CODEWITHSUNDEEP
cmemcpy
Lydon Ch
Lydon Ch

Reputation: 8815

memcpy confusion

I am trying to copy a small array into a bigger array, and I could not figure out how to get it working (program always crashes on Visual studio 2008 x32)

memcpy work for 

   memcpy( raster+(89997000), abyRaster, sizeof(abyRaster));

but not 

   memcpy( raster+(line*3000), abyRaster, sizeof(abyRaster));

I just want to get it working in the for loop, but got confused about the pointer arithmetic and the size of int and unsigned char.

Ideas ?

    unsigned char raster[3000*3000];

    unsigned char abyRaster[3000*1];

    for( int line=0; line<3000;line++ ) {

        int arrayPosition = line*3000;

        memcpy( raster+(arrayPosition), abyRaster, sizeof(abyRaster));          
    }

Upvotes: 0

Views: 937

Answers (4)

Pratap
Pratap

Reputation: 7

The program can not be run directly because of not enough memory 
If the system supports high enough the memory allocated by the program. then  
the program copies bytes of a variable abyRaster to another variable on     
every position (line*3000) of  variable raster.
abyRaster[0] is copied to  raster[0]
abyRaster[1] is copied to  raster[3000]
abyRaster[2] is copied to  raster[6000]
  :                             :
  :                             :
int line=0; line<3000;line++ used to identify only the index values of array

Upvotes: 0

corlettk
corlettk

Reputation: 13574

portoalet,

http://www.cplusplus.com/reference/clibrary/cstring/memcpy/ says:

void * memcpy ( void * destination, const void * source, size_t num );
destination : Pointer to the destination array where the content is to be copied, type-casted to a pointer of type void*.
source      : Pointer to the source of data to be copied, type-casted to a pointer of type void*.
num         : Number of bytes to copy.

I personally find the "address-of-the-element" syntax (below) much more self-explanatory than the equivalent base-of-the-array-plus-the-index syntax... especially once you get into offsets-into-arrays-of-structs.

memcpy( &raster[arrayPosition], abyRaster, sizeof(abyRaster));

And BTW: I agree with the other previous posters... everything bigger than "one line" (say 4096 bytes) should be allocated on the heap... otherwise you VERY quickly run out of stack space. Just DON'T FORGET to free EVERYTHING you malloc... the heap isn't self-cleansing like the stack, and ANSI C doesn't have a garbage collector (to follow you around and clean-up after you).

Cheers. Keith.

Upvotes: 0

unwind
unwind

Reputation: 400109

That raster array is quite large (9 MB) for a stack variable. Try allocating it from the heap.

Upvotes: 3

Alexandre C.
Alexandre C.

Reputation: 56986

The code seems ok, except that

unsigned char raster[3000*3000];

declares a huge array on the stack, and you may run out of stack space for this operation (typical stack sizes are just a few megabytes).

Try to declare raster as a dynamic array, using malloc.

Upvotes: 4

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