C program prints garbage characters - c

Question

I have an assignment to write a program in C that functions similarly to the bash sed 's/oldstring/newstring/g' but only using stdio.h and string.h. We cannot use malloc s we have not yet covered it in class. The program has to continue to take user input until the user enters ^D. We're using GCC so I have it set up to use variable length arrays and I've managed to get the program to find and replace a single instance of oldstring in the user input. However, on occasion the program will output some garbage characters and I am not sure why. I assume it is a memory allocation error or the program is reading past where I want it to read. The code is below:
#include <stdio.h>
#include <string.h>
int isMatch(char * os, char * us){
int i;
char temp[strlen(os)];
for(i=0; i<strlen(us); i++){
int k=0;
for(int j=i; j<i+strlen(os); j++){
temp[k]=us[j];
k++;
}
if(strcmp(temp, os)==0){
return 1;
}
else{
return 0;
}
}
}
void replace(char * os, char * us, char * ns, int loc){
char out[strlen(us) - (strlen(os) - strlen(ns))];
int i;
for(i=0; i<loc; i++){
out[i]=us[i];
}
int k=0;
for(i=loc; i<loc+strlen(ns); i++){
out[i]=ns[k];
k++;
}
k=0;
for(i=loc+strlen(ns); i<strlen(us)-(strlen(os)-strlen(ns)); i++){
out[i]=us[loc+strlen(os)+k];
k++;
}
printf("%s\n", out);
}
int main(int argc, char * argv[]){
char input[100];
int i;
char c;
int match;
while(1){
if(scanf("%c", &c)==EOF){
break;
}
if((input[0]=c) != '\n'){
for(i=1; i<100; i++){
scanf("%c", &input[i]);
if(input[i]=='\n'){
break;
}
}
}
for(i=0; i<100; i++){
match = isMatch(argv[1], &input[i]);
if(match == 1){
replace(argv[1], input, argv[2], i);
}
if(input[i]=='\n'){
break;
}
}
}
}
I call the program with ./a.out aa b for example.
I then enter helaalo and the program spits out helblo which is correct. I then enter libraary and the program outputs librbry followed by some random characters on new lines. I then enter caar and the program outputs cbr followed by even more random letters on new lines. A screenshot of this behavior is included.

Answer 1

Since you can't use malloc, you can force to add terminating \0 for the end of input string and end of out string,
// input
for(i=1; i<100; i++){
scanf("%c", &input[i]);
if(input[i]=='\n'){
input[i] = '\0'; // add terminating \0 for input
break;
}
}
// out
for(i=loc+strlen(ns); i<strlen(us)-(strlen(os)-strlen(ns)); i++){
out[i]=us[loc+strlen(os)+k];
k++;
}
int len = strlen(us)-(strlen(os)-strlen(ns));
out[len] = '\0'; // for out termiante \0
printf("%s\n", out);

Answer 2

The problem is coming from your the way you are filling your input buffer. You don't add a \0 at the end of the string.
There is also an other "problem", when you declare you array out if you want a dynamic size you need to use a malloc. how you are declaring it will not have a dynamic size , the size of the array will be calculated at compilation time. Just to keep it in mind.