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GETC_UNLOCKED(3P) POSIX Programmer's Manual GETC_UNLOCKED(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
getc_unlocked, getchar_unlocked, putc_unlocked, putchar_unlocked —
stdio with explicit client locking
#include <stdio.h>
int getc_unlocked(FILE *stream);
int getchar_unlocked(void);
int putc_unlocked(int c, FILE *stream);
int putchar_unlocked(int c);
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.
If getc_unlocked() or putc_unlocked() are implemented as macros they
may evaluate stream more than once, so the stream argument should
never be an expression with side-effects.
See getc(3p), getchar(3p), putc(3p), and putchar(3p).
See getc(3p), getchar(3p), putc(3p), and putchar(3p).
The following sections are informative.
None.
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(c,*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
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.
None.
Section 2.5, Standard I/O Streams, flockfile(3p), getc(3p),
getchar(3p), putc(3p), putchar(3p)
The Base Definitions volume of POSIX.1‐2008, stdio.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 GETC_UNLOCKED(3P)
Pages that refer to this page: stdio.h(0p), flockfile(3p), getchar_unlocked(3p), putchar_unlocked(3p), putc_unlocked(3p)