Question

I’m trying to self-study C using C Primer Plus from Stephen Prata and one of the end-of-chapter exercises is to “Write a function that replaces the contents of a string with the string reversed.”. This is a chapter on character strings with a good dose of pointers. I’m trying to use pointers as much as possible so I can better understand, but I’m stuck.

My problem is that when I print the value of the return pointer in main, it is garbled.

When I use gdb(just learning how to use that too), I can see that the memory address returned from my function is the same address that was used in the function and it’s getting assigned to my pointer in main okay as far as I can tell.

I’ve tried so many things, what am I missing? FWIW I have not learned about malloc yet in the book, though I see it referenced on various www pages I’ve frequented trying to better understand C.

$ cc -o exercise8 exercise8.c && ./exercise8
This is s1 before: abcd
This is s2 in function: dcba
This is s3 after: d`!

/*   A function that replaces the contents of a string with the string reversed. */
#include <stdio.h>
#include <string.h>
char *str_rev(char * string);
int main(void)
{
   char * s1  = "abcd";
   char * s3;
   printf("This is s1 before: %s\n", s1);

   s3 = str_rev(s1);
   printf("This is s3 after: %s\n", s3);

}

char *str_rev(char * string)
{
   char ar3[5];
   char * s2;
   int len = 0;

   s2 = ar3;

   len = (strlen(string) - 1);
   string = string + len;

   while ( len >= 0 )
   {
      *s2 = *string;
      len--;
      string--;
      s2++;
   }
   s2++;
   *s2 = 0;
   s2 = s2 - 5;

   printf("This is s2 in function: %s\n", s2);
   return s2;
}

$ gdb exercise8
GNU gdb (GDB) 7.1-ubuntu

Reading symbols from exercise8...done.
(gdb) break 12
Breakpoint 1 at 0x804844a: file exercise8.c, line 12.
(gdb) break 40
Breakpoint 2 at 0x80484d9: file exercise8.c, line 40.
(gdb) run
Starting program: exercise8
This is s1 before: abcd         // My original string.
This is s2 in function: dcba        // Good, my reversed string while in the function.

Breakpoint 2, str_rev (string=0xbffff043 "dcba") at exercise8.c:40
40               return s2;
(gdb) print s2
$1 = 0xbffff043 "dcba"          // Location of pointer s2.
(gdb) continue
Continuing.

Breakpoint 1, main () at exercise8.c:12
12               printf("This is s3 after: %s\n", s3);
(gdb) print s3
$2 = 0xbffff043 "dcba"          // Back in main same pointer as s2 from function.
(gdb) step
This is s3 after: d`Q           // Line 12 executed.  Output garbled.
14      }
(gdb)

Answer 1

char ar3[5];
char * s2 = ar3;

The above code will make s2 points to a character string on the stack. This ar3 variable will be deleted once your function finishes.

You should output to some variable that you have pre-allocated. Modify it as follow

int main(void)
{
   char * s1  = "abcd";
   char s3[5];
   printf("This is s1 before: %s\n", s1);

   str_rev(s1, s3);
   printf("This is s3 after: %s\n", s3);

}

void str_rev(char * string, char * s2)
{
   ........

   // don't return

   // Also assign the last character with the NULL terminator
   ar2[strlen(string)] = '\0';
}

Of course, once you get to the chapter regarding malloc, you can allocate the necessary memory for s3 depending on the length of s1. Until then, read on and have fun.