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cpointersdereferencepebble-watchpebble-sdk

Passing dereferenced pointer (ie. by value) to a function expecting a pointer

I was just curious - why does C allow you to pass dereferenced pointers when a function is expecting a pointer. Here's an example:

typedef struct {
char message[255];
} Bla;  

// so he's the function that's expecting the pointer
void cool_function(Bla *_bla) {

}

Bla *woot = (Bla *)malloc(2 * sizeof(Bla));

woot[0] = (Bla) {
    .message = "bla bla bla"
};

// woot itself is a pointer, but it's getting dereferenced when I am adding [0]
cool_function(woot[0]);

Now that compiles fine, but when I go to actually use _bla (ie. _bla->what), BAM, runtime error.

So this confuses me -- what happens to it if it's passed by value to this function that is clearly expecting a pointer? Why does it even compile?

I'm fairly new to C, so please don't jump all over me. :/

* UPDATE *

Sorry for not mentioning earlier, but this is an app for the Pebble Watch, which is using a special version of gcc for specific arm processors (arm-cs-tools).

* UPDATE 2 *

I think I found out why it was doing that. The function 'cool_function' was actually in a separate file, and when I declared the function in the header, it was only declared with void cool_function() - forgot to include Bla *_bla. But the definition was void cool_function(Bla *_bla). It was all my fault for confusing the compiler.

Solution

  • You should get the compiler error:

    error: passing 'Bla' to parameter of incompatible type 'Bla *'; take the address with &
    cool_function(woot[0]);

    If you use gcc or clang, try to compile your program with -WError and -Wall options.