Date: Fri, 28 Aug 2015 07:05:06 -0700 From: Conrad Meyer <cem@FreeBSD.org> To: Xin LI <delphij@freebsd.org> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r287236 - head/bin/df Message-ID: <CAG6CVpXWTcP_%2Bv60Bj-1T=fbissT_rH8YvYeOHzBozr-gFCJ8w@mail.gmail.com> In-Reply-To: <55E01D9A.2090805@delphij.net> References: <201508280044.t7S0ixfW038766@repo.freebsd.org> <20150828044312.GA88538@FreeBSD.org> <55E01D9A.2090805@delphij.net>
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On Fri, Aug 28, 2015 at 1:36 AM, Xin Li <delphij@delphij.net> wrote: > > > On 8/27/15 21:43, Alexey Dokuchaev wrote: >> On Fri, Aug 28, 2015 at 12:44:59AM +0000, Xin LI wrote: >>> New Revision: 287236 >>> URL: https://svnweb.freebsd.org/changeset/base/287236 >>> >>> Log: >>> Use exit() instead of return in main(). >> >> Because?.. > > Because the explicit exit() have a subtle difference from returning from > main(), and that could confuse static analyzers. I thought it was > obvious and too much for trivial changes like this. Let me try again: > > C standard defines that when main() returns to the startup code, the > latter is required by the standard to call exit() with main()'s return > value, should main() be defined to return a type compatible with int. > On FreeBSD, this is done by lib/csu/${ARCH}/crt1.c. > > Note, however, that return'ing from a C function means the stack > contents, for example, variables defined in function scope, are > discarded. Therefore, if, let's say one defined a pointer in the > function scope, like: > > void *p; > > And then later assign a block of memory allocated from heap to it: > > p = malloc(size); > > Since p is in function scope, upon return, it's gone. If there is no > other pointers that referenced the memory block referenced by p, the > memory block is _technically_ leaked. > > This does not matter in practice because exit() or returning from main > are both the points of termination, and the kernel would then reclaim > all memory pages that belongs to the process. However, doing exit() > makes it more explicit that this is the point of no returns, actually, > it hints the compiler or a static analyzer to do the right thing without > needing to make main() a special case. So, a better commit log may have been: "Use exit() instead of return in main() to work around a broken static analyzer" Any C static analyzer must understand main(). Best, Conrad
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