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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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 fully
thread-safe manner. They shall be thread-safe when used within a
scope protected by flockfile() (or ftrylockfile()) and
funlockfile(). These functions can 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.1‐2017, stdio.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 2017 GETC_UNLOCKED(3P)
Pages that refer to this page: stdio.h(0p), flockfile(3p), getchar_unlocked(3p), putchar_unlocked(3p), putc_unlocked(3p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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