diff --git a/vlib/io/os_file_reader_test.v b/vlib/io/os_file_reader_test.v
index 39606c8a03..409845f9f4 100644
--- a/vlib/io/os_file_reader_test.v
+++ b/vlib/io/os_file_reader_test.v
@@ -20,7 +20,7 @@ fn read_file(file string, cap int) []string {
 }
 
 fn test_file_reader() {
-	for cap := 64; cap <= 10000; cap += 256 {
+	for cap := 1; cap <= 10000; cap += 256 {
 		lines := read_file(@FILE, cap)
 		assert lines.last() == '// my last line'
 	}
diff --git a/vlib/os/file.c.v b/vlib/os/file.c.v
index 5e008a3a24..2f88ca0c22 100644
--- a/vlib/os/file.c.v
+++ b/vlib/os/file.c.v
@@ -13,8 +13,132 @@ struct FileInfo {
 	size int
 }
 
+// open_file can be used to open or create a file with custom flags and permissions and returns a `File` object.
+pub fn open_file(path string, mode string, options ...int) ?File {
+	mut flags := 0
+	for m in mode {
+		match m {
+			`w` { flags |= o_create | o_trunc }
+			`a` { flags |= o_create | o_append }
+			`r` { flags |= o_rdonly }
+			`b` { flags |= o_binary }
+			`s` { flags |= o_sync }
+			`n` { flags |= o_nonblock }
+			`c` { flags |= o_noctty }
+			`+` { flags |= o_rdwr }
+			else {}
+		}
+	}
+	if mode == 'r+' {
+		flags = o_rdwr
+	}
+	if mode == 'w' {
+		flags = o_wronly | o_create | o_trunc
+	}
+	if mode == 'a' {
+		flags = o_wronly | o_create | o_append
+	}
+	mut permission := 0o666
+	if options.len > 0 {
+		permission = options[0]
+	}
+	$if windows {
+		if permission < 0o600 {
+			permission = 0x0100
+		} else {
+			permission = 0x0100 | 0x0080
+		}
+	}
+	mut p := path
+	$if windows {
+		p = path.replace('/', '\\')
+	}
+	fd := C.open(charptr(p.str), flags, permission)
+	if fd == -1 {
+		return error(posix_get_error_msg(C.errno))
+	}
+	cfile := C.fdopen(fd, charptr(mode.str))
+	if isnil(cfile) {
+		return error('Failed to open or create file "$path"')
+	}
+	return File{
+		cfile: cfile
+		fd: fd
+		is_opened: true
+	}
+}
+
+// open tries to open a file for reading and returns back a read-only `File` object.
+pub fn open(path string) ?File {
+	/*
+	$if linux {
+		$if !android {
+			fd := C.syscall(sys_open, path.str, 511)
+			if fd == -1 {
+				return error('failed to open file "$path"')
+			}
+			return File{
+				fd: fd
+				is_opened: true
+			}
+		}
+	}
+	*/
+	cfile := vfopen(path, 'rb') ?
+	fd := fileno(cfile)
+	return File{
+		cfile: cfile
+		fd: fd
+		is_opened: true
+	}
+}
+
+// create creates or opens a file at a specified location and returns a write-only `File` object.
+pub fn create(path string) ?File {
+	/*
+	// NB: android/termux/bionic is also a kind of linux,
+	// but linux syscalls there sometimes fail,
+	// while the libc version should work.
+	$if linux {
+		$if !android {
+			//$if macos {
+			//	fd = C.syscall(398, path.str, 0x601, 0x1b6)
+			//}
+			//$if linux {
+			fd = C.syscall(sys_creat, path.str, 511)
+			//}
+			if fd == -1 {
+				return error('failed to create file "$path"')
+			}
+			file = File{
+				fd: fd
+				is_opened: true
+			}
+			return file
+		}
+	}
+	*/
+	cfile := vfopen(path, 'wb') ?
+	fd := fileno(cfile)
+	return File{
+		cfile: cfile
+		fd: fd
+		is_opened: true
+	}
+}
+
+// open_stdin - return an os.File for stdin, so that you can use .get_line on it too.
+pub fn open_stdin() File {
+	return File{
+		fd: 0
+		cfile: C.stdin
+		is_opened: true
+	}
+}
+
 // **************************** Write ops  ***************************
-// write implements the Writer interface
+// write implements the Writer interface.
+// It returns how many bytes were actually written.
 pub fn (mut f File) write(buf []byte) ?int {
 	if !f.is_opened {
 		return error('file is not opened')
@@ -22,8 +146,8 @@ pub fn (mut f File) write(buf []byte) ?int {
 	/*
 	$if linux {
 		$if !android {
-			C.syscall(sys_write, f.fd, s.str, s.len)
-			return
+			res := C.syscall(sys_write, f.fd, s.str, s.len)
+			return res
 		}
 	}
 	*/
@@ -34,6 +158,8 @@ pub fn (mut f File) write(buf []byte) ?int {
 	return written
 }
 
+// writeln writes the string `s` into the file, and appends a \n character.
+// It returns how many bytes were written, including the \n character.
 pub fn (mut f File) writeln(s string) ?int {
 	if !f.is_opened {
 		return error('file is not opened')
@@ -59,6 +185,8 @@ pub fn (mut f File) writeln(s string) ?int {
 	return (written + 1)
 }
 
+// write_string writes the string `s` into the file
+// It returns how many bytes were actually written.
 pub fn (mut f File) write_string(s string) ?int {
 	if !f.is_opened {
 		return error('file is not opened')
@@ -71,19 +199,34 @@ pub fn (mut f File) write_string(s string) ?int {
 	return written
 }
 
-// write_to implements the RandomWriter interface
+// write_to implements the RandomWriter interface.
+// It returns how many bytes were actually written.
+// It resets the seek position to the end of the file.
 pub fn (mut f File) write_to(pos int, buf []byte) ?int {
+	if !f.is_opened {
+		return error('file is not opened')
+	}
 	C.fseek(f.cfile, pos, C.SEEK_SET)
 	res := int(C.fwrite(buf.data, 1, buf.len, f.cfile))
+	if res == 0 && buf.len != 0 {
+		return error('0 bytes written')
+	}
 	C.fseek(f.cfile, 0, C.SEEK_END)
 	return res
 }
 
+// write_bytes writes `size` bytes to the file, starting from the address in `data`.
+// NB: write_bytes is unsafe and should be used carefully, since if you pass invalid
+// pointers to it, it will cause your programs to segfault.
 [unsafe]
 pub fn (mut f File) write_bytes(data voidptr, size int) int {
 	return int(C.fwrite(data, 1, size, f.cfile))
 }
 
+// write_bytes_at writes `size` bytes to the file, starting from the address in `data`,
+// at byte offset `pos`, counting from the start of the file (pos 0).
+// NB: write_bytes_at is unsafe and should be used carefully, since if you pass invalid
+// pointers to it, it will cause your programs to segfault.
 [unsafe]
 pub fn (mut f File) write_bytes_at(data voidptr, size int, pos int) int {
 	C.fseek(f.cfile, pos, C.SEEK_SET)
@@ -93,12 +236,13 @@ pub fn (mut f File) write_bytes_at(data voidptr, size int, pos int) int {
 }
 
 // **************************** Read ops  ***************************
-// read_bytes reads bytes from the beginning of the file
+// read_bytes reads bytes from the beginning of the file.
+// Utility method, same as .read_bytes_at(size, 0).
 pub fn (f &File) read_bytes(size int) []byte {
 	return f.read_bytes_at(size, 0)
 }
 
-// read_bytes_at reads bytes at the given position in the file
+// read_bytes_at reads `size` bytes at the given position in the file.
 pub fn (f &File) read_bytes_at(size int, pos int) []byte {
 	mut arr := []byte{len: size}
 	nreadbytes := f.read_bytes_into(pos, mut arr) or {
@@ -109,8 +253,8 @@ pub fn (f &File) read_bytes_at(size int, pos int) []byte {
 }
 
 // read_bytes_into fills `buf` with bytes at the given position in the file.
-// `buf` must have length greater than zero.
-// Returns number of bytes read or an error.
+// `buf` *must* have length greater than zero.
+// Returns the number of read bytes, or an error.
 pub fn (f &File) read_bytes_into(pos int, mut buf []byte) ?int {
 	if buf.len == 0 {
 		panic(@FN + ': `buf.len` == 0')
@@ -129,7 +273,7 @@ pub fn (f &File) read_bytes_into(pos int, mut buf []byte) ?int {
 	return nbytes
 }
 
-// read implements the Reader interface
+// read implements the Reader interface.
 pub fn (f &File) read(mut buf []byte) ?int {
 	if buf.len == 0 {
 		return 0
@@ -142,7 +286,7 @@ pub fn (f &File) read(mut buf []byte) ?int {
 	return nbytes
 }
 
-// read_at reads buf.len bytes from pos in the file
+// read_at reads `buf.len` bytes starting at file byte offset `pos`, in `buf`.
 pub fn (f &File) read_at(pos int, mut buf []byte) ?int {
 	if buf.len == 0 {
 		return 0
@@ -157,7 +301,7 @@ pub fn (f &File) read_at(pos int, mut buf []byte) ?int {
 }
 
 // **************************** Utility  ops ***********************
-// flush writes any unwritten data in stream's buffer
+// flush writes any buffered unwritten data left in the file stream.
 pub fn (mut f File) flush() {
 	if !f.is_opened {
 		return
@@ -165,18 +309,49 @@ pub fn (mut f File) flush() {
 	C.fflush(f.cfile)
 }
 
-// open_stdin - return an os.File for stdin, so that you can use .get_line on it too.
-pub fn open_stdin() File {
-	return File{
-		fd: 0
-		cfile: C.stdin
-		is_opened: true
-	}
-}
-
+// write_str writes the bytes of a string into a file,
+// *including* the terminating 0 byte.
 pub fn (mut f File) write_str(s string) ? {
 	if !f.is_opened {
 		return error('file is closed')
 	}
 	f.write(s.bytes()) ?
 }
+
+// read_struct reads a single struct of type `T`
+pub fn (mut f File) read_struct<T>(mut t T) ? {
+	if !f.is_opened {
+		return none
+	}
+	tsize := int(sizeof(*t))
+	if tsize == 0 {
+		return none
+	}
+	C.errno = 0
+	nbytes := int(C.fread(t, 1, tsize, f.cfile))
+	if C.errno != 0 {
+		return error(posix_get_error_msg(C.errno))
+	}
+	if nbytes != tsize {
+		return error_with_code('incomplete struct read', nbytes)
+	}
+}
+
+// write_struct writes a single struct of type `T`
+pub fn (mut f File) write_struct<T>(t &T) ? {
+	if !f.is_opened {
+		return error('file is not opened')
+	}
+	tsize := int(sizeof(*t))
+	if tsize == 0 {
+		return error('struct size is 0')
+	}
+	C.errno = 0
+	nbytes := int(C.fwrite(t, 1, tsize, f.cfile))
+	if C.errno != 0 {
+		return error(posix_get_error_msg(C.errno))
+	}
+	if nbytes != tsize {
+		return error_with_code('incomplete struct write', nbytes)
+	}
+}
diff --git a/vlib/os/file_test.v b/vlib/os/file_test.v
new file mode 100644
index 0000000000..47ec3b79c7
--- /dev/null
+++ b/vlib/os/file_test.v
@@ -0,0 +1,56 @@
+import os
+
+struct Point {
+	x f64
+	y f64
+	z f64
+}
+
+const unit_point = Point{1.0, 1.0, 1.0}
+
+const tfolder = os.join_path(os.temp_dir(), 'os_file_test')
+
+const tfile = os.join_path(tfolder, 'test_file')
+
+const another_point = Point{0.25, 2.25, 6.25}
+
+fn testsuite_begin() {
+	os.rmdir_all(tfolder) or { }
+	assert !os.is_dir(tfolder)
+	os.mkdir_all(tfolder) or { panic(err) }
+	os.chdir(tfolder)
+	assert os.is_dir(tfolder)
+}
+
+fn testsuite_end() {
+	os.chdir(os.wd_at_startup)
+	os.rmdir_all(tfolder) or { panic(err) }
+	assert !os.is_dir(tfolder)
+}
+
+fn test_write_struct() {
+	size_of_point := int(sizeof(Point))
+	mut f := os.open_file(tfile, 'w') or { panic(err) }
+	f.write_struct(another_point) or { panic(err) }
+	f.close()
+	x := os.read_file(tfile) or { panic(err) }
+	y := unsafe { byteptr(memdup(&another_point, size_of_point)).vstring_with_len(size_of_point) }
+	assert x == y
+	$if debug {
+		eprintln(x.bytes())
+		eprintln(y.bytes())
+	}
+}
+
+fn test_read_struct() {
+	mut f := os.open_file(tfile, 'w') or { panic(err) }
+	f.write_struct(another_point) or { panic(err) }
+	f.close()
+
+	f = os.open_file(tfile, 'r') or { panic(err) }
+	mut p := Point{}
+	f.read_struct(mut p) or { panic(err) }
+	f.close()
+
+	assert p == another_point
+}
diff --git a/vlib/os/os_c.v b/vlib/os/os_c.v
index e48721c41d..8ed3e98dac 100644
--- a/vlib/os/os_c.v
+++ b/vlib/os/os_c.v
@@ -214,74 +214,6 @@ pub fn fileno(cfile voidptr) int {
 	}
 }
 
-// open_append opens `path` file for appending.
-pub fn open_append(path string) ?File {
-	mut file := File{}
-	$if windows {
-		wpath := path.replace('/', '\\').to_wide()
-		mode := 'ab'
-		file = File{
-			cfile: C._wfopen(wpath, mode.to_wide())
-		}
-	} $else {
-		cpath := path.str
-		file = File{
-			cfile: C.fopen(charptr(cpath), 'ab')
-		}
-	}
-	if isnil(file.cfile) {
-		return error('failed to create(append) file "$path"')
-	}
-	file.is_opened = true
-	return file
-}
-
-// open_file can be used to open or create a file with custom flags and permissions and returns a `File` object.
-pub fn open_file(path string, mode string, options ...int) ?File {
-	mut flags := 0
-	for m in mode {
-		match m {
-			`r` { flags |= o_rdonly }
-			`w` { flags |= o_create | o_trunc }
-			`b` { flags |= o_binary }
-			`a` { flags |= o_create | o_append }
-			`s` { flags |= o_sync }
-			`n` { flags |= o_nonblock }
-			`c` { flags |= o_noctty }
-			`+` { flags |= o_rdwr }
-			else {}
-		}
-	}
-	mut permission := 0o666
-	if options.len > 0 {
-		permission = options[0]
-	}
-	$if windows {
-		if permission < 0o600 {
-			permission = 0x0100
-		} else {
-			permission = 0x0100 | 0x0080
-		}
-	}
-	mut p := path
-	$if windows {
-		p = path.replace('/', '\\')
-	}
-	fd := C.open(charptr(p.str), flags, permission)
-	if fd == -1 {
-		return error(posix_get_error_msg(C.errno))
-	}
-	cfile := C.fdopen(fd, charptr(mode.str))
-	if isnil(cfile) {
-		return error('Failed to open or create file "$path"')
-	}
-	return File{
-		cfile: cfile
-		fd: fd
-		is_opened: true
-	}
-}
-
 // vpopen system starts the specified command, waits for it to complete, and returns its code.
 fn vpopen(path string) voidptr {
 	// *C.FILE {
@@ -850,63 +782,26 @@ pub fn chmod(path string, mode int) {
 	C.chmod(charptr(path.str), mode)
 }
 
-// open tries to open a file for reading and returns back a read-only `File` object.
-pub fn open(path string) ?File {
-	/*
-	$if linux {
-		$if !android {
-			fd := C.syscall(sys_open, path.str, 511)
-			if fd == -1 {
-				return error('failed to open file "$path"')
-			}
-			return File{
-				fd: fd
-				is_opened: true
-			}
+// open_append opens `path` file for appending.
+pub fn open_append(path string) ?File {
+	mut file := File{}
+	$if windows {
+		wpath := path.replace('/', '\\').to_wide()
+		mode := 'ab'
+		file = File{
+			cfile: C._wfopen(wpath, mode.to_wide())
+		}
+	} $else {
+		cpath := path.str
+		file = File{
+			cfile: C.fopen(charptr(cpath), 'ab')
 		}
 	}
-	*/
-	cfile := vfopen(path, 'rb') ?
-	fd := fileno(cfile)
-	return File{
-		cfile: cfile
-		fd: fd
-		is_opened: true
-	}
-}
-
-// create creates or opens a file at a specified location and returns a write-only `File` object.
-pub fn create(path string) ?File {
-	/*
-	// NB: android/termux/bionic is also a kind of linux,
-	// but linux syscalls there sometimes fail,
-	// while the libc version should work.
-	$if linux {
-		$if !android {
-			//$if macos {
-			//	fd = C.syscall(398, path.str, 0x601, 0x1b6)
-			//}
-			//$if linux {
-			fd = C.syscall(sys_creat, path.str, 511)
-			//}
-			if fd == -1 {
-				return error('failed to create file "$path"')
-			}
-			file = File{
-				fd: fd
-				is_opened: true
-			}
-			return file
-		}
-	}
-	*/
-	cfile := vfopen(path, 'wb') ?
-	fd := fileno(cfile)
-	return File{
-		cfile: cfile
-		fd: fd
-		is_opened: true
+	if isnil(file.cfile) {
+		return error('failed to create(append) file "$path"')
 	}
+	file.is_opened = true
+	return file
 }
 
 // execvp - loads and executes a new child process, *in place* of the current process.