fopen / fgets using char* instead of FILE*, why does this work?

Question

I noticed that I had used a char* variable instead of a FILE* variable in my code when using fopen and fgets, but my code works. I am wondering why this is? A section of my code is as follows.

...
char* filePath = ac->filepath;
char* line = malloc(sizeof(char) * MAX_CHAR_PER_LINE) ;
filePath = fopen(filePath, "r"); // we are assigning the result to a char*, not FILE*
if (filePath == NULL) {
    printf ("\n[%s:%d] - error opening file '%s'", __FILE__, __LINE__, filePath);
    printf ("\n\n");
    exit (1);
}

while ((fgets(line, MAX_CHAR_PER_LINE, filePath) != NULL)) {
...

Answer 1

Both a char* and a FILE* simply store a memory address. C has fairly weak typing (Edit: this was a misunderstanding on my part, see comments below) so it lets you assign pointers without worrying about the type they point to.

fopen returns the address of a FILE object and you store that address somewhere (in your case it is in a char* ). When you use the address in fgets it still has the address of the FILE object so everything will work as expected.

Answer 2

Your compiler is very permissive. It should have complained that the FILE* to char* conversion is invalid.

Anyway, assuming the compiler accepts this implicit conversion, the answer is quite simple. By the C standard, a char* and a void* pointer can hold, without loss, any pointer value. So, you can convert from SOMETYPE* to char* and then, back from char* to SOMETYPE*, and you get the same pointer you initially had. Actually, on most systems, all pointers are equivalent and you can freely convert from one to another.

FILE* is a small and opaque value of pointer type. Probably an actual pointer to an internal data structure, but this may be implemented differently (eg it might be a UNIX file descriptor converted to a pointer). The STDIO functions just expect that you use the same opaque FILE* value you got from fopen, in fread/fwrite/fclose.

The FILE* is converted to char* when assigned to filePath. The char* is converted to FILE* as the first parameter of fgets (again, your compiler should complain), and so, gets back to its initial value.

Suggestion: Use higher levels of errors/warnings in your compiler and fix your code.