Thread: char pointers and arrays

while working on a simple little utility I came across something I don't quite understand
why is it that

Code:

 char str[] = "first,second,third";

works for strtok()
but

Code:

  char *str = "first,second,third";

ends in a segfault that even though I trace the stack I cant figure out why for the life of me

Now it's been a while since I was in class for C, since I've been programming professionally for years but to the best of my knowledge
char *blah = "this is a const char *";
and char blah[] = "this is a const char*";
were the same exact thing.

The examples were compiled on a Linux x86 system
anyway, here's the sample codes

Code:

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
	char str[] = "first,second,third";
	char *result = NULL;
	char *ptr = str;
	printf("%s\n",str);
	result=strtok(ptr,",");
	while(result)
	{
		printf("%s\n",result);
		result = strtok(NULL,",");
	}
	return 0;
}

the above WORKS

Code:

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
	char *str = "first,second,third";
	char *result = NULL;
	char *ptr = str;
	printf("%s\n",str);
	result=strtok(ptr,",");
	while(result)
	{
		printf("%s\n",result);
		result = strtok(NULL,",");
	}
	return 0;
}

the above results in a segmentation fault

1. You cannot modify string literal like "helo" it results in undefined behaviour.
e.g char *p = "crash";
*p = 'C'; // undefined behaviour!
2. strtok() modifies the string. RTFM for more details.

while looking at the assemnly I learned that the way the compiler arranges the two char arrays
for char *
the string is set in static memory (i.e. .LC0 .string "blahblah")
where as
char []
pushes the array onto the stack and copies the characters word (4bytes) at a time

Code:

char *
.LC0:
	.string	"first,second,third"
...
	movl	$.LC0, 20(%esp)

Code:

char []
	movl	$1936877926, 21(%esp)
	movl	$1702046836, 25(%esp)
	movl	$1684959075, 29(%esp)
	movl	$1768453164, 33(%esp)
	movw	$25714, 37(%esp)

so the answer that I came up with in a simplified manner is that with a char *blah = "words" the characters are static and cannot be manipulated while char [] can be manipulated similar to the old const char * vs char *

I cant believe it took me to disassemble the program to figure out the obvious.