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       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 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 relationship 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 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 locks.

              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 deadlock.

       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).

       │InterfaceAttributeValue   │
       │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 mandatory.txt)


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