Can functions return strings as their output?

Question

I am trying to implement a code that uses a function called reverse that takes a string and returns a new string that is the reverse of the old string. so far as I can tell the function itself works properly - i.e. I tested to see if the the reversed string: new string had the correct values and it did. However when I try to print this reversed string after it has been returned to the main function I get a bad access code. Does anyone have any ideas? Here is my code:

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

char reverse( char *string)   {
char newstring[50]; //reversed string to be returned to the main function
int size = strlen(string);
int index = 0;
for (index = 0 ; index < size ; index++)   {
    newstring[index] = *(string+size-index-1); //creates a new string that is the reverse of     the old string
}
return newstring; //reurns the new string
}


int main(int argc, const char * argv[]) {

char string[50];
printf("Please enter a string you wish to reverse:\n");
scanf("%49s", string); // user enters a string of maximum length 50
int size = strlen(string);
char *reverse_string;
reverse_string = reverse(string); // sets a pointer called reverse_string to the pointer returned by the reverse function
for ( int index = 0 ; index < size ; index++)   {
    printf("%c", *(reverse_string+index));// prints the reversed string value by value
}
    return 0;
}

Function can return the address of the string, but not the string itself. — chux
– chux, Commented Dec 5, 2014 at 4:05
returning newstring is a problem as newstring is a local variable. — chux
– chux, Commented Dec 5, 2014 at 4:06

TheOmegaPostulate · Accepted Answer · 2014-12-05 04:07:45Z

2

You need to change the declaration of your function so that it returns char* not just a char. In you're main loop you are currently assigning a char to reverse_string not a char*.

answered Dec 5, 2014 at 4:07

TheOmegaPostulate

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2 Comments

Adam Over a year ago

ok so how would I do that? I tried changing the function declaration to: char* reverse( char *string) but when I tried reversing the string: apple all I got was e???0 as a result

TheOmegaPostulate Over a year ago

You will also need to malloc (allocate memory for) the variable newstring within the reverse function as it is a local variable. Remember you can use the sizeof() function here to make things more robust :)

nj-ath · Accepted Answer · 2014-12-05 04:32:26Z

0

Whenever you open braces in c, a new stack frame is created and it is destroyed when you close the the corresponding braces. The problem is all the variables that were created inside these braces are also destroyed and any address you return is useless as it doesn't exist. The workaround for this problem is that you can allocate memory on the heap (using malloc) and return the address (heap is shared by the whole program) or you can pass string as an argument and use that to store the reversed string (pseudo call-by-reference).

answered Dec 5, 2014 at 4:32

nj-ath

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Comments

Markku K. · Accepted Answer · 2014-12-05 04:41:53Z

0

Assuming the declaration of reverse() really has it returning char*, the problem is that you are returning a pointer to a variable (newstring) that was declared locally in reverse(). That means the variable's address is no longer accessible once you exit that function. You have at least these options:

Declare the variable globally (above the function definition).
Create the variable on the heap (e.g. use malloc):

char *newstring = (char*) malloc(50);

answered Dec 5, 2014 at 4:41

Markku K.

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Karthikeyan.R.S · Accepted Answer · 2014-12-05 04:50:58Z

0

char reverse( char *string)

Function returning is a char data type. It will allow you to return a character not a whole string. You can make the data type as returning pointer to char. char *. It will allow you to return a string.

Then make the variable using in that function and allocate the memory with malloc function. If you use the normal variable then it will not work.

edited Dec 5, 2014 at 4:50

answered Dec 5, 2014 at 4:23

Karthikeyan.R.S

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Comments

chux · Accepted Answer · 2014-12-05 14:34:35Z

Code should not attempt to return the address of local variable newstring to be use for later referencing (printf("%c",...) as by that time, the local variable contents and existence are not valid.
When returning a pointer, the function's return type should be a pointer like char *reverse() rather than char reverse().

This all can be solved by reversing the string in place.

void reverse2(char *string) {
  size_t rightp1 = strlen(string);
  size_t left = 0;
  while (left < rightp1) {
    rightp1--; 
    char t = string[left];
    string[left] =  string[rightp1];
    string[rightp1] = t;
    left++;
  }

int main(int argc, const char * argv[]) {
   ...   
   scanf("%49s", string);
   reverse(string);
   fputs(string, stdout);
   return 0;
}

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