LSEEK(2)
NAME
lseek − move read/write pointer
USAGE
#define L_SET0 /* set the seek pointer */
#define L_INCR1 /* increment the seek pointer */
#define L_XTND2 /* extend the file size */
pos = lseek(d, offset, whence)
int pos;
int d, offset, whence;
DESCRIPTION
The descriptor d refers to a file or device open for reading and/or writing. Lseek sets the file pointer of d as follows:
If whence is L_SET, the pointer is set to offset bytes.
If whence is L_INCR, the pointer is set to its current location plus offset.
If whence is L_XTND, the pointer is set to the size of the file plus offset.
Upon successful completion, lseek returns the resulting pointer location, measured in bytes from the beginning of the file. Some devices are incapable of seeking. The value of the pointer associated with such a device is undefined.
The ‘whence’ values are defined in <sys/file.h>.
NOTES
If lseek goes far beyond the end of a file, and then writes, it creates a gap that occupies no physical space and reads as zeros.
Offset ought to be ‘off_t’ or ‘long’, for consistency with the lint library and the kernel.
RETURN VALUE
Upon successful completion, a non-negative integer (the current file pointer value) is returned. Otherwise, a value of −1 is returned and errno is set to indicate the error.
ERRORS
Lseek will fail and the file pointer will remain unchanged if:
[EBADF] D is not an open file descriptor.
[ESPIPE] D is associated with a pipe or a socket.
[EINVAL] Whence is not a proper value.
[EINVAL] The resulting file pointer would be negative.