# read: analogously to write, support reading from in-memory streams in # addition to file descriptors. # # We can pass it either a file descriptor or an address to a stream. If a # file descriptor is passed in, we _read from it using the right syscall. If a # stream is passed in (a fake file descriptor), we read from it instead. This # lets us initialize input for tests. # # A little counter-intuitively, the output of 'read' ends up in.. a stream. So # tests end up doing a redundant copy. Why? Well, consider the alternatives: # # a) Reading into a string, and returning a pointer to the end of the read # region, or a count of bytes written. Now this count or end pointer must be # managed separately by the caller, which can be error-prone. # # b) Having 'read' return a buffer that it allocates. But there's no way to # know in advance how large to make the buffer. If you read less than the # size of the buffer you again end up needing to manage initialized vs # uninitialized memory. # # c) Creating more helpful variants like 'read-byte' or 'read-until' which # also can take a file descriptor or stream, just like 'write'. But such # primitives don't exist in the Linux kernel, so we'd be implementing them # somehow, either with more internal buffering or by making multiple # syscalls. # # Reading into a stream avoids these problems. The buffer is externally # provided and the caller has control over where it's allocated, its lifetime, # and so on. The buffer's read and write pointers are internal to it so it's # easier to keep in a consistent state. And it can now be passed directly to # helpers like 'read-byte' or 'read-until' that only need to support streams, # never file descriptors. # # Like with 'write', we assume our data segment will never begin at an address # shorter than 0x08000000, so any smaller arguments are assumed to be real # file descriptors. # # As a reminder, a stream looks like this: # write: int # index at which to write to next # read: int # index at which to read next # data: (array byte) # prefixed by size as usual == code # instruction effective address register displacement immediate # . op subop mod rm32 base index scale r32 # . 1-3 bytes 3 bits 2 bits 3 bits 3 bits 3 bits 2 bits 2 bits 0/1/2/4 bytes 0/1/2/4 bytes read: # f: fd or (addr stream byte), s: (addr stream byte) -> num-bytes-read/eax: int # . prologue 55/push-ebp 89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp # if (f < 0x08000000) return _read(f, s) # f can't be a user-mode address, so treat it as a kernel file descriptor 81 7/subop/compare 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 0x08000000/imm32 # compare *(ebp+8) 73/jump-if-addr>= $read:fake/disp8 # . . push args ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 0xc/disp8 . # push *(ebp+12) ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 . # push *(ebp+8) # . . call e8/call _read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp # return eb/jump $read:end/disp8 $read:fake: # otherwise, treat 'f' as a stream to scan from # . save registers 56/push-esi 57/push-edi # esi = f 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 8/disp8 . # copy *(ebp+8) to esi # edi = s 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 7/r32/edi 0xc/disp8 . # copy *(ebp+12) to esi # eax = _buffer-4(out = &s->data[s->write], outend = &s->data[s->size], # in = &f->data[f->read], inend = &f->data[f->write]) # . . push &f->data[f->write] 8b/copy 0/mod/indirect 6/rm32/esi . . . 0/r32/eax . . # copy *esi to eax 8d/copy-address 1/mod/*+disp8 4/rm32/sib 6/base/esi 0/index/eax . 0/r32/eax 0xc/disp8 .
package twofer

import "testing"

// Define a function ShareWith(string) string.

var tests = []struct {
	name, expected string
}{
	{"", "One for you, one for me."},
	{"Alice", "One for Alice, one for me."},
	{"Bob", "One for Bob, one for me."},
}

func TestShareWith(t *testing.T) {
	for _, test := range tests {
		if observed := ShareWith(test.name); observed != test.expected {
			t.Fatalf("ShareWith(%s) = \"%v\", want \"%v\"", test.name, observed, test.expected)
		}
	}
}

func BenchmarkShareWith(b *testing.B) {
	for i := 0; i < b.N; i++ {

		for _, test := range tests {
			ShareWith(test.name)
		}

	}
}
# add to esp # write(_test-stream, "Ab") # . . push args 68/push "Ab"/imm32 68/push _test-stream/imm32 # . . call e8/call write/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp # eax = read(_test-stream, _test-tmp-stream) # . . push args 68/push _test-tmp-stream/imm32 68/push _test-stream/imm32 # . . call e8/call read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp # check-ints-equal(eax, 2, msg) # . . push args 68/push "F - test-read-single: return value"/imm32 68/push 2/imm32 50/push-eax # . . call e8/call check-ints-equal/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp # check-stream-equal(_test-tmp-stream, "Ab", msg) # . . push args 68/push "F - test-read-single"/imm32 68/push "Ab"/imm32 68/push _test-tmp-stream/imm32 # . . call e8/call check-stream-equal/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp # end c3/return test-read-is-stateful: # - make two consecutive reads, check that their results are appended # clear-stream(_test-stream) # . . push args 68/push _test-stream/imm32 # . . call e8/call clear-stream/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 4/imm32 # add to esp # clear-stream(_test-tmp-stream) # . . push args 68/push _test-tmp-stream/imm32 # . . call e8/call clear-stream/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 4/imm32 # add to esp # write(_test-stream, "C") # . . push args 68/push "C"/imm32 68/push _test-stream/imm32 # . . call e8/call write/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp # read(_test-stream, _test-tmp-stream) # . . push args 68/push _test-tmp-stream/imm32 68/push _test-stream/imm32 # . . call e8/call read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp # write(_test-stream, "D") # . . push args 68/push "D"/imm32 68/push _test-stream/imm32 # . . call e8/call write/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp # read(_test-stream, _test-tmp-stream) # . . push args 68/push _test-tmp-stream/imm32 68/push _test-stream/imm32 # . . call e8/call read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp # check-stream-equal(_test-tmp-stream, "CD", msg) # . . push args 68/push "F - test-read-is-stateful"/imm32 68/push "CD"/imm32 68/push _test-tmp-stream/imm32 # . . call e8/call check-stream-equal/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp # end c3/return test-read-returns-0-on-end-of-file: # - read after hitting end-of-file, check that result is 0 # setup # . clear-stream(_test-stream) # . . push args 68/push _test-stream/imm32 # . . call e8/call clear-stream/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 4/imm32 # add to esp # . clear-stream(_test-tmp-stream) # . . push args 68/push _test-tmp-stream/imm32 # . . call e8/call clear-stream/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 4/imm32 # add to esp # . write(_test-stream, "Ab") # . . push args 68/push "Ab"/imm32 68/push _test-stream/imm32 # . . call e8/call write/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp # first read gets to end-of-file # . read(_test-stream, _test-tmp-stream) # . . push args 68/push _test-tmp-stream/imm32 68/push _test-stream/imm32 # . . call e8/call read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp # second read # . read(_test-stream, _test-tmp-stream) # . . push args 68/push _test-tmp-stream/imm32 68/push _test-stream/imm32 # . . call e8/call read/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp # check-ints-equal(eax, 0, msg) # . . push args 68/push "F - test-read-returns-0-on-end-of-file"/imm32 68/push 0/imm32 50/push-eax # . . call e8/call check-ints-equal/disp32 # . . discard args 81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp # end c3/return == data _test-tmp-stream: # (stream byte) # current write index 0/imm32 # current read index 0/imm32 # size 8/imm32 # data 00 00 00 00 00 00 00 00 # 8 bytes # . . vim:nowrap:textwidth=0