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lseek(2) [freebsd man page]

LSEEK(2)						      BSD System Calls Manual							  LSEEK(2)

NAME
lseek -- reposition read/write file offset LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> off_t lseek(int fildes, off_t offset, int whence); DESCRIPTION
The lseek() system call 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. The lseek() system call repositions the file position pointer associated with the file descriptor 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() system call 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. 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 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
The lseek() system call will fail and the file position pointer will remain unchanged if: [EBADF] The fildes argument is not an open file descriptor. [EINVAL] The whence argument is not a proper value or the resulting file offset would be negative for a non-character special file. [ENXIO] 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. [EOVERFLOW] The resulting file offset would be a value which cannot be represented correctly in an object of type off_t. [ESPIPE] The fildes argument is associated with a pipe, socket, or FIFO. SEE ALSO
dup(2), open(2), pathconf(2) STANDARDS
The lseek() system call is expected to conform to ISO/IEC 9945-1:1990 (``POSIX.1''). HISTORY
The lseek() function appeared in Version 7 AT&T UNIX. BUGS
This document's use of whence is incorrect English, but is maintained for historical reasons. BSD
May 26, 2012 BSD

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LSEEK(2)						     Linux Programmer's Manual							  LSEEK(2)

NAME
lseek - reposition read/write file offset SYNOPSIS
#include <sys/types.h> #include <unistd.h> off_t lseek(int fd, off_t offset, int whence); DESCRIPTION
The lseek() function repositions the offset of the open file associated with the file descriptor fd to the argument offset according to the directive whence as follows: SEEK_SET The offset is set to offset bytes. SEEK_CUR The offset is set to its current location plus offset bytes. SEEK_END The offset is set to the size of the file plus offset bytes. The lseek() function allows the file offset to be set beyond the end of the file (but this does not change the size of the file). If data is later written at this point, subsequent reads of the data in the gap (a "hole") return null bytes ('') until data is actually written into the gap. Seeking file data and holes Since version 3.1, Linux supports the following additional values for whence: SEEK_DATA Adjust the file offset to the next location in the file greater than or equal to offset containing data. If offset points to data, then the file offset is set to offset. SEEK_HOLE Adjust the file offset to the next hole in the file greater than or equal to offset. If offset points into the middle of a hole, then the file offset is set to offset. If there is no hole past offset, then the file offset is adjusted to the end of the file (i.e., there is an implicit hole at the end of any file). In both of the above cases, lseek() fails if offset points past the end of the file. These operations allow applications to map holes in a sparsely allocated file. This can be useful for applications such as file backup tools, which can save space when creating backups and preserve holes, if they have a mechanism for discovering holes. For the purposes of these operations, a hole is a sequence of zeros that (normally) has not been allocated in the underlying file storage. However, a file system is not obliged to report holes, so these operations are not a guaranteed mechanism for mapping the storage space actually allocated to a file. (Furthermore, a sequence of zeros that actually has been written to the underlying storage may not be reported as a hole.) In the simplest implementation, a file system can support the operations by making SEEK_HOLE always return the offset of the end of the file, and making SEEK_DATA always return offset (i.e., even if the location referred to by offset is a hole, it can be considered to consist of data that is a sequence of zeros). The _GNU_SOURCE feature test macro must be defined in order to obtain the definitions of SEEK_DATA and SEEK_HOLE from <unistd.h>. RETURN VALUE
Upon successful completion, lseek() returns the resulting offset location as measured in bytes from the beginning of the file. On error, the value (off_t) -1 is returned and errno is set to indicate the error. ERRORS
EBADF fd is not an open file descriptor. EINVAL whence is not valid. Or: the resulting file offset would be negative, or beyond the end of a seekable device. EOVERFLOW The resulting file offset cannot be represented in an off_t. ESPIPE fd is associated with a pipe, socket, or FIFO. ENXIO whence is SEEK_DATA or SEEK_HOLE, and the current file offset is beyond the end of the file. CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. SEEK_DATA and SEEK_HOLE are nonstandard extensions also present in Solaris, FreeBSD, and DragonFly BSD; they are proposed for inclusion in the next POSIX revision (Issue 8). NOTES
Some devices are incapable of seeking and POSIX does not specify which devices must support lseek(). On Linux, using lseek() on a terminal device returns ESPIPE. When converting old code, substitute values for whence with the following macros: old new 0 SEEK_SET 1 SEEK_CUR 2 SEEK_END L_SET SEEK_SET L_INCR SEEK_CUR L_XTND SEEK_END Note that file descriptors created by dup(2) or fork(2) share the current file position pointer, so seeking on such files may be subject to race conditions. SEE ALSO
dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2013-03-27 LSEEK(2)

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