Editing C dynamic memory allocation (section)

==Type safety==
<code>malloc</code> returns a [[void pointer]] (<code>void *</code>), which indicates that it is a pointer to a region of unknown data type. The use of casting is required in C++ due to the strong type system, whereas this is not the case in C. One may "cast" (see [[type conversion]]) this pointer to a specific type:

<syntaxhighlight lang="c">
int *ptr, *ptr2;
ptr = malloc(10 * sizeof(*ptr)); /* without a cast */
ptr2 = (int *)malloc(10 * sizeof(*ptr)); /* with a cast */
</syntaxhighlight>

There are advantages and disadvantages to performing such a cast.

===Advantages to casting===
* Including the cast may allow a C program or function to compile as C++.
* The cast allows for [[ANSI C|pre-1989 versions]] of <code>malloc</code> that originally returned a <code>char *</code>.<ref name="Cprog_malloc">{{cite web |url=http://faq.cprogramming.com/cgi-bin/smartfaq.cgi?id=1043284351&answer=1047673478 |title=Casting malloc |publisher=Cprogramming.com |access-date=9 March 2007 }}</ref>
* Casting can help the developer identify inconsistencies in type sizing should the destination pointer type change, particularly if the pointer is declared far from the <code>malloc()</code> call (although modern compilers and static analysers can warn on such behaviour without requiring the cast<ref>{{cite web|url=http://clang.llvm.org/doxygen/MallocSizeofChecker_8cpp_source.html|title=clang: lib/StaticAnalyzer/Checkers/MallocSizeofChecker.cpp Source File|website=clang.llvm.org|access-date=1 April 2018}}</ref>).

===Disadvantages to casting===
* Under the C standard, the cast is redundant.
* Adding the cast may mask failure to include the header <code>stdlib.h</code>, in which the [[function prototype]] for <code>malloc</code> is found.<ref name="Cprog_malloc" /><ref>{{cite web |url=http://www.c-faq.com/malloc/mallocnocast.html |title=comp.lang.c FAQ list · Question 7.7b |publisher=C-FAQ |access-date=9 March 2007 }}</ref> In the absence of a prototype for <code>malloc</code>, the C90 standard requires that the C compiler assume <code>malloc</code> returns an <code>int</code>. If there is no cast, C90 requires a diagnostic when this integer is assigned to the pointer; however, with the cast, this diagnostic would not be produced, hiding a bug.  On certain architectures and data models (such as LP64 on 64-bit systems, where <code>long</code> and pointers are 64-bit and <code>int</code> is 32-bit), this error can actually result in undefined behaviour, as the implicitly declared <code>malloc</code> returns a 32-bit value whereas the actually defined function returns a 64-bit value. Depending on calling conventions and memory layout, this may result in [[stack smashing]]. This issue is less likely to go unnoticed in modern compilers, as C99 does not permit implicit declarations, so the compiler must produce a diagnostic even if it does assume <code>int</code> return.
* If the type of the pointer is changed at its declaration, one may also need to change all lines where <code>malloc</code> is called and cast.