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HP-UX 11i Version 3: February 2007

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lseek — move read/write file pointer; seek


#include <unistd.h>

off_t lseek(int fildes, off_t offset, int whence);


lseek() sets the file pointer associated with the file descriptor as follows:

  • If whence is SEEK_SET, the pointer is set to offset bytes.

  • If whence is SEEK_CUR, the pointer is set to its current location plus offset.

  • If whence is SEEK_END, the pointer is set to the size of the file plus offset.

These symbolic constants are defined in <unistd.h>.


When lseek() completes successfully, it returns an integer, which is the resulting file offset as measured in bytes from the beginning of the file. Otherwise, a value of -1 is returned and errno is set to indicate the error.

For all files that are not character or block special files, the integer returned on successful completion is non-negative. For character or block special files that correspond to disk sections larger than 2 gigabytes, a non-negative integer is returned for successful seeks beyond 2 gigabytes. This value is the resulting file offset as measured in bytes from the beginning of the file, when taken as an unsigned value. -1 always indicates an error return, even when encountered on greater than 2 gigabyte disk sections. The lseek() call succeeds for NFS directories even if the resulting file offset becomes negative.


lseek() fails and the file offset remains unchanged if one or more of the following is true:


fildes is not an open file descriptor.


fildes is associated with a pipe, socket, or FIFO.


whence is not one of the supported values.


The resulting file offset would be negative.


The resulting file offset would be a value which cannot be represented correctly in an object of type off_t.


Some devices are incapable of seeking. The value of the file offset associated with such a device is undefined.

Using lseek() with a whence of SEEK_END on device special files is not supported and the results are not defined.


creat(2), dup(2), fcntl(2), lseek64(2), open(2), unistd(5).


lseek(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1

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