flockfile, ftrylockfile, funlockfile — lock FILE for stdio
#include <stdio.h>
void
flockfile( |
FILE * | filehandle) ; |
int
ftrylockfile( |
FILE * | filehandle) ; |
void
funlockfile( |
FILE * | filehandle) ; |
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Note | |
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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.
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Note |
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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 non-locking 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 non-blocking version of flockfile
(). It does nothing in case some
other thread owns *filehandle
, and it obtains
ownership and increments the lockcount otherwise.
The ftrylockfile
() function
returns zero for success (the lock was obtained), and nonzero
for failure.
These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS
is defined.
They are in libc since libc 5.1.1 and in glibc since glibc
2.0.