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 versions <= 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 fu-
       ture  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 relationship between lockf() and fcntl(2) locks unspecified.
       A  portable application should probably avoid mixing calls to these in-

       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 ear-
              lier  locked  section, both are merged.  File locks are released
              as soon as the process holding the locks closes  some  file  de-
              scriptor  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 appropriately.

              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 de-
              livery 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  at-

       |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 direc-
       tory Documentation/filesystems (on older kernels, these files  are  di-
       rectly  under the Documentation directory, and mandatory-locking.txt is
       called mandatory.txt)

       This page is part of release 5.05 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                               2019-03-06                          LOCKF(3)
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