NAME

lseek − move read/write file pointer

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

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

DESCRIPTION

fildes is a file descriptor returned from a creat, open, dup, fcntl, pipe, or ioctl system call.  lseek sets the file pointer associated with fildes 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.

On success, lseek returns the resulting pointer location, as measured in bytes from the beginning of the file.  Note that if fildes is a remote file descriptor and offset is negative, lseek returns the file pointer even if it is negative. 

lseek allows the file pointer to be set beyond the existing data in the file. If data are later written at this point, subsequent reads in the gap between the previous end of data and the newly written data will return bytes of value 0 until data are written into the gap. 

lseek fails and the file pointer remains unchanged if one or more of the following are true:

EBADF fildes is not an open file descriptor. 

ESPIPE fildes is associated with a pipe or fifo. 

EINVAL whence is not SEEK_SET, SEEK_CUR, or SEEK_END.  The process also gets a SIGSYS signal. 

EINVAL fildes is not a remote file descriptor, and the resulting file pointer would be negative. 

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

SEE ALSO

creat(2), dup(2), fcntl(2), open(2)

DIAGNOSTICS

Upon successful completion, a non-negative integer indicating the file pointer value is returned.  Otherwise, a value of −1 is returned and errno is set to ­indicate the error.