Something I stumbled upon and made me wonder. Why does this work?
void foo (int* a) { int x = 3; *a = x; } int main() { int a; foo(&a); return 0; } But this causes a segmentation fault (both on Visual Studio 2008 and gcc)?
void foo (int* a) { int x = 3; *a = x; } int main() { int* a; foo(a); return 0; } Is it something defined in the language or just an implementation issue?
When you declare
int* a; You are declaring a pointer variable a but you are not making it point to anything. Then in the function, you do
*a = x; Which dereferences the pointer and tries to assign what it points to the value of x. But since it doesn't point to anything, you get undefined behaviour, manifested in a segmentation fault.
You should do this:
int i; // the actual integer variable int* a = &i; // a points to i The difference between that and the first one is that int a; declares a real integer variable, then you take its address with &a and passes it to the function. The pointer a inside the function foo points to the variable a in main, and so dereferencing it and assigning to it is perfectly fine.
printf("%d\n", a);, since "%d" expects type int but a is type int *.
printf, it's because you're creating a pointer that doesn't point to anything and then assigning x to what it points to (which is nothing)%d specifier with pointers because pointers and integers may be different sizes. It was the second thing that is incorrect in the second program.int a; Assigns memory as soon as you declare it but this not the case with int *a;
int *a; is pointer declaration (MEMORY not yet allocated for that).
int *a = (int*)malloc(sizeof(int)); // allocate memory 