Question

Upon compiling and running this small program to reverse a string, I get a Segmentation Fault before any output occurs. Forgive me if this is an obvious question, I’m still very new to C.

#include <stdio.h>

int reverse(char string[], int length); 

int main() {
char string[] = "reversed";

  printf("String at start of main = %s", string);
  reverse(string, sizeof(string));
  printf("%s\n", string);

return 0;

}

// Reverse string 
int reverse(char string[], int length) {
 int i;
 char reversed[] = {};
 int temp;

 for(i = 0; i < length; ++i) {
 temp = string[i];
 reversed[length - i] = temp;

 }
 return 0; 
}

Answer 1

Because of this:

First you create an array with zero elements:

char reversed[] = {};

And later you attempt to write to that array beyond its bounds:

reversed[length - i] = temp;

Update:

The above means that you need to allocate memory whose size is only known at runtime (it’s length). The usual C-style way of doing this is… by pushing the burden of memory allocation to your caller:

int reverse(const char* string, char* destination, int length);

This function would write to a buffer provided by the caller, who now also must ensure that:

1. The buffer is large enough
2. The memory for buffer gets freed when it should be