Why do compilers not warn about out-of-bounds static array indices?

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GCC does warn about this. But you need to do two things:

  1. Enable optimization. Without at least -O2, GCC is not doing enough analysis to know what a is, and that you ran off the edge.
  2. Change your example so that a[] is actually used, otherwise GCC generates a no-op program and has completely discarded your assignment.

.

$ cat foo.c 
int main(void)
{
  int a[10];
  a[13] = 3;  // oops, overwrote the return address
  return a[1];
}
$ gcc -Wall -Wextra  -O2 -c foo.c 
foo.c: In function ‘main’:
foo.c:4: warning: array subscript is above array bounds

BTW: If you returned a[13] in your test program, that wouldn’t work either, as GCC optimizes out the array again.

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