> asking for 1 byte 65536 times, is indeed different than asking for 65536 bytes, 1 time.

Yes it's different. As others have noted, the difference is what is returned if less than 65536 are available to read in the file: total failure vs partial read.

There is, unsurprisingly, no requirement that it has an unnecessarily inefficient implementation to meet this behavioral requirement. (The C standard doesn't talk about such things as syscalls but, even if it did, it surely wouldn't require such a thing.)

The irony is that that partial read is actually the default on both Windows and Posix (i.e. both ReadFile and read() will read up to the number of bytes specified). So a one-syscall implementation for fread would have been easier than multiple calls, and certainly would be standard compliant.

The dd example isn't comparable because dd is much lower level, and you really are specifying how the syscalls should be made.