Reading uninitialized memory

[Andrew Vaught]

Consider the following code:

#include <gmp.h>

int main(void) {
int m, k;
mpz_t n;

  mpz_init(n);
  mpz_set_si(n, 0);

  k = mpz_get_si(n);

  mpz_clear(n);

  return 0;
}


When I run it under checkergcc, I get:

>From Checker (pid:06275): (ruh) read uninitialized byte(s) in a block.
When Reading 4 byte(s) at address 0x0805e850, inside the heap (sbrk).
0 byte(s) into a block (start: 0x805e850, length: 4, mdesc: 0x0).
The block was allocated from:
        pc=0x080558a9 in chkr_malloc at stubs-malloc.c:57
        pc=0x08048ed5 in __gmp_default_allocate at memory.c:50
        pc=0x08048c27 in __gmpz_init at init.c:29
        pc=0x08048685 in main at tst.c:8
        pc=0x0805594f in this_main at stubs-main.c:13
        pc=0x4002ea42 in checker_text_stubs_end at stubs/end-stubs.c:7
        pc=0x080485c8 in *unknown* at *unknown*:0
Stack frames are:
        pc=0x080489f5 in __gmpz_get_si at get_si.c:29
        pc=0x08048724 in main at tst.c:11
        pc=0x0805594f in this_main at stubs-main.c:13
        pc=0x4002ea42 in checker_text_stubs_end at stubs/end-stubs.c:7
        pc=0x080485c8 in *unknown* at *unknown*:0
Initialisation of detector...
Searching in data
Searching in stack
Searching in registers
>From Checker (pid:06275): (gar) garbage detector results.
There are no leaks.
Initialisation of detector...
Searching in data
Searching in stack
Searching in registers
>From Checker (pid:06275): (gar) garbage detector results.
There are no leaks.


   I'm guessing that zero is stored without limbs and that mpz_get_si()
always reads whatever passes for a non-limb.  The value read from
uninitialized memory is not used and doesn't affect the value of zero, but
you could get rid of a read and the spurious error message from this and
other bounds-checking programs.

       Cheers,
          Andy

-----------------                        XOLD(K,IC,I)=
Andy Vaught               ....        DO ITERS=1, 10  XOLD(K,IC,I)
address@hidden     |  |   /CALLMSOLVE(A,B,X,I,ITERS,TOL)+(RANNYU(0)
Arizona State University  ======|WRITE(6,'(I5,2X,F12.6)')ITERS,TOL -HALF)
Tempe, Arizona USA        OOOOOO \ENDDORETURN PARAMETER(ZERO=1.D0)*TENTH*DELTA