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getc_unlocked.3p (6412B)

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.TH GETC_UNLOCKED "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
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.
.\"
.SH NAME
getc_unlocked,
getchar_unlocked,
putc_unlocked,
putchar_unlocked
\(em stdio with explicit client locking
.SH SYNOPSIS
.LP
.nf
#include <stdio.h>
.P
int getc_unlocked(FILE *\fIstream\fP);
int getchar_unlocked(void);
int putc_unlocked(int \fIc\fP, FILE *\fIstream\fP);
int putchar_unlocked(int \fIc\fP);
.fi
.SH DESCRIPTION
Versions of the functions
\fIgetc\fR(),
\fIgetchar\fR(),
\fIputc\fR(),
and
\fIputchar\fR()
respectively named
\fIgetc_unlocked\fR(),
\fIgetchar_unlocked\fR(),
\fIputc_unlocked\fR(),
and
\fIputchar_unlocked\fR()
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
\fIflockfile\fR()
(or
\fIftrylockfile\fR())
and
\fIfunlockfile\fR().
These functions can safely be used in a multi-threaded program if and
only if they are called while the invoking thread owns the (\c
.BR "FILE *" )
object, as is the case after a successful call to the
\fIflockfile\fR()
or
\fIftrylockfile\fR()
functions.
.P
If
\fIgetc_unlocked\fR()
or
\fIputc_unlocked\fR()
are implemented as macros they may evaluate
.IR stream
more than once, so the
.IR stream
argument should never be an expression with side-effects.
.SH "RETURN VALUE"
See
.IR "\fIgetc\fR\^(\|)",
.IR "\fIgetchar\fR\^(\|)",
.IR "\fIputc\fR\^(\|)",
and
.IR "\fIputchar\fR\^(\|)".
.SH ERRORS
See
.IR "\fIgetc\fR\^(\|)",
.IR "\fIgetchar\fR\^(\|)",
.IR "\fIputc\fR\^(\|)",
and
.IR "\fIputchar\fR\^(\|)".
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
Since they may be implemented as macros,
\fIgetc_unlocked\fR()
and
\fIputc_unlocked\fR()
may treat incorrectly a
.IR stream
argument with side-effects. In particular, \fIgetc_unlocked\fP(*f++)
and \fIputc_unlocked\fP(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:
.sp
.RS 4
.nf
#undef getc_unlocked
#undef putc_unlocked
.fi
.P
.RE
.SH RATIONALE
Some I/O functions are typically implemented as macros for performance
reasons (for example,
\fIputc\fR()
and
\fIgetc\fR()).
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\fR(),
\fIgetchar\fR(),
\fIputc\fR(),
and
\fIputchar\fR()
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 4
.nf
flockfile(fileptr);
putc_unlocked(\(aq1\(aq, fileptr);
putc_unlocked(\(aq\en\(aq, fileptr);
fprintf(fileptr, "Line 2\en");
funlockfile(fileptr);
.fi
.P
.RE
.br
.P
is permissible, and results in the text sequence:
.sp
.RS 4
.nf
1
Line 2
.fi
.P
.RE
.P
being printed without being interspersed with output from other
threads.
.P
It would be wrong to have the standard names such as
\fIgetc\fR(),
\fIputc\fR(),
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\fR(),
\fIputc\fR(),
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
.IR stdio
(\c
.BR "FILE *" )
structures or buffers are also safely synchronized.
.P
Note that there is no need for functions of the form
\fIgetc_locked\fR(),
\fIputc_locked\fR(),
and so on, since this is the functionality of
\fIgetc\fR(),
\fIputc\fR(),
.IR "et al" .
It would be inappropriate to use a feature test macro to
switch a macro definition of
\fIgetc\fR()
between
\fIgetc_locked\fR()
and
\fIgetc_unlocked\fR(),
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\fR()
and
\fIxxx_unlocked\fR()
forms leads to the confusion of whether the suffix describes the
behavior of the function or the circumstances under which it should be
used.
.P
Three additional routines,
\fIflockfile\fR(),
\fIftrylockfile\fR(),
and
\fIfunlockfile\fR()
(which may be macros), are provided to allow the user to delineate a
sequence of I/O statements that are executed synchronously.
.P
The
\fIungetc\fR()
function is infrequently called relative to the other functions/macros
so no unlocked variation is needed.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.5" ", " "Standard I/O Streams",
.IR "\fIflockfile\fR\^(\|)",
.IR "\fIgetc\fR\^(\|)",
.IR "\fIgetchar\fR\^(\|)",
.IR "\fIputc\fR\^(\|)",
.IR "\fIputchar\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<stdio.h>\fP"
.\"
.SH COPYRIGHT
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 .
.PP
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 .