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

   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

   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

   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 mandatory.txt)


   This  page  is  part of release 4.09 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


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.