Copying a struct's contents to another

Question

I am trying to copy a struct's contents into another struct of the same type.

I would like to be able to change the values of one struct without it affecting the other later though.

I am dealing with reading and editing PPM files. I have a struct:

typedef struct {
    char format[4];
    char comments[MAX_COMMENT_LENGTH];
    int width, height, maxColourValue;
    PPMPixel **pixels;
} PPMImage;

And then I have a copy function to copy the values over but I get an error when assigning different fields.

I am trying to copy the fields of newPPM into messagePPM.

Error:

incompatible types when assigning to type 'char[4]' from type 'char *'
    messagePPM->format = newPPM->format;
incompatible types when assigning to type 'char[100]' from type 'char *'
    messagePPM->comments = newPPM->comments;

Copy Function:

//A function to copy contents of one PPMImage to another
void copyPPM(PPMImage *newPPM, PPMImage *messagePPM) {

    messagePPM->format = newPPM->format;
    messagePPM->comments = newPPM->comments;
    messagePPM->width = newPPM->width;
    messagePPM->height = newPPM->height;
    messagePPM->maxColourValue = newPPM->maxColourValue;
    messagePPM->pixels = newPPM->pixels;

}

How do I fix my error? Will copying fields this way achieve what I am aiming for?

you should use memcpy to copy the values of the format and comments fields. You could use strcpy if the values are null terminated. — bruceg, Feb 10 '16 at 23:14
This has been answered here:- http://stackoverflow.com/questions/4931123/copying-one-structure-to-another — BryanT, Feb 10 '16 at 23:16

chqrlie · Accepted Answer · 2016-02-11T22:15:07.343

You can copy the contents of one structure to the other with a simple assignment:

void copyPPM(PPMImage *newPPM, PPMImage *messagePPM)  {
    *newPPM = *messagePPM;
}

This means you do not even need a function.

Yet the structures will share the pixels array. If you want to duplicate that, you will need to allocate a copy and copy the contents.

Copying one structure over another one may also cause the pixels array of the destination to be lost.

If you want to make a deep copy of the structure, you need to allocate new arrays for the pixels this way:

void copyPPM(PPMImage *newPPM, PPMImage *messagePPM)  {
    *newPPM = *messagePPM;
    if (newPPM->pixels) {
        newPPM->pixels = malloc(newPPM->height * sizeof(*newPPM->pixels));
        for (int i = 0; i < newPPM->height; i++) {
            newPPM->pixels[i] = malloc(newPPM->width * sizeof(*newPPM->pixels[i]);
            memcpy(newPPM->pixels[i], messagePPM->pixels[i],
                   newPPM->width * sizeof(*newPPM->pixels[i]));
        }
    }
}

How would I go about copying the array over so that I have a version of it in each struct? It's an array using malloc for each cell and that points to a pixel, so it works like a 2d array of pixels. — Porteous96, Feb 11 '16 at 02:02

score 0 · Answer 2 · answered Feb 10 '16 at 23:10

0

You can simply do a = b, where and b are variables of type PPImage.

answered Feb 10 '16 at 23:10

Cecilio Pardo

1,717
10
13

This just copies the address, do you mean *a = *b? – David Ranieri Feb 10 '16 at 23:12
So in this case does this make a's fields point to what b's fields point to or does it actually copy the values? I'm new to C and still trying to understand the whole pointer thing. – Porteous96 Feb 10 '16 at 23:13
@CecilioPardo, `a` and `b` are pointers in the function, so you need to dereference. – David Ranieri Feb 10 '16 at 23:13
@Porteous96: It's make a bit-by-by copy of the struct, copying all the values. – Cecilio Pardo Feb 10 '16 at 23:14
1

@AlterMann: I know, by in my answer I said they are variables of the struct type. He doesn't need such function. – Cecilio Pardo Feb 10 '16 at 23:15

Copying a struct's contents to another

2 Answers2