The dangers of replacing malloc()

D. J. Bernstein djb at
Wed Jun 25 20:41:07 PDT 2003

Suppose that, taking the advice of inexperienced programmers who trumpet
weak linking, I use sbrk() to write my own malloc(), free(), etc. Here's
what can go horribly wrong.

Suppose the OS distributor doesn't go to the effort of dealing with
competition for sbrk(). This is the normal situation; it has been quite
explicitly tolerated by the sbrk() documentation for several centuries.

Suppose the OS distributor decides to write somedorkyosfunction() using
some funky new allocation function that I haven't replaced because I
haven't heard of it. Yesterday it was valloc(); tomorrow xyzalloc().
This happens all the time: look at the FreeBSD reallocf(), for example.

Suppose the OS distributor decides that valloc() or xyzalloc() should do
its own thing, rather than calling malloc(). This happens too: I tried
the sample program shown below under Linux, and somedorkyosfunction()
ended up calling brk() rather than my own malloc().

Finally, suppose the OS distributor decides that some syscall I use
should be replaced by a library routine that uses somedorkyosfunction().
This happens too. Note for the reading-impaired: I'm not saying that the
name malloc() has ever been used for a syscall; I'm saying that poll(),
socket(), et al. have been used for allocating library routines.

Result: My program innocently calls that library routine, which calls
somedorkyosfunction(), which calls valloc() or xyzalloc(), which
incorrectly assumes that its sbrk() results are contiguous, destroying
the data allocated by my own malloc().

As I said before, I encounter more than enough portability problems
without going out of my way to look for them. I wish OS distributors
would put a little more thought into the needs of people who _don't_
spend their entire lives working with a single platform.

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

#include <stdlib.h>

void somedorkyosfunction(void)

void *malloc(size_t n)
{ write(1,"malloc\n",7); return 0; }
void *calloc(size_t n,size_t m)
{ write(1,"calloc\n",7); return 0; }
void *realloc(void *x,size_t n)
{ write(1,"realloc\n",8); return 0; }
void free(void *x)
{ write(1,"free\n",5); }

int main()
  return 0;

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