Program works but outputs trailing garbage values - c

Question

I am working on a code that gives the steps to change one string into another and it works fine but gives garbage values.
The code is:
void steps(char str1[],char str2[]) {
int belongs,req;
strlwr(str1);
strlwr(str2);
if(strlen(str1)==strlen(str2)) {
for(int i=0;i<=strlen(str1);i++) {
if(str2[i]!=str1[i]) {
++changes;
printf("%d::SUBSTITUTION:: %c <--- %c\n",changes,str1[i],str2[i]);
str1[i]=str2[i];
printf("--->%s\n",str1);
}
}
}
if(strlen(str1)>strlen(str2)){
for(int i=strlen(str1);i>=strlen(str2);i--) {
if(str1[i]=='\0') {
}
else {
++changes;
printf("%d::DELETE:: %c\n",changes,str1[i]);
str1[i]=0;
printf("--->%s\n",str1);
}
}
}
if(strlen(str1)<strlen(str2)) {
for(int i=(strlen(str1));i<=(strlen(str2)-1);i++) {
++changes;
printf("%d::ADD:: %c\n",changes,str2[i]);
str1[i]=str2[i];
printf("-->%s\n",str1);
}
}
steps(str1,str2);
}
The output for the input strings, suppose 'sym' and 'symbiosis' is:
1::ADD:: b
-->symb
2::ADD:: i
-->symbi
3::ADD:: o
-->symbio
4::ADD:: s
-->symbios
5::ADD:: i
*-->symbiosiDzb
6::ADD:: s
-->symbiosis²b
7::DELETE:: b
--->symbiosis²
8::DELETE:: ²
--->symbiosis*
At step 5 and beyond, why is it showing me garbage values?
I tried the best I could and due to my limited knowledge in C (and
pointers) I am reluctant to use pointers and hence hesitate a bit
with DMA. Also, a strange thing that I noticed is that the garbage
values occur only when the difference between the strings is large.
Please help me fix it!

Answer 1

if(strlen(str1)<strlen(str2)){
for(int i=(strlen(str1));i<=(strlen(str2)-1);i++){
++changes;
printf("%d::ADD:: %c\n",changes,str2[i]);
str1[i]=str2[i];
printf("-->%s\n",str1);
}}
In that snippet, the part where you do str1[i]=str2[i]; is probably illegal.
First, strings in C are nullterminated. That means a string ends when a nullbyte comes. So if you add a character, you are overwriting the nullbyte. Therefore, you have to make sure that the character following it is a nullbyte, otherwise the string ends when the next nullbyte comes in memory. That's where the garbage data comes from.
So it would be:
str1[i]=str2[i];
str1[i + 1]=0;
Additionally, you have to make sure that the buffer is large enough. That means that the memory you reserved for str1 when calling steps might not be enough for you to append characters. Technically you can still add new characters, but what you are probably doing is overflowing the buffer and writing to memory that you don't 'own'. Which could be critical.
What you could do is allocate a new buffer with enough memory which holds your new string. You can then copy str1 into it and then append the characters from str2.
For example like this:
char buffer[512];
buffer[0] = 0;
// Write str1 into the buffer
strcat(buffer, str1);
// [...]
// Instead of 'str1[i]=str2[i];'
buffer[i]=str2[i];
buffer[i + 1] = 0;
In this example, you still have to make sure that both str1 and str2 are smaller than 512 bytes though, otherwhise you have to increase the buffer or allocate buffer on the heap using malloc (and free afterwards).
To provide a better example on how to do it, it might be helpful to see how you call the steps function.

Answer 2

Thanks everybody!
I was able to resolve the problem...
The working code is:
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
int changes=0;
void steps(char str1[],char str2[]){
char* temp;
temp=(char *)malloc((strlen(str2)));//Buffer of size of string 2 to store string 1
unsigned int length_1=strlen(str1);
unsigned int length_2=strlen(str2);
int belongs,req;
strlwr(str1);
strlwr(str2);//To avoid conversion of same letters but in different cases
//Strings of equal length:
if(strlen(str1)==strlen(str2)){
for(int i=0;i<=strlen(str1);i++){
//Strings with no swapping possible:
if(str2[i]!=str1[i]){
++changes;
printf("%d::SUBSTITUTION:: %c <--- %c\n",changes,str1[i],str2[i]);
str1[i]=str2[i];
printf("--->%s\n",str1);
}}}
//String 1 is longer than string 2, hence deletion:
if(strlen(str1)>strlen(str2)){
for(int i=strlen(str1);i>=strlen(str2);i--){
++changes;
printf("%d::DELETE:: %c\n",changes,str1[i]);
str1[i]=0;
printf("--->%s\n",str1);
}
}
//String 2 is longer than string 1, hence addition:
if(strlen(str1)<strlen(str2)){
strcpy(temp,str1);
for(int i=(strlen(str1));i<strlen(str2);i++){
if (str2[i]=='\0'){
}
else{
++changes;
printf("%d::ADD:: %c\n",changes,str2[i]);
*(temp+i)=str2[i];
*(temp+1+i)=0;
printf("-->%s\n",temp);
}}
char newstr[length_2];//Use to store 'temp' and use in steps function again
strcpy(newstr,temp);
steps(newstr,str2);
}
free(temp); //Release memory
}//End!