Why is one seg faulting and the other not?

Question

Hi I have a program that needs to compare a string array to a predefined string but when I use the variable args[0] it works in my function strcmp as below

int gash_execute(char **args){
    int i;

    if(args[0] == NULL){
        return 1;
    }

    for(i = 0; i < gash_command_num(); i++){
        if(strcmp(args[0], functions[i]) == 0){
            return(*function_address[i])(args);     
        } 
    }
    return gash_launch(args);
}

However when trying to strcmp args[i] such as below I get a seg fault. Can anyone help me find a solution to this problem?

int gash_execute(char **args){
    int i;

    if(args[0] == NULL){
        return 1;
    }

    for(i = 0; i < gash_command_num(); i++){
        if(strcmp(args[i], functions[i]) == 0){
            return(*function_address[i])(args);     
        } 
    }
    return gash_launch(args);
}

args[] is an array of strings that were previously separated by spaces, this program is for a custom shell, so pretend in my shell command line I input "cat echo ls" args[0] would be "cat" and so on. However now I need to implement i/o redirection. So i need to check every element of args to check if they represent the symbols "<" ">" "|" and if one of them is we can take it from there

Answer 1

Without seeing all the code, or a report from a tool such as valgrind , I can't say for sure. But I can tell you that this loop is full of potential problems.

for(i = 0; i < gash_command_num(); i++){
    if(strcmp(args[i], functions[i]) == 0){
        return(*function_address[i])(args);     
    } 
}

It's iterating through three arrays ( args , functions , and function_address ) based on some function call which takes none of those as variables ( gash_command_num() ) which has an unknown relation to how many elements are actually in those arrays.

And it's using two global variables ( functions and function_addresses ) which could contain anything.

If all those things are related, I would suggest making that explicit... but I suspect they're not. I suspect your loop logic is wrong. It's comparing args[i] with functions[i] . I suspect gash_command_num() is actually the size of functions and so the loop is walking off args .

What I suspect you really want to do is to see if args[0] matches any function name in functions and then call the related function . If args[0] is ls then you want to check if there's a built in shell function for ls and call it with all the arguments.

Rather than searching a list over and over again, and having to manage two parallel lists, this would be much better served with a hash table . The keys are the function names, the values are the function pointers. C doesn't have a hash table built in, but there's plenty of libraries for that. Gnome Lib is a solid choice for this and many other basic functionality that C is missing.

Using a hash table, and eliminating the globals, your code reduces to this:

/* For encapsulation */
typedef int(*shell_function_t)(char **);

int gash_execute(char **args, GHashTable *shell_functions){
    int i;

    if(args[0] == NULL){
        return 1;
    }

    shell_function_t func = g_hash_table_lookup(shell_functions, args[0]);
    if( func ) {
        return(*func)(args);
    }
    else {
        return gash_launch(args);
    }
}

Scanning for pipes is now a separate problem. For that you do want to loop through args looking for special characters. That is made much simpler if, when you create args , you ensure it ends with a null pointer.

for(int i = 0; args[i] != NULL; i++) {
    ...check if args[i] is special...
}