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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 ('\0') 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 allo-
       cated 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  opera-
       tions  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 loca-
       tion 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 follow-
       ing 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  cur-
       rent  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)