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    <p>The last one--<code>realloc(ptr, 0)</code>--comes close. It will free any allocated block and replace it with a minimal allocation (says my Mac OS X 10.5 manpage). Check your local manpage to see what it does on your system.</p> <p>That is, if <code>ptr</code> pointed at a substantial object, you'll get back most of its memory.</p> <hr> <p>The man page on Debian Lenny agrees with <a href="https://stackoverflow.com/questions/750060/is-there-an-alternative-way-to-use-the-free-function-in-c/750068#750068">Mitch</a> and <a href="https://stackoverflow.com/questions/750060/is-there-an-alternative-way-to-use-the-free-function-in-c/750075#750075">Jonathan</a>...does BSD really diverge from Linux on this?</p> <hr> <p>From the offending man page:</p> <blockquote> <p>The realloc() function tries to change the size of the allocation pointed to by ptr to size, and returns ptr. [...] If size is zero and ptr is not NULL, a new, minimum sized object is allocated and the original object is freed.</p> </blockquote> <hr> <p>The linux and solaris man pages are very clean, and the '89 standard: <code>realloc(ptr,0)</code> works like <code>free(ptr)</code>. The Mac OS manpage above, and the standard as quoted by Jonathan are less clear but seems to leave room to break the equivalence.</p> <p>I've been wondering <em>why</em> the difference: the "act like free" interpretation seems very natural to me. Both of the implementations I have access to include <em>some</em> environment variable driven tunablity, but the BSD version accepts many more options Some examples:</p> <blockquote> <pre><code> MallocGuardEdges If set, add a guard page before and after each large block. </code></pre> </blockquote> <pre><code> MallocDoNotProtectPrelude If set, do not add a guard page before large blocks, even if the MallocGuardEdges envi- ronment variable is set. MallocDoNotProtectPostlude If set, do not add a guard page after large blocks, even if the MallocGuardEdges envi- ronment variable is set. </code></pre> <p>and</p> <blockquote> <pre><code> MallocPreScribble If set, fill memory that has been allocated with 0xaa bytes. This increases the likeli- hood that a program making assumptions about the contents of freshly allocated memory will fail. </code></pre> </blockquote> <pre><code> MallocScribble If set, fill memory that has been deallo- cated with 0x55 bytes. This increases the likelihood that a program will fail due to accessing memory that is no longer allo- cated. </code></pre> <p>Possibly the "minimum sized object" is nothing (i.e. equivalent to <code>free</code>) in the normal modes, but <em>something</em> with some of the guards in place. Take that for what it's worth.</p>
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    1. POIs there an alternative way to free dynamically allocated memory in C - not using the free() function?
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    1. COWhich version of MacOS X? My (getting dated) 10.4.11 version does not say anything specific about zero size realloc(), though the statement "realloc() returns a NULL pointer if there is an error, and the allocation pointed to by ptr is still valid" implies what you sugggest. ...continued...
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      1. USJonathan Leffler
    2. COThe standard, I believe , leaves it either implementation defined or undefined whether a null pointer or a pointer to some valid location of size zero (that cannot, therefore, be dereferenced safely) is returned.
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      1. USJonathan Leffler
    3. CO@Jonathon: That makes sense to me, but I found this rather weird wording in man page and now I'm confused.
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