What happens if I don't call fclose() in a C program?

As long as your program is running, if you keep opening files without closing them, the most likely result is that you will run out of file descriptors/handles available for your process, and attempting to open more files will fail eventually. On Windows, this can also prevent other processes from opening or deleting the files you have open, since by default, files are opened in an exclusive sharing mode that prevents other processes from opening them.

Once your program exits, the operating system will clean up after you. It will close any files you left open when it terminates your process, and perform any other cleanup that is necessary (e.g. if a file was marked delete-on-close, it will delete the file then; note that that sort of thing is platform-specific).

However, another issue to be careful of is buffered data. Most file streams buffer data in memory before writing it out to disk. If you're using FILE* streams from the stdio library, then there are two possibilities:

1. Your program exited normally, either by calling the exit(3) function, or by returning from main (which implicitly calls exit(3)).
2. Your program exited abnormally; this can be via calling abort(3) or _Exit(3), dying from a signal/exception, etc.

If your program exited normally, the C runtime will take care of flushing any buffered streams that were open. So, if you had buffered data written to a FILE* that wasn't flushed, it will be flushed on normal exit.

Conversely, if your program exited abnormally, any buffered data will not be flushed. The OS just says "oh dear me, you left a file descriptor open, I better close that for you" when the process terminates; it has no idea there's some random data lying somewhere in memory that the program intended to write to disk but did not. So be careful about that.

The C standard says that calling exit (or, equivalently, returning from main) causes all open FILE objects to be closed as-if by fclose. So this is perfectly fine, except that you forfeit the opportunity to detect write errors.

EDIT: There is no such guarantee for abnormal termination (abort, a failed assert, receipt of a signal whose default behavior is to abnormally terminate the program -- note that there aren't necessarily any such signals -- and other implementation-defined means). As others have said, modern operating systems will clean up all externally visible resources, such as open OS-level file handles, regardless; however, FILEs are likely not to be flushed in that case.

There certainly have been OSes that did not clean up externally visible resources on abnormal termination; it tends to go along with not enforcing hard privilege boundaries between "kernel" and "user" code and/or between distinct user space "processes", simply because if you don't have those boundaries it may not be possible to do so safely in all cases. (Consider, for instance, what happens if you write garbage over the open-file table in MS-DOS, as you are perfectly able to do.)

Assuming you exit under control, using the exit() system call or returning from main(), then the open file streams are closed after flushing. The C Standard (and POSIX) mandate this.

If you exit out of control (core dump, SIGKILL) etc, or if you use _exit() or _Exit(), then the open file streams are not flushed (but the file descriptors end up closed, assuming a POSIX-like system with file descriptors - Standard C does not mandate file descriptors). Note that _Exit() is mandated by the C99 standard, but _exit() is mandated by POSIX (but they behave the same on POSIX systems). Note that file descriptors are separate from file streams. See the discussion of 'Consequences of Program Termination' on the POSIX page for _exit() to see what happens when a program terminates under Unix.