os: implement File.seek/2 and File.tell/0

vlib/os/file.c.v

@@ -16,6 +16,7 @@ struct FileInfo {
 fn C.fseeko(&C.FILE, u64, int) int
 
 fn C._fseeki64(&C.FILE, u64, int) int
+fn C._ftelli64(&C.FILE) i64
 
 fn C.getc(&C.FILE) int
 
@@ -727,3 +728,60 @@ pub fn (mut f File) write_raw_at<T>(t &T, pos u64) ? {
 		return error_with_code('incomplete struct write', nbytes)
 	}
 }
+
+pub enum SeekMode {
+	start
+	current
+	end
+}
+
+// seek moves the file cursor (if any) associated with a file
+// to a new location, offset `pos` bytes from the origin. The origin
+// is dependent on the `mode` and can be:
+//   .start   -> the origin is the start of the file
+//   .current -> the current position/cursor in the file
+//   .end     -> the end of the file
+// If the file is not seek-able, or an error occures, the error will
+// be returned to the caller.
+// A successful call to the fseek() function clears the end-of-file
+// indicator for the file.
+pub fn (mut f File) seek(pos i64, mode SeekMode) ? {
+	if !f.is_opened {
+		return error_file_not_opened()
+	}
+	whence := int(mode)
+	mut res := 0
+	$if x64 {
+		$if windows {
+			res = C._fseeki64(f.cfile, pos, whence)
+		} $else {
+			res = C.fseeko(f.cfile, pos, whence)
+		}
+	}
+	$if x32 {
+		res = C.fseek(f.cfile, pos, whence)
+	}
+	if res == -1 {
+		return error(posix_get_error_msg(C.errno))
+	}
+}
+
+// tell will return the current offset of the file cursor measured from
+// the start of the file, in bytes. It is complementary to seek, i.e.
+// you can use the return value as the `pos` parameter to .seek( pos, .start ),
+// so that your next read will happen from the same place.
+pub fn (f &File) tell() ?i64 {
+	if !f.is_opened {
+		return error_file_not_opened()
+	}
+	mut pos := i64(0)
+	$if windows && x64 {
+		pos = C._ftelli64(f.cfile)
+	} $else {
+		pos = C.ftell(f.cfile)
+	}
+	if pos == -1 {
+		return error(posix_get_error_msg(C.errno))
+	}
+	return pos
+}

vlib/os/file_test.v

@@ -331,3 +331,31 @@ fn test_read_raw_at_negative_pos() ? {
 	f.read_raw_at<Point>(-234) or { assert err.msg == 'Invalid argument' }
 	f.close()
 }
+
+fn test_seek() ? {
+	mut f := os.open_file(tfile, 'w') ?
+	f.write_raw(another_point) ?
+	f.write_raw(another_byte) ?
+	f.write_raw(another_color) ?
+	f.write_raw(another_permission) ?
+	f.close()
+
+	// println('> ${sizeof(Point)} ${sizeof(byte)} ${sizeof(Color)} ${sizeof(Permissions)}')
+	f = os.open_file(tfile, 'r') ?
+	defer {
+		f.close()
+	}
+	f.seek(i64(sizeof(Point)), .start) ?
+	assert f.tell() ? == sizeof(Point)
+	b := f.read_raw<byte>() ?
+
+	f.seek(i64(sizeof(Color)), .current) ?
+	x := f.read_raw<Permissions>() ?
+
+	f.seek(-i64(sizeof(Permissions) + sizeof(Color)), .end) ?
+	assert f.tell() ? == sizeof(Point) + sizeof(byte)
+	cc := f.read_raw<Color>() ?
+	assert b == another_byte
+	assert x == another_permission
+	assert cc == another_color
+}

vlib/os/os_c.v

@@ -13,7 +13,7 @@ fn C.readlink(pathname &char, buf &char, bufsiz size_t) int
 
 fn C.getline(voidptr, voidptr, voidptr) int
 
-fn C.ftell(fp voidptr) int
+fn C.ftell(fp voidptr) i64
 
 fn C.sigaction(int, voidptr, int) int
 
@@ -83,12 +83,12 @@ pub fn read_bytes(path string) ?[]byte {
 		return error('ftell failed')
 	}
 	C.rewind(fp)
-	mut res := []byte{len: fsize}
+	mut res := []byte{len: int(fsize)}
 	nr_read_elements := int(C.fread(res.data, fsize, 1, fp))
 	if nr_read_elements == 0 && fsize > 0 {
 		return error('fread failed')
 	}
-	res.trim(nr_read_elements * fsize)
+	res.trim(nr_read_elements * int(fsize))
 	return res
 }
 
@@ -110,7 +110,7 @@ pub fn read_file(path string) ?string {
 	// C.fseek(fp, 0, SEEK_SET)  // same as `C.rewind(fp)` below
 	C.rewind(fp)
 	unsafe {
-		mut str := malloc_noscan(fsize + 1)
+		mut str := malloc_noscan(int(fsize) + 1)
 		nelements := int(C.fread(str, 1, fsize, fp))
 		is_eof := int(C.feof(fp))
 		is_error := int(C.ferror(fp))
@@ -586,7 +586,7 @@ pub fn read_file_array<T>(path string) []T {
 	C.rewind(fp)
 	// read the actual data from the file
 	len := fsize / tsize
-	buf := unsafe { malloc_noscan(fsize) }
+	buf := unsafe { malloc_noscan(int(fsize)) }
 	nread := C.fread(buf, tsize, len, fp)
 	C.fclose(fp)
 	return unsafe {
@@ -594,7 +594,7 @@ pub fn read_file_array<T>(path string) []T {
 			element_size: tsize
 			data: buf
 			len: int(nread)
-			cap: len
+			cap: int(len)
 		}
 	}
 }

vlib/os/os_nix.c.v

@@ -304,10 +304,7 @@ pub fn is_dir(path string) bool {
 	return res
 }
 */
-/*
-pub fn (mut f File) fseek(pos, mode int) {
-}
-*/
+
 // mkdir creates a new directory with the specified path.
 pub fn mkdir(path string) ?bool {
 	if path == '.' {