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

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

       #include <unistd.h>

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

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

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

       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

       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 relationship between lockf() and fcntl(2) locks unspecified.  A
       portable application should probably avoid mixing calls to these

       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 released.  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

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

              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.

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

              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.

              The command was F_LOCK and this lock operation would cause a

       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.

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

       │Interface                                     Attribute     Value   │
       │lockf()                                       │ Thread safety │ MT-Safe │

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

       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 Documentation directory, and mandatory-locking.txt is called

       This page is part of release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

GNU                                2021-03-22                           LOCKF(3)