NAME
getc_unlocked, getchar_unlocked, putc_unlocked, putchar_unlocked -
stdio with explicit client locking
SYNOPSIS
#include <stdio.h>
int getc_unlocked(FILE *stream);
int getchar_unlocked(void);
int putc_unlocked(int c, FILE *stream);
int putchar_unlocked(int c);
DESCRIPTION
Versions of the functions getc(), getchar(), putc(), and putchar()
respectively named getc_unlocked(), getchar_unlocked(),
putc_unlocked(), and putchar_unlocked() shall be provided which are
functionally equivalent to the original versions, with the exception
that they are not required to be implemented in a thread-safe manner.
They may only safely be used within a scope protected by flockfile()
(or ftrylockfile()) and funlockfile(). These functions may safely be
used in a multi-threaded program if and only if they are called while
the invoking thread owns the ( FILE *) object, as is the case after a
successful call to the flockfile() or ftrylockfile() functions.
RETURN VALUE
See getc() , getchar() , putc() , and putchar() .
ERRORS
See getc() , getchar() , putc() , and putchar() .
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
Since they may be implemented as macros, getc_unlocked() and
putc_unlocked() may treat incorrectly a stream argument with side
effects. In particular, getc_unlocked(*f++) and putc_unlocked(*f++) do
not necessarily work as expected. Therefore, use of these functions in
such situations should be preceded by the following statement as
appropriate:
#undef getc_unlocked
#undef putc_unlocked
RATIONALE
Some I/O functions are typically implemented as macros for performance
reasons (for example, putc() and getc()). For safety, they need to be
synchronized, but it is often too expensive to synchronize on every
character. Nevertheless, it was felt that the safety concerns were more
important; consequently, the getc(), getchar(), putc(), and putchar()
functions are required to be thread-safe. However, unlocked versions
are also provided with names that clearly indicate the unsafe nature of
their operation but can be used to exploit their higher performance.
These unlocked versions can be safely used only within explicitly
locked program regions, using exported locking primitives. In
particular, a sequence such as:
flockfile(fileptr);
putc_unlocked(’1’, fileptr);
putc_unlocked(’\n’, fileptr);
fprintf(fileptr, "Line 2\n");
funlockfile(fileptr);
is permissible, and results in the text sequence:
1
Line 2
being printed without being interspersed with output from other
threads.
It would be wrong to have the standard names such as getc(), putc(),
and so on, map to the "faster, but unsafe" rather than the "slower, but
safe’’ versions. In either case, you would still want to inspect all
uses of getc(), putc(), and so on, by hand when converting existing
code. Choosing the safe bindings as the default, at least, results in
correct code and maintains the "atomicity at the function" invariant.
To do otherwise would introduce gratuitous synchronization errors into
converted code. Other routines that modify the stdio ( FILE *)
structures or buffers are also safely synchronized.
Note that there is no need for functions of the form getc_locked(),
putc_locked(), and so on, since this is the functionality of getc(),
putc(), et al. It would be inappropriate to use a feature test macro to
switch a macro definition of getc() between getc_locked() and
getc_unlocked(), since the ISO C standard requires an actual function
to exist, a function whose behavior could not be changed by the feature
test macro. Also, providing both the xxx_locked() and xxx_unlocked()
forms leads to the confusion of whether the suffix describes the
behavior of the function or the circumstances under which it should be
used.
Three additional routines, flockfile(), ftrylockfile(), and
funlockfile() (which may be macros), are provided to allow the user to
delineate a sequence of I/O statements that are executed synchronously.
The ungetc() function is infrequently called relative to the other
functions/macros so no unlocked variation is needed.
FUTURE DIRECTIONS
None.
SEE ALSO
getc() , getchar() , putc() , putchar() , the Base Definitions volume
of IEEE Std 1003.1-2001, <stdio.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .