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<html><!-- Mirrored from c-faq.com/malloc/alloca.glb.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 08:00:17 GMT --><head><title></title></head><body>From: Gordon Burditt<br>Newsgroups: comp.lang.c<br>Subject: Re: <TT>alloca()</TT><br>Date: 25 Jan 1999 09:34:09 GMT<br>Message-ID: <16167775DFB6601E.39220BF5766349A7.B20FE25E4B4173B2@library-proxy.airnews.net><p>>I'm curious to hear a little about a function called <TT>alloca()</TT>.<br>>The (Solaris) man page I have here tells me that it allocates <br>>memory on the stack, which is then automatically freed when you <br>>return. Now, I'm well aware that <TT>alloca()</TT> is Not Standard And<br>>thus System Dependant and Beyond The Scope Of This Group,<br>>so please don't feel the need to reply just to tell me that. On<br>>the other hand, I'm interested to know how portable this is <em>in</em><br>><em>practice</em>, and what advantages and disadvantages it has w.r.t <br>><TT>malloc()</TT> et al. I've seen it used a fair bit, and presumably the<br>>authors of the code must have had <em>some</em> reason for using it.<br><p>This function is one of the most horribly unportable abominationsI have ever tried to implement. There are ways to try toimplement it with <TT>malloc()</TT>, but if the memory is actuallyallocated on the stack, it causes lots of problems. You'repretty much stuck with needing to implement this in the compileras a built-in. <p>You can write a function that knows about linkage conventions that adjusts the stack, but on one architecture (which didn't use a stackframe pointer), I ended up having to allocate something like 128 bytes extra on each call, due to possible register saving. That 128 byte figure was a <strong>guess</strong> as to what the worst case was, and I think it involved some (unreasonable) assumptions about the maximum number of arguments to functions that call <TT>alloca()</TT>. You did getthose bytes back when the function exited. If it had beenpractical to modify the compiler-generated function exit sequences, life would have been much simpler.<p>What's the problem? Well, if you start allocating stuff on thestack when there's already stuff on the stack, you have<TT>alloca()</TT> allocating stuff in the middle of it, which reallymesses up argument passing, where stuff pushed on the stackis supposed to be contiguous. Horrible example:<p><pre> printf("%p %p %p %p %p\n", alloca(1), alloca(2), alloca(10), alloca(8), alloca(20));</pre><p>Now, where can you use <TT>alloca</TT>? I think this is reasonably safe:<p>You may use alloca() in the form of:<pre> pointer_variable = alloca(expression);</pre>as an expression statement in the outermost block of a function. <p>You are asking for trouble if you use it anywhere that doesn't fit this description. You are likely to run into trouble if you use <TT>alloca()</TT> in any of these places, because there might be something on the stack at the point alloca() is called:<p><ol><li>Inside a loop.<li>Inside any block that begins with local variables, exceptthe outermost block of a function, especially if theallocated memory is used after exiting this block.<li>Using any expression more complicated than a pointer variableon the left hand side of an assignment, including oneelement of an array of pointers. <li>Where the return value of alloca() is used as a function argument.<li>In any context where the value of the = operator is used,such as <pre> if ((pointer_variable = alloca(sizeof(struct something))) == NULL) { .... }</pre></ol><p>And I expect that someone will call me on even THAT highly restrictivelimitation not being conservative enough for the code generated bysome compilers. Now, if it's done as a compiler builtin, you mightmanage to get around the problems. <p>Once I finally got that <TT>alloca()</TT> function figured out, it workedreasonably well - as I recall, the primary use for it was in aBison parser. That 128 bytes wasted per invocation combinedwith a fixed stack size could be a nuisance. Why didn't I justuse GCC? Because this was an attempt to <strong>port</strong> GCC, initiallyusing cross-compilers, to a machine that turned out to barelyhave enough memory to compile GCC (1.35 or so) natively. When GCC 2 came out, it turned out to be enough of a memory pigthat natively compiling itself was out of the question.<p> Gordon L. Burditt</body><!-- Mirrored from c-faq.com/malloc/alloca.glb.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 08:00:17 GMT --></html>⌨️ 快捷键说明
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