C program prints garbage characters

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

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.

Answer 2

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);