Interpreting valgrind error

Question

I'm trying to debug my code using valgrind. Most of the message I get are:

Conditional jump or move depends on uninitialised value(s)

or

Invalid read of size 8

I'm mainly concerned about the first, if the value was truly uninitialized I believe segmentation fault would occur. I tested this by sending the same pointer to another function along with uninitialized pointer to a function which I know throws a segmentation fault and only the truly uninitialized pointer has cause a segmentation fault.

What also might be the meaning of this error message.

Also, what does the second error means?

Edit1
Here is a model code, would that give error 1 (assume that the header files are legal)?

a.cpp

B b;
C c;
int main(){
  return 0;
}

B.cpp

extern C c;
//    double t; //canceld, declared in the header.
B::B(){
  this->t = 1;
  c.test(t);
}
B::test(){
  c.test(this->t);
}

B.cpp

C::C(){
}

C::test(double t){
  printf("%f\n",t);
}

Answer 1

Conditional jump or move depends on uninitialised value(s)

This means you are trying to do something to an uninitialized variable. For example:

int main()
{
    int x;
    if (x == 5)
        printf("%d\n", x);
    return 0;
}

should do the trick. You can't compare/print or do something to an uninitialized variable.

Invalid read of size 8

This means you are trying to read from memory that isn't there i.e. hasn't been allocated.

int main()
{
    char* x = malloc(10);
    x[10] = '@';    //this is an invalid write
    printf("%c\n", x[10]); //this is an invalid read
    return 0;
}

Would cause an error because you've only allocated space for 10 characters, but you're writing/reading at the 11th character (remember, arrays are 0 indexed, so you can only write to 0-9).

"size X" in general is the amount of memory you're trying to read, so size 8 means you are trying to read 8 bytes.

Hope it helps. Post more specific code if you want debugging help. Valgrind generally tells you where the error occurs so you can figure out what to do.