Question

I’m creating a program to open .txt files in a given directory, I have an array with all the absolute paths of the files inside the directory in question and I’m creating a function to extract and return the name of the files, the function is written as follows:

char *name(char *string) {
    int i = strlen(string);
    char *aux;
    while(string[i-1] != '/'){
        i--;
    }
    strcpy(aux, &string[i]); 
    return aux;
}

The above function is giving a Segmentation Fault error, but if I add the following line ” int j = 0;” before the declaration of aux the mistake is gone, the new and working code is

char *name(char *string) {
    int i = strlen(string);
    int j = 0;
    char *aux;
    while(string[i-1] != '/'){
        i--;
    }
    strcpy(aux, &string[i]); 
    return aux;
}

input: C:\test\a.txt
output: a.txt

Why the addition of “int j = 0;” solves the problem? I’m stuck with that and can’t continue because I don’t know if this inconsistency might lead to bigger problems later, I’m thinking about writing my own function to copy the strings, but before that I really want to understand that error.

Answer 1

You never allocate aux. aux needs to point to a valid memory location before you attempt to copy anything to it.

Instead of char *aux, you need something like char *aux = malloc(i+1);. Note that i+1 is overkill because in your case aux will always be at least 3 characters shorter than string (it won’t contain C:\), but you probably don’t care for such small strings. Remember to free() the pointer once you’re done with it.

Also, the reason you found it works by switching orders of declarations and/or adding a declaration is probably that you got lucky and somehow the location to which aux points to is valid (if you do just char *aux;, aux points to a random location). This is pure luck however, and is still invalid code even though it seems to work.

In the future, you might want to use a tool like Valgrind to help you diagnose memory problems. You should also read a tutorial on basic memory management and pointers in C.