Thread: can't copy value back into pointer

Hi,

I want to implement a version of strlwr() but can't seem to be copying the lower case value back into the string.
My test program with the function looks like:

Code:

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

int strlwr(char* s)
{
    char* tmp = s;
    char r;

    while (*tmp) {
        printf("tmp %c\n",*tmp);
        r = tolower(*tmp);
        printf("r %c\n",r);
        *tmp = r;
        tmp++;
    }

    return 0;
}

int main (void)
{
  char *addr = "AB:CD:EF:00:12:34";
  printf("addr: %s\n", addr);
  strlwr(addr);
  printf("lower: %s\n", addr);

return 0;
}

and I get a seg fault at *tmp = r; but I don't undersatand why, any help would be greatly appreciated!

Thanks!

Code:

char *addr = "AB:CD:EF:00:12:34";

You cannot modify a string literal (which is what "addr" points to).

Either make "addr" an array, or allocate memory, so you can work with a modifiable string.

Originally Posted by Matticus

Code:

char *addr = "AB:CD:EF:00:12:34";

You cannot modify a string literal (which is what "addr" points to).

Either make "addr" an array, or allocate memory, so you can work with a modifiable string.

Oh yes, that's what it was! Thank you!

What's the advantage (if any) from using a temporary pointer (tmp) rather than working with the s directly?

In this case, there are no advantages to using "tmp", since any modifications made to the value of pointer "s" are local to the function and not seen by the caller.

Code:

#include <stdio.h>

void print_string(const char *s);

int main(void)
{
    const char *s = "Test";

    puts(s);
    print_string(s);
    puts(s);

    return 0;
}

void print_string(const char *s)
{
    while( *s )
        putchar(*(s++));
    putchar('\n');
}