Malloc + 2d arrays + C

[Giorgos Keramidas]

On 2002-03-19 11:04, Ioannis Sidiropoulos wrote:
>
> Prospa8w na grapsw ena (to prwto mou) programma pou exei na kanei me
> images. Exw loipon mia synarthsh pou pairnei enan pointer se 2d
> array (to image) kai to epeksergazetai. Mesa s'ayth thn synarthsh kanw
> malloc gia enan temp 2d array wste na ginei olh h epksergasia s'ayton kai
> prin epistrepsw ap'thn synarthsh na ton kanw copy sto actual image (ksexasa
> na pw oti einai char arrays):
>
> void process(int size, char **image) {
>   int arraySize = 2*size+1; //arraySize = to mhkos ths pleyras toy#
>                             // TETRAGWNOU array

Se C h se C++ grafeis?  Ti einai to // ...?
H C den exei sxolia se C++ stul.

Giati kaneis ton `periergo' pollaplasiasmo me to 2 kai prostheteis 1?
To `size' einai telika, h den einai to megethos tou pinaka?

>   char **tmp = malloc(arraySize*sizeof(char *));
>
>   for (l;l<arraySize;l++) {

Duo lathi:
- undeclared variable `l'.
- uninitialized variable `l' used as a loop sentinel.

Ah, epishs.  Grafeis me athlio stul.
Whitespace.  Use whitespace.  It's free and available in large quantities.
To parapano einai POLU pio euanagnosto an to grapseis san:

	for (x = 0; x < arraySize; x++) {
		...
	}

>     tmp[i] = malloc(arraySize*sizeof(char));
>  }

Edo exeis kataferei kai ekanes allocate ena array megethous P x P,
opou P = 2 * size + 1.  Akoma den katalabaino giati o periergos
pollaplasiasmos me to 2 kai to + 1, alla telospantwn.

>  // edw ginetai to processing ston tmp
>  // kai twra 8a ton kanw copy ston image
>  // kai 8a eley8erwsw th mnhmh
>
>  memmove (image, tmp, arraySize);

Oriste?  Posa bytes einai auto?
Koita ti leei to manpage ths memmove() edo pera...

	SYNOPSIS
	     #include <string.h>

	     void *
	     memmove(void *dst, const void *src, size_t len);

	DESCRIPTION
	     The memmove() function copies len bytes from string src to
	     string dst.  The two strings may overlap; the copy is always
	     done in a non-destructive manner.

Diabase prosektika... "The memmove() function copies len bytes".
Esena poios sou eipe oti ena array me X entries apo `char *' einai X bytes?

	hades:~$ cat > sizeof.c
	#include <stdio.h>

	int main (void)
	{
		printf("%d\n", sizeof(char *));
		return 0;
	}
	hades:~$ cc -Wall sizeof.c
	hades:~$ ./a.out
	4

>  free(tmp);

Kalh fash.  Ekanes free ton `tmp' pinaka, opote kapoios mporei na parei th
mnhmh pou palia htan o `tmp' sto akribos epomeno malloc(), efoson einai
pleon free memory :)  Mono pou ksexases pos den ekanes mono:

	tmp = malloc(...);

alla ekanes kai ena malloc() gia KATHE stoixeio tou tmp array.  Pisteuo pos
mallon tha exeis ena mikro memory leak.

> }
>
> Parolo pou kanei compile to programma me petaei Segm. Fault
> otan to trexw, ti exw kanei la8os???

Xoris na doume olo to programma?  Sobarologeis tora?
Ki ego pou ksero ti sto gero-diaolo kaneis ekei pou egrapses:

>  // edw ginetai to processing ston tmp
>  // kai twra 8a ton kanw copy ston image
>  // kai 8a eley8erwsw th mnhmh

Show us the code, darling.  All of it...

Giorgos Keramidas                       FreeBSD Documentation Project
keramida@{freebsd.org,ceid.upatras.gr}  http://www.FreeBSD.org/docproj/