Why isn't my dynamically allocated struct array in C working?

Question

I have been trying to create a dynamically allocated array of struct type label and have been failing miserably. In my .h file, I have:

    typedef struct _label {
            char name[256];
            char type[256];
            int address;
} label;

and in my .c file, I have this at the top:

    label* allLabels = (label*) malloc(sizeof(label) * 10); // line 10
    int arrayIndex = 0;

and finally, I have a function in the same .c file that is meant to add these struct objects to the array to be used by other methods in the file:

    void addLabel(char line[], char type[], int addr) {
            label database;
            database.name = line; // line 805
            database.type = type; // line 806
            database.address = addr;
            allLabels[arrayIndex] = database;
            arrayIndex++;
        }

Basically I just want to have a collection of accessible labels. Can someone help me understand what I'm doing wrong?

I get these errors, and I haven't forgotten any of the necessary #include statements either:

formatBuilder.c:10:3: error: initializer element is not constant
formatBuilder.c: In function 'addLabel':
formatBuilder.c:805:18: error: incompatible types when assigning to type 'char[256]' from type 'char *'
formatBuilder.c:806.18: error: incompatible types when assigning to type 'char[256]' from type 'char *'

Answer 1

You can't assign to char arrays like that, you need one of the string operations, such as:

strcpy (database.name, line);  // or "->" if database is pointer

(preferably checking the length beforehand to ensure no buffer overflow, or using a safer function depending on your needs).

It's also bad form to cast the return value from malloc in C since it can hide certain subtle errors. It's acceptable if your code has to also compile in C++ but you just need to ensure you have the correct prototype in scope.

---

In terms of the initialisation error, I suspect that you have the declaration at file level (outside of any function). This means you can't use a function call to initialise it since it has static storage duration and wants to be set before any code runs.

You can get around that problem thus:

// At file level:

label* allLabels = NULL;

// In your function:

void addLabel(char line[], char type[], int addr) {
    if (allLabels == NULL) {
        allLabels = malloc (sizeof(label) * 10);
        if (allLabels == NULL) {
            // malloc failed, do something to recover.
        }
    }
    // And we don't need local storage here, hit the array directly.

    strcpy (allLabels[arrayIndex].name, line);
    strcpy (allLabels[arrayIndex].type, type);
    allLabels[arrayIndex].address = addr;
    arrayIndex++;
}

This uses a constant initialiser NULL to set the value and you then just need to ensure it's allocated before the first time you use it.

Answer 2

I suggest to use memcpy.

memcpy(&allLabels[arrayIndex], &database, sizeof(label));