📄 libc-thread-safety.html
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>Thread safety</TITLE
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><H1
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><A
NAME="LIBC-THREAD-SAFETY">Thread safety</H1
><P
>The ISO C library has configuration options that control thread
safety, i.e. working behavior if multiple threads call the same
function at the same time.</P
><P
>The following functionality has to be configured correctly,
or used carefully in a multi-threaded environment:</P
><P
></P
><UL
><LI
><P
><TT
CLASS="FUNCTION"
>mblen()</TT
></P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
>mbtowc()</TT
></P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
>wctomb()</TT
></P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
>printf()</TT
>
(and all standard I/O functions except for
<TT
CLASS="FUNCTION"
>sprintf()</TT
>
and
<TT
CLASS="FUNCTION"
>sscanf()</TT
></P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
>strtok()</TT
></P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
>rand()</TT
>
and
<TT
CLASS="FUNCTION"
>srand()</TT
></P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
>signal()</TT
>
and
<TT
CLASS="FUNCTION"
>raise()</TT
></P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
>asctime()</TT
>
,
<TT
CLASS="FUNCTION"
>ctime()</TT
>
,
<TT
CLASS="FUNCTION"
>gmtime()</TT
>
, and
<TT
CLASS="FUNCTION"
>localtime()</TT
></P
></LI
><LI
><P
>the
<TT
CLASS="FUNCTION"
>errno</TT
>
variable</P
></LI
><LI
><P
>the
<TT
CLASS="FUNCTION"
>environ</TT
>
variable</P
></LI
><LI
><P
>date and time settings</P
></LI
></UL
><P
>In some cases, to make <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
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></SPAN
> development
easier, functions are provided (as specified by POSIX 1003.1) that define
re-entrant alternatives, i.e. <TT
CLASS="FUNCTION"
>rand_r()</TT
>, <TT
CLASS="FUNCTION"
>strtok_r()</TT
>, <TT
CLASS="FUNCTION"
>asctime_r()</TT
>, <TT
CLASS="FUNCTION"
>ctime_r()</TT
>, <TT
CLASS="FUNCTION"
>gmtime_r()</TT
>,
and <TT
CLASS="FUNCTION"
>localtime_r()</TT
>. In other cases,
configuration options are provided that control either locking of functions
or their shared data, such as with standard I/O streams,
or by using per-thread data, such as with the <TT
CLASS="FUNCTION"
>errno</TT
> variable.</P
><P
>In some other cases, like the setting of date and time, no
re-entrant or thread-safe alternative or configuration is provided
as it is simply not a worthwhile addition (date and time should
rarely need to be set.)</P
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