Fix OpenBSD implementations of `read_at` and `write_at`

core/os/os_openbsd.odin

@@ -351,7 +351,9 @@ foreign libc {
 	@(link_name="open")	_unix_open	:: proc(path: cstring, flags: c.int, #c_vararg mode: ..u32) -> Handle ---
 	@(link_name="close")	_unix_close	:: proc(fd: Handle) -> c.int ---
 	@(link_name="read")	_unix_read	:: proc(fd: Handle, buf: rawptr, size: c.size_t) -> c.ssize_t ---
+	@(link_name="pread")            _unix_pread         :: proc(fd: Handle, buf: rawptr, size: c.size_t, offset: i64) -> c.ssize_t ---
 	@(link_name="write")	_unix_write	:: proc(fd: Handle, buf: rawptr, size: c.size_t) -> c.ssize_t ---
+	@(link_name="pwrite")           _unix_pwrite        :: proc(fd: Handle, buf: rawptr, size: c.size_t, offset: i64) -> c.ssize_t ---
 	@(link_name="lseek")	_unix_seek	:: proc(fd: Handle, offset: off_t, whence: c.int) -> off_t ---
 	@(link_name="stat")	_unix_stat	:: proc(path: cstring, sb: ^OS_Stat) -> c.int ---
 	@(link_name="fstat")	_unix_fstat	:: proc(fd: Handle, sb: ^OS_Stat) -> c.int ---
@@ -463,23 +465,31 @@ write :: proc(fd: Handle, data: []byte) -> (int, Error) {
 }
 
 read_at :: proc(fd: Handle, data: []byte, offset: i64) -> (n: int, err: Error) {
-	curr := seek(fd, offset, SEEK_CUR) or_return
-	n, err = read(fd, data)
-	_, err1 := seek(fd, curr, SEEK_SET)
-	if err1 != nil && err == nil {
-		err = err1
+	if len(data) == 0 {
+		return 0, nil
 	}
-	return
+
+	to_read := min(uint(len(data)), MAX_RW)
+
+	bytes_read := _unix_pread(fd, raw_data(data), to_read, offset)
+	if bytes_read < 0 {
+		return -1, get_last_error()
+	}
+	return bytes_read, nil
 }
 
 write_at :: proc(fd: Handle, data: []byte, offset: i64) -> (n: int, err: Error) {
-	curr := seek(fd, offset, SEEK_CUR) or_return
-	n, err = write(fd, data)
-	_, err1 := seek(fd, curr, SEEK_SET)
-	if err1 != nil && err == nil {
-		err = err1
+	if len(data) == 0 {
+		return 0, nil
 	}
-	return
+
+	to_write := min(uint(len(data)), MAX_RW)
+
+	bytes_written := _unix_pwrite(fd, raw_data(data), to_write, offset)
+	if bytes_written < 0 {
+		return -1, get_last_error()
+	}
+	return bytes_written, nil
 }
 
 seek :: proc(fd: Handle, offset: i64, whence: int) -> (i64, Error) {