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Technical Reference: Base Operating System and Extensions, Volume 1

flockfile, ftrylockfile, funlockfile Subroutine


Provides for explicit application-level locking of stdio (FILE*) objects.


Standard Library (libc.a)


#include <stdio.h>
void flockfile (FILE * file)
int ftrylockfile (FILE * file)
void funlockfile (FILE * file)


The flockfile, ftrylockfile and funlockfile functions provide for explicit application-level locking of stdio (FILE*) objects. These functions can be used by a thread to delineate a sequence of I/O statements that are to be executed as a unit.

The flockfile function is used by a thread to acquire ownership of a (FILE*) object.

The ftrylockfile function is used by a thread to acquire ownership of a (FILE*) object if the object is available; ftrylockfile is a non-blocking version of flockfile.

The funlockfile function is used to relinquish the ownership granted to the thread. The behavior is undefined if a thread other than the current owner calls the funlockfile function.

Logically, there is a lock count associated with each (FILE*) object. This count is implicitly initialised to zero when the (FILE*) object is created. The (FILE*) object is unlocked when the count is zero. When the count is positive, a single thread owns the (FILE*) object. When the flockfile function is called, if the count is zero or if the count is positive and the caller owns the (FILE*) object, the count is incremented. Otherwise, the calling thread is suspended, waiting for the count to return to zero. Each call to funlockfile decrements the count. This allows matching calls to flockfile (or successful calls to ftrylockfile ) and funlockfile to be nested.

All functions that reference (FILE*) objects behave as if they use flockfile and funlockfile internally to obtain ownership of these (FILE*) objects.

Return Values

None for flockfile and funlockfile. The function ftrylock returns zero for success and non-zero to indicate that the lock cannot be acquired.

Implementation Specifics

These subroutines are part of Base Operating System (BOS) subroutines.

Realtime applications may encounter priority inversion when using FILE locks. The problem occurs when a high priority thread locks a file that is about to be unlocked by a low priority thread, but the low priority thread is preempted by a medium priority thread. This scenario leads to priority inversion; a high priority thread is blocked by lower priority threads for an unlimited period of time. During system design, realtime programmers must take into account the possibility of this kind of priority inversion. They can deal with it in a number of 7434 ways, such as by having critical sections that are guarded by file locks execute at a high priority, so that a thread cannot be preempted while executing in its critical section.

Related Information

The getc_unlocked, getchar_unlocked, putc_unlocked, putchar_unlocked (getc_unlocked, getchar_unlocked, putc_unlocked, putchar_unlocked Subroutines) subroutine.

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