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Finding file positions at a low level

If you want to find a particular file position within a file, using a low-level file routine, you can call the lseek function. This is very similar to the high-level file routine fseek, except that it accepts a file descriptor rather than a stream as an argument.

The lseek function specifies the file position for the next read or write operation. (See File position, for more information on file positions.)

The lseek function takes three parameters. The first parameter is the file descriptor. The second is of type off_t and specifies the number of bytes to move the file position indicator. The third argument and the third parameter is a constant that specifies whether the offset is relative to the beginning of the file (SEEK_SET), to the current file position (SEEK_CUR), or to the end of the file (SEEK_END). If SEEK_CUR or SEEK_END is used, the offset specified can be positive or negative. If you specify SEEK_END, set the position past the current end of the file, and actually write data, you will extend the file with zeroes up to the position you specify. However, the blocks of zeroes are not actually written to disk, so the file takes up less space on disk than it seems to; this is called a sparse file.

The return value of lseek is of type off_t and normally contains the resulting file position, as measured in bytes from the beginning of the file. If you wish to read the current file position, therefore, you can specify an offset of 0 and a third parameter of SEEK_CUR, as follows:

file_position = lseek (file_descriptor, 0, SEEK_CUR);

If there was an error, lseek returns a -1 and sets the system variable errno to one of the following values:

The file descriptor specified is invalid.
Either the third parameter of lseek is invalid, or the file offset is invalid.
The file descriptor corresponds to an object that cannot be positioned, such as a terminal device.

The lseek function is called by many high-level file position functions, including fseek, rewind, and ftell.