os: tag some File methods that take voidptr as unsafe - write_bytes, write_bytes_at (#8985)

examples/wkhtmltopdf.v

@@ -78,7 +78,7 @@ fn main() {
 			println('ERR: $err')
 			return
 		}
-		wrote := file.write_bytes(data, size)
+		wrote := unsafe { file.write_bytes(data, size) }
 		println('write_bytes: $wrote [./google.pdf]')
 		file.flush()
 		file.close()

vlib/os/file.c.v

@@ -79,10 +79,12 @@ pub fn (mut f File) write_to(pos int, buf []byte) ?int {
 	return res
 }
 
+[unsafe]
 pub fn (mut f File) write_bytes(data voidptr, size int) int {
 	return int(C.fwrite(data, 1, size, f.cfile))
 }
 
+[unsafe]
 pub fn (mut f File) write_bytes_at(data voidptr, size int, pos int) int {
 	C.fseek(f.cfile, pos, C.SEEK_SET)
 	res := int(C.fwrite(data, 1, size, f.cfile))

vlib/os/os.v

@@ -325,7 +325,7 @@ pub fn write_file(path string, text string) ? {
 // write_file_array writes the data in `buffer` to a file in `path`.
 pub fn write_file_array(path string, buffer array) ? {
 	mut f := create(path) ?
-	f.write_bytes_at(buffer.data, (buffer.len * buffer.element_size), 0)
+	unsafe { f.write_bytes_at(buffer.data, (buffer.len * buffer.element_size), 0) }
 	f.close()
 }
 

vlib/v/gen/x64/elf.v

@@ -101,7 +101,7 @@ pub fn (mut g Gen) generate_elf_footer() {
 	// Create the binary
 	mut f := os.create(g.out_name) or { panic(err) }
 	os.chmod(g.out_name, 0o775) // make it an executable
-	f.write_bytes(g.buf.data, g.buf.len)
+	unsafe { f.write_bytes(g.buf.data, g.buf.len) }
 	f.close()
 	println('\nx64 elf binary has been successfully generated')
 }