CODEWITHSUNDEEP
cpointersparameter-passingampersand
Leon
Leon

Reputation: 584

pointers in argument, ampersand?

I pass a pointer to a function, I can access struct's members from the pointer.

In this function I need to call another function with the same pointer as argument. I use the ampersand but application stopped.

How to call anotherfunction() from myfunction() ?

bool anotherfunction( const ovrVkGraphicsPipelineParms * parms )
{
    myStruct info;
    info.layout = parms->program->parmLayout.pipelineLayout; // application error
}

bool myfunction( const ovrVkGraphicsPipelineParms * parms )
{
    myStruct info;
    info.layout = parms->program->parmLayout.pipelineLayout; // Works

    anotherfunction( &parms ); // compile, but application stopped
}

void main()
{
    myfunction( &pipelineParms );
}

Upvotes: 1

Views: 258

Answers (2)

Steve Summit
Steve Summit

Reputation: 47942

In C, the & character before a variable does not mean "this thing is a pointer". No, when you put & before a variable, it means "make me a pointer to this".

So when you called

myfunction(&pipelineParms);

you had a variable pipelineParms, and you created a pointer to it, and it was that pointer you passed to myfunction.

Then, in myfunction, parms was a pointer (specifically, a pointer to type ovrVkGraphicsPipelineParms. Hopefully, pipelineParms in main was type ovrVkGraphicsPipelineParms also.)

So in myfunction, if you want to pass the pointer on to anotherfunction, just pass it:

anotherfunction(parms);

We can see that anotherfunction also accepts a pointer to type ovrVkGraphicsPipelineParms, so this is correct.

When you tried to call anotherfunction(&parms), the & said, "make me a pointer to this". So you got a pointer to a pointer to type ovrVkGraphicsPipelineParms, or ovrVkGraphicsPipelineParms **. That might be fine if that's what anotherfunction expects, but it's not.

It's not that C has regular variables, and pointers to regular variables. You can have pointers to anything. You can have pointers to pointers, and pointers to pointers to pointers, and theoretically you can have 4- and 5- and 6-level pointers, too, all the way up to 12 or more (although in practice anything beyond 3 is extremely rare).

One more thing: You said

anotherfunction( &parms ); // compile, but application stopped

but I'm guessing it did not, in fact, compile properly. I'm guessing your compiler gave you a warning. Mine says:

warning: incompatible pointer types passing 'const ovrVkGraphicsPipelineParms **' to parameter of type 'const ovrVkGraphicsPipelineParms *'; remove &

If your compiler didn't give you a warning on that line, I encourage you to switch to a better compiler, if at all possible. (A modern compiler would also warn you about void main, which is also wrong, although it's not causing you any problems.)

Upvotes: 6

kiran Biradar
kiran Biradar

Reputation: 12732

anotherfunction( &parms ); // compile, but application stopped

You already have parms as pointer, you don't need &.

anotherfunction( parms ); // compile, but application stopped

If you append & to parms it will become onst ovrVkGraphicsPipelineParms **

Upvotes: 1

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