The dangers of replacing malloc()

D. J. Bernstein djb at
Thu Jun 26 14:04:36 PDT 2003

Terry proposed that I obtain the desired sequence of sbrk() calls by
writing my own version of malloc(). I explained how this can destroy
data; it isn't portable. Terry now makes the rather strange claim that
POSIX guarantees the portability of the resulting programs---even though
POSIX doesn't specify sbrk() or various other functions that I use.

Terry Lambert writes:
> Memeory allocation in the C library is *defined by standards*,
> all of which require the ability to replace memory allocation and
> freeing functions with equivalents, *as a unit*.

You obviously aren't claiming that POSIX requires _all_ system functions
to use a replacement malloc()/realloc()/free() library for allocation.
That would prohibit sbrk() itself, for example.

Are you trying to say that POSIX requires all _POSIX functions_ to do
their memory allocation via malloc()/realloc()/free()? This has no
relevance to anything I said. We aren't talking about programs that
restrict themselves to the POSIX functions.

> > Suppose the OS distributor decides that valloc() or xyzalloc() should do
> > its own thing, rather than calling malloc().
> Then that OS distributor's OS no longer complies with standards.

I already showed you code demonstrating that the Linux valloc() works
this way. If, as you claim, there's a ``standard'' prohibiting the Linux
behavior, then that ``standard'' is not useful for people who care about
real-world portability. Anyway, I see no evidence supporting your claim.

> If you are in non-compliance with the Intel Application Binary Interface
> specification, you should expect to *minimally* be required to relink,
> recompile, or have to modify your program source code, each time the
> OS major version number changes

If you link statically, upgrades can break your _source code_?

Wow. I never realized that the -static option had such power. Will it
also cause hair to grow on your palms?

---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago

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