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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
.TH "GETC_UNLOCKED" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" getc_unlocked
.SH NAME
getc_unlocked, getchar_unlocked, putc_unlocked, putchar_unlocked \-
stdio with explicit client locking
.SH SYNOPSIS
.LP
\fB#include <stdio.h>
.br
.sp
int getc_unlocked(FILE *\fP\fIstream\fP\fB);
.br
int getchar_unlocked(void);
.br
int putc_unlocked(int\fP \fIc\fP\fB, FILE *\fP\fIstream\fP\fB);
.br
int putchar_unlocked(int\fP \fIc\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
Versions of the functions \fIgetc\fP(), \fIgetchar\fP(), \fIputc\fP(),
and \fIputchar\fP() respectively named \fIgetc_unlocked\fP(), \fIgetchar_unlocked\fP(),
\fIputc_unlocked\fP(), and \fIputchar_unlocked\fP() 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 \fIflockfile\fP() (or \fIftrylockfile\fP()) and
\fIfunlockfile\fP().
These functions may safely be used in a multi-threaded program if
and only if they are called while the invoking thread owns the (
\fBFILE *\fP) object, as is the case after a successful call to the
\fIflockfile\fP()
or \fIftrylockfile\fP() functions.
.SH RETURN VALUE
.LP
See \fIgetc\fP() , \fIgetchar\fP() , \fIputc\fP()
, and \fIputchar\fP() .
.SH ERRORS
.LP
See \fIgetc\fP() , \fIgetchar\fP() , \fIputc\fP()
, and \fIputchar\fP() .
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
Since they may be implemented as macros, \fIgetc_unlocked\fP() and
\fIputc_unlocked\fP() may treat incorrectly a \fIstream\fP
argument with side effects. In particular, \fIgetc_unlocked\fP(*f++)
and \fIputc_unlocked\fP(*f++) do not necessarily work as
expected. Therefore, use of these functions in such situations should
be preceded by the following statement as appropriate:
.sp
.RS
.nf
\fB#undef getc_unlocked
#undef putc_unlocked
\fP
.fi
.RE
.SH RATIONALE
.LP
Some I/O functions are typically implemented as macros for performance
reasons (for example, \fIputc\fP() and \fIgetc\fP()). 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 \fIgetc\fP(), \fIgetchar\fP(),
\fIputc\fP(), and \fIputchar\fP() 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:
.sp
.RS
.nf
\fBflockfile(fileptr);
putc_unlocked('1', fileptr);
putc_unlocked('\\n', fileptr);
fprintf(fileptr, "Line 2\\n");
funlockfile(fileptr);
\fP
.fi
.RE
.LP
is permissible, and results in the text sequence:
.sp
.RS
.nf
\fB1
Line 2
\fP
.fi
.RE
.LP
being printed without being interspersed with output from other threads.
.LP
It would be wrong to have the standard names such as \fIgetc\fP(),
\fIputc\fP(), 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 \fIgetc\fP(), \fIputc\fP(), 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 \fIstdio\fP ( \fBFILE *\fP)
structures or buffers are also safely synchronized.
.LP
Note that there is no need for functions of the form \fIgetc_locked\fP(),
\fIputc_locked\fP(), and so on, since this is the
functionality of \fIgetc\fP(), \fIputc\fP(), \fIet
al\fP. It would be inappropriate to use a feature test macro to switch
a macro definition of \fIgetc\fP() between \fIgetc_locked\fP() and
\fIgetc_unlocked\fP(), 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 \fIxxx_locked\fP() and \fIxxx_unlocked\fP() forms leads to
the confusion of whether the suffix describes the behavior of
the function or the circumstances under which it should be used.
.LP
Three additional routines, \fIflockfile\fP(), \fIftrylockfile\fP(),
and \fIfunlockfile\fP()
(which may be macros), are provided to allow the user to delineate
a sequence of I/O statements that are executed
synchronously.
.LP
The \fIungetc\fP() function is infrequently called relative to the
other
functions/macros so no unlocked variation is needed.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIgetc\fP() , \fIgetchar\fP() , \fIputc\fP() , \fIputchar\fP() ,
the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<stdio.h>\fP
.SH 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 .
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