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    <p>Building on <a href="https://stackoverflow.com/users/5353/sylvain-defresne">Sylvain</a>'s <a href="https://stackoverflow.com/a/4899487/15168">answer</a>, here is a simple implementation with both <code>asprintf()</code> and <code>vasprintf()</code> because where you need one, you usually end up needing the other too. And, given the <code>va_copy()</code> macro from C99, it is easy to implement <code>asprintf()</code> in terms of <code>vasprintf()</code>. Indeed, when writing varargs functions, it is very often helpful to have them in pairs, one with the ellipsis notation and one with the <code>va_list</code> argument in place of the ellipsis, and you trivially implement the former in terms of the latter.</p> <p>This leads to the code:</p> <pre><code>int vasprintf(char **ret, const char *format, va_list args) { va_list copy; va_copy(copy, args); /* Make sure it is determinate, despite manuals indicating otherwise */ *ret = NULL; int count = vsnprintf(NULL, 0, format, args); if (count &gt;= 0) { char *buffer = malloc(count + 1); if (buffer == NULL) count = -1; else if ((count = vsnprintf(buffer, count + 1, format, copy)) &lt; 0) free(buffer); else *ret = buffer; } va_end(copy); // Each va_start() or va_copy() needs a va_end() return count; } int asprintf(char **ret, const char *format, ...) { va_list args; va_start(args, format); int count = vasprintf(ret, format, args); va_end(args); return(count); } </code></pre> <p>The tricky part of using these functions in a system where they are not provided is deciding where the functions should be declared. Ideally, they'd be in <code>&lt;stdio.h&gt;</code>, but then you wouldn't need to write them. So, you have to have some other header which includes <code>&lt;stdio.h&gt;</code> but declares these functions if they are not declared in <code>&lt;stdio.h&gt;</code>. And, ideally, the code should semi-automatically detect this. Maybe the header is <code>"missing.h"</code>, and contains (in part):</p> <pre><code>#ifdef HAVE_CONFIG_H #include "config.h" #endif /* HAVE_CONFIG_H */ #include &lt;stdio.h&gt; #include &lt;stdarg.h&gt; #ifndef HAVE_ASPRINTF extern int asprintf(char **ret, const char *format, ...); extern int vasprintf(char **ret, const char *format, va_list args); #endif /* HAVE_ASPRINTF */ </code></pre> <p>Also, note that this man page for <a href="http://linux.die.net/man/3/asprintf" rel="nofollow noreferrer">asprintf()</a> says that the return value in the pointer is indeterminate in case of error. Other <a href="http://www.manpagez.com/man/3/asprintf/" rel="nofollow noreferrer">man pages</a>, including the one referenced in the question, indicate that it is explicitly set to NULL on error. The C Standard committee document (<a href="http://open-std.org/jtc1/sc22/wg14/www/docs/n1337.pdf" rel="nofollow noreferrer">n1337.pdf</a>) does not specify the error behaviour on lack of memory.</p> <ul> <li>If using asprintf(), do not assume that the pointer is initialized if the function fails.</li> <li>If implementing asprintf(), ensure that the pointer is set to null on error to give deterministic behaviour. </li> </ul>
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    1. POSubstitute or workaround for asprintf on AIX
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    1. COThanks Jonathan. You are right that AIX doesn't have vasprint on a fresh load either. I tested this as well and it seemed to compile fine. I would upvote both this and Sylvain's answers, but I don't actually have enough rep because I'm not a very good SO participant.
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    2. CO@bobwood: you should accept - and did accept - Sylvain's answer; in my view, that is completely correct. He did the donkey work (with a little assistance); my answer is strictly derivative. AFAIK, you can always upvote answers - and upvote more than one answer if you feel that more than one answer is helpful. The accepted answer is for the 'most helpful' answer and gives the answerer (and you, as the asker) an extra bonus. (One draft version of my answer started with "Do not upvote this". I got greedy and removed it.)
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    3. COReputation gained and upvotes awarded on the answers to this question.
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