cimg_forXY(img,x,y) version CImg 2.91

[pwplus7]

Can't figure out why this code runs OK :

include "CImg.h"

using namespace cimg_library; typedef unsigned char u8 ;

int main (int argc,char *argv) { u8 m= 0; int del = 20; int W=del 16, H=del *16, x, y, k, kk ; m= 100; CImg img= CImg (W, H,1,1) ;

for (y=0; y< H; y += del) for (x=0; x< W; x +=del) // cimg_forXY(img,x,y) { for (k= 0; k<del; ++k) for (kk= 0; kk<del; ++kk) img(x+kk, y+k)= m ; m++ ; }

BUT this code generates runtime error " malloc(): corrupted top size, Aborted " ... // for (y=0; y< H; y += del) for (x=0; x< W; x +=del) cimg_forXY(img,x,y) ...

[dtschump]

The second loop cimg_forXY(img,x,y) is definitely not the same a the first for (y=0; y< H; y += del) for (x=0; x< W; x +=del), as the latter uses a step of del for both x and y variables. With the second loop, you'll be doing invalid memory accesses (when x==W-1 and y==H-1, and k>1 or kk>1 for instance). It's not so surprising then that you end up scrambling your memory content and getting some weird errors.

[pwplus7]

OK, I get it now ... namy thanks

On Mon, Sep 7, 2020 at 3:14 PM David Tschumperlé notifications@github.com wrote:

The second loop cimg_forXY(img,x,y) is definitely not the same a the first for (y=0; y< H; y += del) for (x=0; x< W; x +=del), as the latter uses a step of del for both x and y variables. With the second loop, you'll be doing invalid memory accesses (when x==W-1 and y==H-1, and k>1 or kk>1 for instance). It's not so surprising then that you end up scrambling your memory content and getting some weird errors.

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