will lead to errors with a descriptor in small file mode.

> When the source file is compiled with `_FILE_OFFSET_BITS == 64` on a 32 bits machine this function is actually available under the name `lseek` and so transparently replaces the 32 bit interface.

You can have multiple descriptors for the same file if you open the file more than once, or if you duplicate a descriptor with `dup`. Descriptors that come from separate calls to `open` have independent file positions; using `lseek` on one descriptor has no effect on the other. For example,

         {
           int d1, d2;
           char buf[4];
           d1 = open ("foo", O_RDONLY);
           d2 = open ("foo", O_RDONLY);
           lseek (d1, 1024, SEEK_SET);
           read (d2, buf, 4);
         }

will read the first four characters of the file `foo`. (The error-checking code necessary for a real program has been omitted here for brevity.)

By contrast, descriptors made by duplication share a common file position with the original descriptor that was duplicated. Anything which alters the file position of one of the duplicates, including reading or writing data, affects all of them alike. Thus, for example,

         {
           int d1, d2, d3;
           char buf1[4], buf2[4];
           d1 = open ("foo", O_RDONLY);
           d2 = dup (d1);
           d3 = dup (d2);
           lseek (d3, 1024, SEEK_SET);
           read (d1, buf1, 4);
           read (d2, buf2, 4);
         }

will read four characters starting with the 1024'th character of `foo`, and then four more characters starting with the 1028'th character.

— Data Type: off_t

> This is a signed integer type used to represent file sizes. In the GNU C Library, this type is no narrower than `int`.

> If the source is compiled with `_FILE_OFFSET_BITS == 64` this type is transparently replaced by `off64_t`.

— Data Type: off64_t

> This type is used similar to `off_t`. The difference is that even on 32 bit machines, where the `off_t` type would have 32 bits, `off64_t` has 64 bits and so is able to address files up to 2^63 bytes in length.

> When compiling with `_FILE_OFFSET_BITS == 64` this type is available under the name `off_t`.

These 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`.

`L_SET`
  An alias for `SEEK_SET`.
`L_INCR`
  An alias for `SEEK_CUR`.
`L_XTND`
  An alias for `SEEK_END`.