Thread: pointer confusion

This is my theory. It crashes because the strings you are using are consts and are probably stored in read only memory. When you do *b=*a you are simply trying to copy the first character of array a, to the first character of array b. If a is pointing to read only memory, the prog will core.

Here's some code to backup this. example 1 is your code that will crash, example 2 uses malloc to obtain memory for the second array (and therefore it won't be in read only memory).

Code:

#include <stdio.h>

int main(void)
{
	char *a="abcd"; 
	char *b="efgh";
	
	printf("Variable a/Char 1 is %c\n", *a);
	printf("Variable b/Char 1 is %c\n", *b);
	printf("Variable a as a string is %s\n", a);
	printf("Variable b as a string is %s\n", b);
	
	*b=*a;
	
	printf ("After assignment...\n");
	printf("Variable a/Char 1 is %c\n", *a);
	printf("Variable b/Char 1 is %c\n", *b);
	printf("Variable a as a string is %s\n", a);
	printf("Variable b as a string is %s\n", b);
	
	return(0);
	
}
/* Program output
Variable a/Char 1 is a
Variable b/Char 1 is e
Variable a as a string is abcd
Variable b as a string is efgh
Segmentation fault (core dumped)
*/

Code:

#include <stdio.h>

int main(void)
{
	char *a="abcd"; 
	char *b=malloc(10);
	strcpy (b, "efgh");
	
	printf("Variable a/Char 1 is %c\n", *a);
	printf("Variable b/Char 1 is %c\n", *b);
	printf("Variable a as a string is %s\n", a);
	printf("Variable b as a string is %s\n", b);
	
	*b=*a;
	
	printf ("After assignment...\n");
	printf("Variable a/Char 1 is %c\n", *a);
	printf("Variable b/Char 1 is %c\n", *b);
	printf("Variable a as a string is %s\n", a);
	printf("Variable b as a string is %s\n", b);
	
	return(0);
	
}
/* Program output
Variable a/Char 1 is a
Variable b/Char 1 is e
Variable a as a string is abcd
Variable b as a string is efgh
After assignment...
Variable a/Char 1 is a
Variable b/Char 1 is a
Variable a as a string is abcd
Variable b as a string is afgh
*/

Originally posted by Monster
Okay, but what about char a[]="abcdf"

In the examples, a is the source, and is therefore allowed to be in read only memory.

I expect you meant to ask about char b[]="efgh" ?
In this case, you have an array, not a pointer. The declaration has a different meaning. In short:

>char *b="efgh"
is a pointer to a const array of chars.

>char b[]="efgh"
is an array of chars, containing a default string. The array itself will be in read/write memory.

You can read about this in the C FAQ from comp.lang.c.
Here's a link to it.