My invertString function doesn't seem to work properly - c

Question

So, I have to solve the problem above. I have written some code in C, but the compiler doesn't give me any outputs. Here's the code below.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void invertString(char *s1, char **s2);
void invertString(char *s1, char **s2){
int len = strlen(s1);
*s2 = (char*)malloc((len+1) * sizeof(char));
int start = 0;
int end = 0;
int index = 0;
while (end < len){
while(s1[end]<s1[end+1] && end < len-1)
end++;
for (int i=end;i>=start;i--)
(*s2)[index]=s1[i];
index++;
start = end + 1;
while(s1[end]>=s1[end+1] && end < len-1){
(*s2)[index]=s1[end];
index++;
end++;
start = end;
}
}
(*s2)[index] = '\0';
}
int main() {
char s1[] = "A B C D D D D Z Y X W E F G";
char *s2 = NULL;
invertString(s1, &s2);
printf("Input string: %s\n", s1);
printf("Output string: %s\n", s2);
free(s2);
return 0;
}`
I was expecting the code to output the string such that the substrings in ascending order would be reversed to descending, while the rest of the string would remain the same. Instead, I receive no output. I think the function is stuck in a loop because when I try to re-run, it says the function is already running.

Answer 1

Turns out that because my input string had spaces between all letters, my traverser was completely useless. I had to either iterate +2 every time or delete all spaces in the input. This updated code works fine.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void invertString(char *s1, char **s2);
void invertString(char *s1, char **s2){
int len = strlen(s1);
*s2 = (char*)malloc((len) * sizeof(char));
int start = 0;
int end = 0;
int index = 0;
while (end < len){
printf("%d%d\n", end, len);
while(s1[end]<s1[end+1])
end++;
for (int i=end;i>=start;i--){
(*s2)[index]=s1[i];
index++;
}
end++;
start = end;
while(s1[end]>=s1[end+1]){
(*s2)[index]=s1[end];
index++;
end++;
start = end;
}
}
(*s2)[index] = '\0';
}
int main() {
char s1[] = "ABCDDDDZYXWEFG";
char *s2 = NULL;
invertString(s1, &s2);
printf("Input string: %s\n", s1);
printf("Output string: %s\n", s2);
free(s2);
return 0;
}

Answer 2

Code accesses outside array bounds.
Viewable with some tests using OP's corrected code.
Last error certainly due to insufficient allocation.
// *s2 = (char*)malloc((len) * sizeof(char));
*s2 = malloc((len + 1u) * sizeof(char));
Note: char may be negative.
Perhaps additional errors.
void invertString(char *s1, char **s2){
int len = strlen(s1);
*s2 = (char*)malloc((len) * sizeof(char));
int start = 0;
int end = 0;
int index = 0;
while (end < len){
printf("%d%d\n", end, len);
while(s1[end]<s1[end+1])
end++;
for (int i=end;i>=start;i--){
(*s2)[index]=s1[i];
index++;
}
end++;
start = end;
if (end + 1 > len) {
fprintf(stderr, "%d %d %d\n", __LINE__, end + 1, len);
//exit(-1);
}
while(s1[end]>=s1[end+1]){
(*s2)[index]=s1[end];
index++;
end++;
start = end;
}
}
if (index >= len) {
fprintf(stderr, "%d %d %d\n", __LINE__, index, len);
exit(-1);
}
(*s2)[index] = '\0';
}