以奇怪的方式使用 fwrite

Question

If I write

if((fp = fopen(some_path, "wb")))
{   
    int a = 50000;
    fwrite("c",sizeof(char),a,fp);
    fclose(fp);
    fp = fopen(some_path, "rb");
    char arr[50000];
    fread(arr, sizeof(char), a, fp);
    printf("%s\n", arr);
    fclose(fp);
}

it prints "c" of course but the file is ~50kb

My questions are:

1. How is this actually working?
2. If I modify var a to 60000 the executable crashes, so i`m thinking about some internal buffer of fwrite. How do i get its max capacity?
3. What does fwrite() write to the file in order to get the file to ~50kb of size and still print only "c"(I was expecting some mambo-jumbo characters here)?
4. How wrong is this usage of the function, I want to write a blank file of a certain size really fast(with dummy data), would I be wrong exploiting this behaviour in order not to make a big buffer and use up memory to write "real" data but still reduce fwrite calls(I may need to write a 10 gb file for ex.)?

Answer 1

How is this actually working?

I would argue that it isn't working. You did something nonsense, and it went uncaught. The gave you the impression that it will work in the future. That's a failure.

If I modify var a to 60000 the executable crashes, so i`m thinking about some internal buffer of fwrite. How do i get its max capacity?

There's no buffer. You are merely accessing whatever is in memory after the c␀ created by "c" . When it crashes, it's because you've reached a memory page that can't be read (eg hasn't been allocated).

What does fwrite() write to the file in order to get the file to ~50kb of size

Whatever happens to be in memory at the address returned by "c" and beyond.

and still print only "c"(I was expecting some mambo-jumbo characters here)?

It doesn't print only c . Try something like hexdump -C file or od -c file

How wrong is this usage of the function

Incredibly. It could crash for any value of a larger than 2.

I want to write a blank file of a certain size really fast(with dummy data)

The docs for truncate says: " If the file previously was shorter, it is extended, and the extended part reads as null bytes ('\0'). " So you could use the following:

if (truncate(path, length)) {
   perror("truncate");
   exit(1);
}