Man Linux: Main Page and Category List


       lseek - reposition read/write file offset


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

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


       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:

              The offset is set to offset bytes.

              The offset is set to its current location plus offset bytes.

              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.


       Upon  successful  completion,  lseek()  returns  the  resulting  offset
       location  as  measured  in  bytes  from  the  beginning  of  the  file.
       Otherwise,  a  value  of  (off_t) -1  is  returned  and errno is set to
       indicate the error.


       EBADF  fd is not an open file descriptor.

       EINVAL whence is not  one  of  SEEK_SET,  SEEK_CUR,  SEEK_END;  or  the
              resulting  file offset would be negative, or beyond the end of a
              seekable device.

              The resulting file offset cannot be represented in an off_t.

       ESPIPE fd is associated with a pipe, socket, or FIFO.


       SVr4, 4.3BSD, POSIX.1-2001.


       This document’s use of whence is incorrect English, but maintained  for
       historical reasons.

       Some  devices are incapable of seeking and POSIX does not specify which
       devices must support lseek().

       On Linux, using lseek() on a tty 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

       SVr1-3 returns long instead of off_t, BSD returns int.

       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.


       dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)


       This  page  is  part of release 3.24 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at