flockfile(3)


NAME

   flockfile, ftrylockfile, funlockfile - lock FILE for stdio

SYNOPSIS

   #include <stdio.h>

   void flockfile(FILE *filehandle);
   int ftrylockfile(FILE *filehandle);
   void funlockfile(FILE *filehandle);

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

   All functions shown above:
       _POSIX_C_SOURCE
           || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION

   The  stdio functions are thread-safe.  This is achieved by assigning to
   each FILE object a lockcount and  (if  the  lockcount  is  nonzero)  an
   owning  thread.   For each library call, these functions wait until the
   FILE object is no longer locked by a different thread, then lock it, do
   the requested I/O, and unlock the object again.

   (Note:  this  locking  has  nothing to do with the file locking done by
   functions like flock(2) and lockf(3).)

   All this is invisible to the C-programmer, but there may be two reasons
   to  wish for more detailed control.  On the one hand, maybe a series of
   I/O  actions  by  one  thread  belongs  together,  and  should  not  be
   interrupted  by the I/O of some other thread.  On the other hand, maybe
   the locking overhead should be avoided for greater efficiency.

   To this end, a thread can explicitly lock the FILE object, then do  its
   series  of  I/O actions, then unlock.  This prevents other threads from
   coming in between.  If the reason for doing this was to achieve greater
   efficiency,  one does the I/O with the nonlocking versions of the stdio
   functions:  with  getc_unlocked(3)  and  putc_unlocked(3)  instead   of
   getc(3) and putc(3).

   The  flockfile()  function waits for *filehandle to be no longer locked
   by  a  different  thread,  then  makes  the  current  thread  owner  of
   *filehandle, and increments the lockcount.

   The funlockfile() function decrements the lock count.

   The  ftrylockfile()  function  is a nonblocking version of flockfile().
   It does nothing in case some other  thread  owns  *filehandle,  and  it
   obtains ownership and increments the lockcount otherwise.

RETURN VALUE

   The  ftrylockfile()  function  returns  zero  for success (the lock was
   obtained), and nonzero for failure.

ERRORS

   None.

ATTRIBUTES

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

   
   Interface                      Attribute      Value   
   
   flockfile(),  ftrylockfile(),  Thread safety  MT-Safe 
   funlockfile()                                         
   

CONFORMING TO

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

AVAILABILITY

   These functions  are  available  when  _POSIX_THREAD_SAFE_FUNCTIONS  is
   defined.

SEE ALSO

   unlocked_stdio(3)

COLOPHON

   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
   https://www.kernel.org/doc/man-pages/.

                              2016-03-15                      FLOCKFILE(3)





Opportunity


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.





Education


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.