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 section 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)

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 must be one of the symbolic constants SEEK_SET, SEEK_CUR, or SEEK_END.

SEEK_SET

Specifies that whence is a count of characters from the beginning of the file.

SEEK_CUR

Specifies that whence is a count of characters from the current file position. This count may be positive or negative.

SEEK_END

Specifies that whence 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.@end table 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 section I/O 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 so; 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 in the GNU system, you always get ESPIPE if the object is not seekable.)

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

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 an arithmetic data type used to represent file sizes. In the GNU system, this is equivalent to fpos_t or long int.

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.

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