lseek(2) [mojave man page]
LSEEK(2) BSD System Calls Manual LSEEK(2) NAME
lseek -- reposition read/write file offset SYNOPSIS
#include <unistd.h> off_t lseek(int fildes, off_t offset, int whence); DESCRIPTION
The lseek() function repositions the offset of the file descriptor fildes to the argument offset, according to the directive whence. The argument fildes must be an open file descriptor. lseek() repositions the file pointer fildes as follows: If whence is SEEK_SET, the offset is set to offset bytes. If whence is SEEK_CUR, the offset is set to its current location plus offset bytes. If whence is SEEK_END, the offset is set to the size of the file plus offset bytes. If whence is SEEK_HOLE, the offset is set to the start of the next hole greater than or equal to the supplied offset. The definition of a hole is provided below. If whence is SEEK_DATA, the offset is set to the start of the next non-hole file region greater than or equal to the supplied offset. The lseek() function allows the file offset to be set beyond the end of the existing end-of-file of the file. If data is later written at this point, subsequent reads of the data in the gap return bytes of zeros (until data is actually written into the gap). Some devices are incapable of seeking. The value of the pointer associated with such a device is undefined. 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. File systems 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. Each file is presented as having a zero-size virtual hole at the very end of the file. The existence of a hole at the end of every data region allows for easy programming and also provides compatibility to the original implementation in Solaris. It also causes the current file size (i.e., end-of-file offset) to be returned to indicate that there are no more holes past the supplied offset. Applications should use fpathconf(_PC_MIN_HOLE_SIZE) or pathconf(_PC_MIN_HOLE_SIZE) to determine if a file system supports SEEK_HOLE. See pathconf(2). For file systems that do not supply information about holes, the file will be represented as one entire data region. RETURN VALUES
Upon successful completion, lseek() returns the resulting offset location 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. ERRORS
lseek() will fail and the file pointer will remain unchanged if: [EBADF] Fildes is not an open file descriptor. [EINVAL] Whence is not a proper value. [EINVAL] The seek location (calculated from offset and whence) is negative. [ENXIO] For SEEK_DATA, there are no more data regions past the supplied offset. Due to existence of the hole at the end of the file, for SEEK_HOLE this error is only returned when the offset already points to the end-of-file position. [EOVERFLOW] The seek location is too large to be stored in an object of type off_t. [ESPIPE] Fildes is associated with a pipe, socket, or FIFO. SEE ALSO
dup(2), open(2) BUGS
This document's use of whence is incorrect English, but is maintained for historical reasons. STANDARDS
The lseek() function conforms to IEEE Std 1003.1-1988 (``POSIX.1''). 4th Berkeley Distribution April 19, 1994 4th Berkeley Distribution
Check Out this Related Man Page
lseek(2) System Calls lseek(2) 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
The lseek() function sets the file pointer associated with the open file descriptor specified by fildes as follows: o If whence is SEEK_SET, the pointer is set to offset bytes. o If whence is SEEK_CUR, the pointer is set to its current location plus offset. o If whence is SEEK_END, the pointer is set to the size of the file plus offset. o 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. o 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 sup- plied 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, sub- sequent 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() func- tion 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. RETURN VALUES
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. ERRORS
The lseek() function will fail if: EBADF The fildes argument is not an open file descriptor. EINVAL 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. ENXIO For SEEK_DATA, there are no more data regions past the supplied offset. For SEEK_HOLE, there are no more holes past the sup- plied offset. EOVERFLOW The resulting file offset would be a value which cannot be represented correctly in an object of type off_t for regular files. ESPIPE The fildes argument is associated with a pipe, a FIFO, or a socket. USAGE
The lseek() function has a transitional interface for 64-bit file offsets. See lf64(5). 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(). ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ |MT-Level |Async-Signal-Safe | +-----------------------------+-----------------------------+ SEE ALSO
creat(2), dup(2), fcntl(2), fpathconf(2), open(2), read(2), write(2), attributes(5), lf64(5), standards(5) SunOS 5.11 4 May 2005 lseek(2)