CODEWITHSUNDEEP
carrayspointersstructgtk
Jack Pollock
Jack Pollock

Reputation: 334

C - Trouble passing arrays by reference using structs

I'm trying to pass a 2D char* array, a 1D int array and an integer to a function using a struct, however I am having trouble wrapping my head round how to pass them by reference using pointers, rather than just by value. I need all variables to be editable by the functions they are passed into, and have that value reflected throughout the whole program, not just within the function scope. Essentially like a global variable, but passed from function to function using structs, defined initially in the main function.

I initially was using global variables during development, as it worked and it was easy, however I ran into some issues accessing the values in one of the arrays (when accessed from a certain function it would return empty), and I know that global variables are generally a bad idea.

I am using GTK, so as far as I'm aware the only way to pass multiple arguments into a callback is to use structs, hence why I need to pass them via a struct, rather than passing them directly into the function. Unless I'm wrong?

I need to define the following:

char* queuedHashes[100][101];
int queuedHashTypes[100] = {(int)NULL};
int hashCount = 0;

I've been having trouble understanding the pointer and struct syntax required to achieve this and the methods I have tried have led to me running into the char* array type being not assignable, so have not been able to implement anything that works so far.

Any help would be greatly appreciated, thanks.

Upvotes: 0

Views: 126

Answers (1)

Some programmer dude
Some programmer dude

Reputation: 409196

To pass a structure by "reference" (I put it in quotes because C doesn't have "references") you simply pass a pointer to the structure. The contents of the structure is in the memory pointed to by the structure pointer.

So if you have a structure like

struct myStruct
{
    char* queuedHashes[100][101];
    int queuedHashTypes[100];
    int hashCount;
};

Then you could have a function like

void myFunction(struct myStruct *theStructure)
{
    theStructure->queuedHashTypes[0] = 1;
}

And use the structure and the function something like this:

int main(void)
{
    struct myStruct aStructure;  // Define a structure object

    aStructure.queuedHashTypes[0] = 0;

    printf("Before calling the function queuedHashTypes[0] is %d\n",
           aStructure.queuedHashTypes[0]);

    myFunction(&aStructure);  // Pass a pointer to the structure

    printf("The function initialized queuedHashTypes[0] to %d\n",
           aStructure.queuedHashTypes[0]);
}

The program above should print that queuedHashTypes[0] is 0 before the function call, and 1 after the call.

Upvotes: 1

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