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LOCKF(3)                                                                Linux Programmer's Manual                                                               LOCKF(3)

NAME
       lockf - apply, test or remove a POSIX lock on an open file

SYNOPSIS
       #include <unistd.h>

       int lockf(int fd, int cmd, off_t len);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       lockf():
           _XOPEN_SOURCE >= 500
               || /* Glibc since 2.19: */ _DEFAULT_SOURCE
               || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION
       Apply,  test,  or  remove a POSIX lock on a section of an open file.  The file is specified by fd, a file descriptor open for writing, the action by cmd, and the
       section consists of byte positions pos..pos+len-1 if len is positive, and pos-len..pos-1 if len is negative, where pos is the current file position, and  if  len
       is  zero,  the  section extends from the current file position to infinity, encompassing the present and future end-of-file positions.  In all cases, the section
       may extend past current end-of-file.

       On Linux, lockf() is just an interface on top of fcntl(2) locking.  Many other systems implement lockf() in this way, but note that POSIX.1 leaves the  relation‐
       ship between lockf() and fcntl(2) locks unspecified.  A portable application should probably avoid mixing calls to these interfaces.

       Valid operations are given below:

       F_LOCK Set  an  exclusive lock on the specified section of the file.  If (part of) this section is already locked, the call blocks until the previous lock is re‐
              leased.  If this section overlaps an earlier locked section, both are merged.  File locks are released as soon as the process  holding  the  locks  closes
              some file descriptor for the file.  A child process does not inherit these locks.

       F_TLOCK
              Same as F_LOCK but the call never blocks and returns an error instead if the file is already locked.

       F_ULOCK
              Unlock the indicated section of the file.  This may cause a locked section to be split into two locked sections.

       F_TEST Test  the lock: return 0 if the specified section is unlocked or locked by this process; return -1, set errno to EAGAIN (EACCES on some other systems), if
              another process holds a lock.

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and errno is set to indicate the error.

ERRORS
       EACCES or EAGAIN
              The file is locked and F_TLOCK or F_TEST was specified, or the operation is prohibited because the file has been memory-mapped by another process.

       EBADF  fd is not an open file descriptor; or cmd is F_LOCK or F_TLOCK and fd is not a writable file descriptor.

       EDEADLK
              The command was F_LOCK and this lock operation would cause a deadlock.

       EINTR  While waiting to acquire a lock, the call was interrupted by delivery of a signal caught by a handler; see signal(7).

       EINVAL An invalid operation was specified in cmd.

       ENOLCK Too many segment locks open, lock table is full.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚Interface                                                                                                                             β”‚ Attribute     β”‚ Value   β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚lockf()                                                                                                                               β”‚ Thread safety β”‚ MT-Safe β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, SVr4.

SEE ALSO
       fcntl(2), flock(2)

       locks.txt and mandatory-locking.txt in the Linux kernel source directory Documentation/filesystems (on older kernels, these files are directly under the Documen‐
       tation directory, and mandatory-locking.txt is called mandatory.txt)

GNU                                                                            2021-03-22                                                                       LOCKF(3)