Question

I don’t understand how the reallocation of memory for a struct allows me to insert a larger char array into my struct.

Struct definition:

typedef struct props
{
    char northTexture[1];
    char southTexture[1];
    char eastTexture[1];
    char westTexture[1];
    char floorTexture[1];
    char ceilingTexture[1];
} PROPDATA;

example:

void function SetNorthTexture( PROPDATA* propData, char* northTexture )
{
    if( strlen( northTexture ) != strlen( propData->northTexture ) )
    {
        PROPDATA* propPtr = (PROPDATA*)realloc( propData, sizeof( PROPDATA ) +
            sizeof( northTexture ) );
        if( propPtr != NULL )
        {
            strcpy( propData->northTexture, northTexture );
        } 
    }
    else
    {
        strcpy( propData->northTexture, northTexture );
    }
}

I have tested something similar to this and it appears to work, I just don’t understand how it does work. Now I expect some people are thinking “just use a char*” but I can’t for whatever reason. The string has to be stored in the struct itself.

My confusion comes from the fact that I haven’t resized my struct for any specific purpose. I haven’t somehow indicated that I want the extra space to be allocated to the north texture char array in that example. I imagine the extra bit of memory I allocated is used for actually storing the string, and somehow when I call strcpy, it realises there is not enough space…

Any explanations on how this works (or how this is flawed even) would be great.

Answer 1

Is this C or C++? The code you’ve posted is C, but if it’s actually C++ (as the tag implies) then use std::string. If it’s C, then there are two options.

If (as you say) you must store the strings in the structure itself, then you can’t resize them. C structures simply don’t allow that. That “array of size 1” trick is sometimes used to bolt a single variable-length field onto the end of a structure, but can’t be used anywhere else because each field has a fixed offset within the structure. The best you can do is decide on a maximum size, and make each an array of that size.

Otherwise, store each string as a char*, and resize with realloc.