As a hacker-in-training, I decided to go about making my own string_reverse function that takes a string, allocates memory for a new string, and returns a pointer to a new string, but I’m not getting what I desire, as this returns a segmentation fault.
#include <stdio.h>
#include <stdlib.h>
char* string_reverse(char* string);
char* string_reverse(char* string) {
int len = 0;
for (int i = 0; *(string + i) != '\0'; ++i)
len++;
char* result = (char*)malloc(len * sizeof(char));
if (result == NULL){
puts("Pointer failure");
exit(EXIT_FAILURE);
}
for (int i = 0; *(string + i) != '\0'; ++i)
*(result + (len - i)) = *(string + i);
return *result;
}
int main() {
char* str= "Ni Hao!";
char* result = string_reverse(str);
printf("%s\n", result);
free(result);
return 0;
}
In return, I get this debugging message:
Starting program: /home/tmo/string_reverse
Program received signal SIGSEGV, Segmentation fault.
0xb7e5b3b3 in strlen () from /lib/i686/cmov/libc.so.6
How should I interpret this result?
,
Your code didn’t add the null terminator to the reversed string. As a result the printf function crashed trying to calculate it’s length.
Change the malloc line to the following
char* result = (char*)malloc((len+1) * sizeof(char));
And you need to add the following line to the end of the string_reverse function in order to ensure the string has a null terminator.
resultlen = '\0';
Couple of other comments
- sizeof(char) is not needed. The size of the char is one of the few types defined by the C standard and it’s value is 1.
- The first loop can be replaced by a simple call to strlen
EDIT
Two other issues. The line which actually does the character copy appears to be incorrect. I believe it should be (len – i – 1). Otherwise the initial character write will occur at (result + len) which is the place of the null terminator.
*(result + ((len - i) - 1)) = *(string + i);
Also, don’t dereference result on return
,
Also you shouldn’t be dereferencing result at the end of the function because its allready a pointer to your resulting string.
return result;