os.File: add .read_struct_at and .write_struct_at (#9114)

vlib/os/file.c.v

@@ -337,6 +337,27 @@ pub fn (mut f File) read_struct<T>(mut t T) ? {
 	}
 }
 
+// read_struct_at reads a single struct of type `T` at position specified in file
+pub fn (mut f File) read_struct_at<T>(mut t T, pos int) ? {
+	if !f.is_opened {
+		return none
+	}
+	tsize := int(sizeof(*t))
+	if tsize == 0 {
+		return none
+	}
+	C.errno = 0
+	C.fseek(f.cfile, pos, C.SEEK_SET)
+	nbytes := int(C.fread(t, 1, tsize, f.cfile))
+	C.fseek(f.cfile, 0, C.SEEK_END)
+	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 {
@@ -355,3 +376,24 @@ pub fn (mut f File) write_struct<T>(t &T) ? {
 		return error_with_code('incomplete struct write', nbytes)
 	}
 }
+
+// write_struct_at writes a single struct of type `T` at position specified in file
+pub fn (mut f File) write_struct_at<T>(t &T, pos int) ? {
+	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
+	C.fseek(f.cfile, pos, C.SEEK_SET)
+	nbytes := int(C.fwrite(t, 1, tsize, f.cfile))
+	C.fseek(f.cfile, 0, C.SEEK_END)
+	if C.errno != 0 {
+		return error(posix_get_error_msg(C.errno))
+	}
+	if nbytes != tsize {
+		return error_with_code('incomplete struct write', nbytes)
+	}
+}

vlib/os/file_test.v

@@ -6,6 +6,18 @@ struct Point {
 	z f64
 }
 
+struct Extended_Point {
+	a f64
+	b f64
+	c f64
+	d f64
+	e f64
+	f f64
+	g f64
+	h f64
+	i f64
+}
+
 const unit_point = Point{1.0, 1.0, 1.0}
 
 const tfolder = os.join_path(os.temp_dir(), 'os_file_test')
@@ -14,6 +26,8 @@ const tfile = os.join_path(tfolder, 'test_file')
 
 const another_point = Point{0.25, 2.25, 6.25}
 
+const extended_point = Extended_Point{1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0}
+
 fn testsuite_begin() {
 	os.rmdir_all(tfolder) or { }
 	assert !os.is_dir(tfolder)
@@ -54,3 +68,29 @@ fn test_read_struct() {
 
 	assert p == another_point
 }
+
+fn test_read_struct_at() {
+	mut f := os.open_file(tfile, 'w') or { panic(err) }
+	f.write([byte(1), 2, 3]) 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_at(mut p, 3) or { panic(err) }
+	f.close()
+
+	assert p == another_point
+}
+
+fn test_write_struct_at() {
+	mut f := os.open_file(tfile, 'w') or { panic(err) }
+	f.write_struct(extended_point) or { panic(err) }
+	f.write_struct_at(another_point, 3) or { panic(err) }
+	f.close()
+	f = os.open_file(tfile, 'r') or { panic(err) }
+	mut p := Point{}
+	f.read_struct_at(mut p, 3) or { panic(err) }
+	f.close()
+
+	assert p == another_point
+}