Question

I am trying to find out filetypes using c code, here is the code

char *get_file_type(char *path, char *filename)
{
    FILE *fp;
    char command[100];
    char file_details[100];
    char *filetype;

    sprintf(command, "file -i %s%s", path, filename);
    fp = popen(command, "r");
    if (fp == NULL) {
        printf("Failed to run command\n" );
        exit(1);
    }
    while (fgets(file_details,  sizeof(file_details)-1, fp) != NULL) {
         filetype = (strtok(strstr(file_details, " "), ";"));
    }

    pclose(fp);
    return filetype;
}

here instead of declaring command[], can I use *command? I tried to use it, but it throwed an exception. we dont need to free up variables declared like command[]? if yes how?

Answer 1

You can use char *command;, but then, you must allocate some memory for commandto refer to with a call to malloc() and when you are done ith that memory, it has to be freed again with a call to free().

As you can see, that is a lot more work than using a fixed-size array (as you do now), but it can be made a lot safer as well, because you could create a buffer of exactly the right size, instead of hoping that the total length of the command won’t exceed 100 characters.

Aside from that, your code has a problem: The filetype pointer that the function returns points to a location within the array file_details, but that array will be cleaned up by the compiler when executing the return statement, so the pointer that gets returned by the function refers to some memory that is marked as “free to be used for other purposes”.

If it is not a problem that the result of get_file_type is only valid for one file at a time, you can declare the file_details array as static, so that it will be preserved across calls to the function.