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cpointersdereference

Passing pointers into another function in C

Say I have a variable

int * foo;

I pass this variable into

func(int *x)
{
  *x = bar;
}

However, this does not work. However, if I put a non-pointer (say change it to int foo;) and directly put the address in, this works.

Why doesn't my first case work?

Thanks!

Solution

  • By declaring int * foo you've only created a pointer. foo is not actually pointing at anything yet - it's uninitialised. You might even get a segmentation fault from code like that, as the pointer could point outside of program memory. If you do:

    int * foo = (int *)malloc(sizeof(int));

    You will have initialised the pointer, malloc allocates some memory from the heap of the size passed to it. You can also do:

    int bar = 0;
int * foo = &bar;

    Which will make foo a pointer to bar (& returns the address of the variable).