need help with the following C function ...

purpleburple · 11-12-2002, 09:40 PM

Hi. Can someone help me with the following code? I know something is wrong but not what it is.

gcc gives me these errors>
reverse_str.c: In function `reverse_string':
reverse_str.c:31: warning: return makes integer from pointer without a cast
reverse_str.c:31: warning: function returns address of local variable

--------------------------------------------------------
#include <stdio.h>

char reverse_string(char *str);

int main(void)
{

char *string = "This will print backwards";

printf("%s\n", reverse_string(string));

return 0;

}
char reverse_string(char *str)
{

char str2[40];
int x = 0;
int y = 0;

while (str[x] != '\0')
x++;

while ( x >= 0)
str2[y++] = str[x--];

/* append NULL character to string */
str2[y] = '\0';

return str2;

}

thanks

niac · 11-13-2002, 07:49 AM

Your algo is not correct x get the value where str[x]='\0'
then you make str2[0]=str[x++]
str2[0]='\0' not what you want. Thus printf() will stop at the first cell.

Moreover you return a pointer which is local : try to avoid this. Local varilable are deleted after exiting the function.

Here will be the correct way :

#include <stdio.h>

void reverse_string(char *str, char *str2);

void reverse_string(char *str, char *str2) {
int x = 0;
int y = 0;
while (str[x] != '\0')
x++;
while ( x >= 0)
str2[y++] = str[--x];
/* append NULL character to string */
str2[y] = '\0';
}

int main(void) {
char *string = "This will print backwards";
char str2[40];
reverse_string(string,str2);
printf("%s\n", str2);
return 0;
}

Mik · 11-13-2002, 07:54 AM

Well there are many things which are wrong with your program.
- You should be returning a char * not just a character in the reverse_string function.
- You shouldn't return an address to a local variable. You could copy str2 into str and then return str instead of str2.
- You shouldn't assign a literal string to a string pointer. Use strcpy for that.
- The second while loop starts with copying the \0 into the string so the result string will always be empty. You could use --x instead of x-- to solve this.
- The null character is appended one place to far so you should use y - 1 for that.

Well after you change all that it should work. It did here at least.

Hmm looks like I was too slow again.

GtkUser · 11-13-2002, 07:59 PM

Code:

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

char *  func (char *, int);

int main()
{

  char str [] = "Here is a string";
  printf("%s\n", func ( str, strlen(str) - 1) ) ;

  return 0;
}

char *  func (char  * s, int size)
{
  char *ptr = s;
  char * rStr = malloc (256);
  int index = 0;

  while( size >= 0)
    {
      rStr[index] = *( ptr + size);
      ++index;
      --size;
      ptr = s;
    }
  rStr[index] = '\0';
  return rStr;
}

Here is one version. Geez, Gnome terminal is a piece of junk. I had to use XTerm to get this program to display properly. Anyway, the main point is that you have to declare the return string as static storage space on the free store. If you just make a local variable than the pointer that you are returning to it will point to unallocated memory as soon as the function returns, the stack is released.

BTW I'm sure there are nicer examples, but this one does work.