C Function returning Array and pointer without a cast [duplicate]

Question

I got a problem with one of my final function. My goal is too treat a picture. I declare in a header :

#define PICTURESIZE 1024
typedef unsigned char Picture[PICTURESIZE][PICTURESIZE];
typedef unsigned char LinearBlocPicture[PICTURESIZE*PICTURESIZE];

I include this header in an other file, where i got only one function :

LinearBlocPicture picture2BlocVector (Picture image){
    LinearBlocPicture pictureModify;
    int curseurLinearBlocPicture = 0; 
    unsigned char blocPixel[8][8]; 
    unsigned char* bufferLinearBlocPicture;
    int x,y,i,j,k;  


    for(y=0; y < 1024; y=y+8) { 
        for(x=0; x < 1024; x=x+8) { 
            for(j=0; j < 8; j++) { 
                for(i=0; i<8; i++) { 
                    blocPixel[i][j] = image[x+i][y+j];
                } 
            }  

            bufferLinearBlocPicture = bloc2Vector (blocPixel);

            for (k=0; k<64; k++){
                pictureModify[curseurLinearBlocPicture+k] = bufferLinearBlocPicture[k];
            }

            curseurLinearBlocPicture = curseurLinearBlocPicture + 64;
        } 
    }

    return pictureModify;
}

This function apply my treatment, the problem come from the return value, and what is expected to be return. I got this when i try to compilate :

First line error: ‘picture2BlocVector’ declared as function returning an array
Application.c: In function ‘picture2BlocVector’:
Last line: warning: return makes integer from pointer without a cast
Last line: warning: function returns address of local variable

I don't understand why there is a problem for the return type, because i declare properly my type LinearBlocPicture. The pointer problem come from this line i think :

pictureModify[curseurLinearBlocPicture+k] = bufferLinearBlocPicture[k];

I'm not sure to be allowed to do this. Thanks for your help.

No, you cant do this in C this way. Declare result as a pointer, allocate in your function, and return that. — OldProgrammer
– OldProgrammer, Commented Sep 24, 2013 at 21:16

yan · Accepted Answer · 2013-09-24 21:20:08Z

In C, arrays are often "decayed" to pointer values, so when you return an array, you're essentially returning the address of its first element. In your sample code, you allocate pictureModify on a local stack, and attempt to return its address. When your function returns, all variables declared during its execution will be cleaned up -- this includes your local buffer.

To get around this you have a few options:

Use malloc() to allocate a block of memory, write your results to it and then return a pointer to it. When you do this, you need to make sure your calling code will call free() on it.
Allocate memory in your calling code, and pass a pointer to that buffer to picture2BlocVector. This way, you're localizing all memory management to one place. In this scenario, you can even create a local variable and pass its address.

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