C program to reverse a string

This is a very simple C program to reverse all the characters in a given string. As for example considering the string “hello world”, the program should output “dlrow olleh”. The string reversal problem is a common embedded software interview question asked for both entry level and middle level firmware engineering positions.

#include <stdio.h>

void strRev(char *str)
{
  char temp;
  char *begin = str;
  char *end = str;

  if (!str || !(*str))
    goto error;

  // Go to the end of the string
  while (*(++end))
    ;

  // Discard the null character
  end--;

  while (begin != end)
  {
    // Swap
    temp = *end;
    *end = *begin;
    *begin = temp;
    
    // Move the pointers
    begin++;
    end--;
  }

 error:
  return;
}

int main(void)
{
  char str[] = "hello world";

  puts(str);
  strRev(str);
  puts(str);
    
  return 0;
}

Code walk-through

This simple C program is to reverse a string. Reversing a string is a classic interview question which is asked commonly. In this given example, the function strRev() reverses the characters present in the character array.

The character pointers begin and end are assigned with the address of the received string str.

char *begin = str;
  char *end = str;

The lines,

if (!str || !(*str))
    goto error;

checks for a null pointer or for a null value and jumps to error: label.

A character pointer end is moved to the last character (excluding the null character) in the received string in the following lines,

// Go to the end of the string
  while (*(++end))
    ;
 
  // Discard the null character
  end--;

Finally the begin and end pointers’ dereferenced characters are swapped until all characters are swapped.

while (begin != end)
  {
    // Swap
    temp = *end;
    *end = *begin;
    *begin = temp;
    
    // Move the pointers
    begin++;
    end--;
  }

Now the string characters are reversed and the new string is printed from main().