C :: Uninitialized Value Was Created By A Heap Allocation / Memcheck With Valgrind

I discovered valgrind and started using it for my c code. But I get following error message at almost every malloc position, :

==19505== 40 errors in context 10 of 12: ==19505== Use of uninitialised value of size 8 ==19505== at 0x10000416E: my_method (main.c:662) ==19505== by 0x10000159E: main (main.c:182) ==19505== Uninitialised value was created by a heap allocation ==19505== at 0x47F1: malloc (vg_replace_malloc.c:302) ==19505== by 0x100001C21: my_method (main.c:333) ==19505== by 0x10000159E: main (main.c:182)

and I really don't understand what it means. I already googled it but I didn't find out what is my mistake.SO here i just put one example:

Code:

int main(int argc, char** argv) {

//i declare my variables at this position
Uint *used, *forbidden_jumps, *forbidden_jumpsV,
*forbidden_jump;

/*now i want to allocate one of them, this is my line 333 from the error message*/

//a_num is set during the execution of the program,
ALLOC(used, Uint, a_num);
}

[code].....

Is there any support page for the output of valgrind? I found it on the homepage.