Repository : http://git.fedorahosted.org/git/?p=secure-coding.git On branch : master commit fa41f38b864810921f947e08904a299928e80223 Author: Florian Weimer <fweimer(a)redhat.com&gt; Date: Mon Aug 26 16:15:53 2013 +0200 C: Add material on global variables defensive-coding/en-US/C-Language.xml | 46 ++++++++++++++++++++ ...ntials-Close.xml => C-Globals-String_Array.xml} | 7 +++- defensive-coding/src/C-Globals.c | 17 +++++++ defensive-coding/src/src.mk | 1 + 4 files changed, 70 insertions(+), 1 deletions(-) diff --git a/defensive-coding/en-US/C-Language.xml b/defensive-coding/en-US/C-Language.xml index 83f6da0..b039ed2 100644 --- a/defensive-coding/en-US/C-Language.xml +++ b/defensive-coding/en-US/C-Language.xml @@ -147,4 +147,50 @@ integer overflow. </para> </section> + + <section id="sect-Defensive_Coding-C-Globals"> + <title>Global variables</title> + <para> + Global variables should be avoided because they usually lead to + thread safety hazards. In any case, they should be declared + <literal>static</literal>, so that access is restricted to a + single translation unit. + </para> + <para> + Global constants are not a problem, but declaring them can be + tricky. <xref linkend="ex-Defensive_Coding-C-Globals-String_Array"/> + shows how to declare a constant array of constant strings. + The second <literal>const</literal> is needed to make the + array constant, and not just the strings. It must be placed + after the <literal>*</literal>, and not before it. + </para> + <example id="ex-Defensive_Coding-C-Globals-String_Array"> + <title>Declaring a constant array of constant strings</title> + <xi:include href="snippets/C-Globals-String_Array.xml" + xmlns:xi="http://www.w3.org/2001/XInclude" /> + </example> + <para> + Sometimes, static variables local to functions are used as a + replacement for proper memory management. Unlike non-static + local variables, it is possible to return a pointer to static + local variables to the caller. But such variables are + well-hidden, but effectively global (just as static variables at + file scope). It is difficult to add thread safety afterwards if + such interfaces are used. Merely dropping the + <literal>static</literal> keyword in such cases leads to + undefined behavior. + </para> + <para> + Another source for static local variables is a desire to reduce + stack space usage on embedded platforms, where the stack may + span only a few hundred bytes. If this is the only reason why + the <literal>static</literal> keyword is used, it can just be + dropped, unless the object is very large (larger than + 128 kilobytes on 32 bit platforms). In the latter case, it is + recommended to allocate the object using + <literal>malloc</literal>, to obtain proper array checking, for + the same reasons outlined in <xref + linkend="sect-Defensive_Coding-C-Allocators-alloca"/>. + </para> + </section> </section> diff --git a/defensive-coding/en-US/snippets/Features-TLS-GNUTLS-Credentials-Close.xml b/defensive-coding/en-US/snippets/C-Globals-String_Array.xml similarity index 75% copy from defensive-coding/en-US/snippets/Features-TLS-GNUTLS-Credentials-Close.xml copy to defensive-coding/en-US/snippets/C-Globals-String_Array.xml index 8c28b0f..2f05b7d 100644 --- a/defensive-coding/en-US/snippets/Features-TLS-GNUTLS-Credentials-Close.xml +++ b/defensive-coding/en-US/snippets/C-Globals-String_Array.xml @@ -3,5 +3,10 @@ ]> <!-- Automatically generated file. Do not edit. --> <programlisting language="C"> -gnutls_certificate_free_credentials(cred); +static const char *const string_list[] = { + "first", + "second", + "third", + NULL +}; </programlisting> diff --git a/defensive-coding/src/C-Globals.c b/defensive-coding/src/C-Globals.c new file mode 100644 index 0000000..75b33b4 --- /dev/null +++ b/defensive-coding/src/C-Globals.c @@ -0,0 +1,17 @@ +#include <stddef.h> + +//+ C Globals-String_Array +static const char *const string_list[] = { + "first", + "second", + "third", + NULL +}; +//- + +// Silence compiler warning +const char *const * +get_string_list() +{ + return string_list; +} diff --git a/defensive-coding/src/src.mk b/defensive-coding/src/src.mk index 219e70b..d47fc09 100644 --- a/defensive-coding/src/src.mk +++ b/defensive-coding/src/src.mk @@ -12,6 +12,7 @@ LDFLAGS = -g compile_only += C-Pointers-remaining compile_only += C-Arithmetic-add compile_only += C-Arithmetic-mult +compile_only += C-Globals compile_only += Java-JNI-Pointers CFLAGS_Java-JNI-Pointers = \