lseek - move read/write file pointer
#include <sys/types.h> #include <unistd.h> off_t lseek(int fildes, off_t offset, int whence);
The lseek() function sets the file pointer associated with the open file descriptor specified by 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.
If whence is SEEK_HOLE, the offset of the start of the next hole greater than or equal to the supplied offset is returned. The definition of a hole is provided near the end of the DESCRIPTION.
If whence is SEEK_DATA, the file pointer is set to the start of the next non-hole file region greater than or equal to the supplied offset.
The symbolic constants SEEK_SET, SEEK_CUR, SEEK_END, SEEK_HOLE, and SEEK_DATA are defined in the header <unistd.h>.
Some devices are incapable of seeking. The value of the file pointer associated with such a device is undefined.
The lseek() function 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.
If fildes is a remote file descriptor and offset is negative, lseek() returns the file pointer even if it is negative. The lseek() function will not, by itself, extend the size of a file.
If fildes refers to a shared memory object, lseek() behaves as if fildes referred to a regular file.
A “hole” is defined as a contiguous range of bytes in a file, all having the value of zero, but not all zeros in a file are guaranteed to be represented as holes returned with SEEK_HOLE. Filesystems are allowed to expose ranges of zeros with SEEK_HOLE, but not required to. Applications can use SEEK_HOLE to optimise their behavior for ranges of zeros, but must not depend on it to find all such ranges in a file. The existence of a hole at the end of every data region allows for easy programming and implies that a virtual hole exists at the end of the file. Applications should use fpathconf(_PC_MIN_HOLE_SIZE) or pathconf(_PC_MIN_HOLE_SIZE) to determine if a filesystem supports SEEK_HOLE. See fpathconf(2).
For filesystems that do not supply information about holes, the file will be represented as one entire data region.
Upon successful completion, the resulting offset, as measured in bytes from the beginning of the file, is returned. Otherwise, (off_t)−1 is returned, the file offset remains unchanged, and errno is set to indicate the error.
The lseek() function will fail if:
The fildes argument is not an open file descriptor.
The whence argument is not SEEK_SET, SEEK_CUR, or SEEK_END; or the fildes argument is not a remote file descriptor and the resulting file pointer would be negative.
For SEEK_DATA, there are no more data regions past the supplied offset. For SEEK_HOLE, there are no more holes past the supplied offset.
The resulting file offset would be a value which cannot be represented correctly in an object of type off_t for regular files.
The fildes argument is associated with a pipe, a FIFO, a socket, or a STREAMS device.
The lseek() function has a transitional interface for 64-bit file offsets. See lf64(7).
In multithreaded applications, using lseek() in conjunction with a read(2) or write(2) call on a file descriptor shared by more than one thread is not an atomic operation. To ensure atomicity, use pread() or pwrite().
See attributes(7) for descriptions of the following attributes: