From f7e92c596c17a2ba3e473a63c479b8f9338046dd Mon Sep 17 00:00:00 2001
From: Russ Cox <rsc@golang.org>
Date: Thu, 27 Aug 2009 18:36:45 -0700
Subject: [PATCH] os.File.ReadAt/WriteAt

R=r
DELTA=84  (81 added, 0 deleted, 3 changed)
OCL=34006
CL=34006
---
 src/pkg/os/file.go    | 47 ++++++++++++++++++++++++++++++++++++++++---
 src/pkg/os/os_test.go | 40 ++++++++++++++++++++++++++++++++++++
 2 files changed, 84 insertions(+), 3 deletions(-)

diff --git a/src/pkg/os/file.go b/src/pkg/os/file.go
index 958a8230c6..c9c00788ff 100644
--- a/src/pkg/os/file.go
+++ b/src/pkg/os/file.go
@@ -115,7 +115,7 @@ var EOF Error = eofError(0)
 // It returns the number of bytes read and an Error, if any.
 // EOF is signaled by a zero count with err set to EOF.
 // TODO(r): Add Pread, Pwrite (maybe ReadAt, WriteAt).
-func (file *File) Read(b []byte) (ret int, err Error) {
+func (file *File) Read(b []byte) (n int, err Error) {
 	if file == nil {
 		return 0, EINVAL
 	}
@@ -132,10 +132,31 @@ func (file *File) Read(b []byte) (ret int, err Error) {
 	return n, err
 }
 
+// ReadAt reads len(b) bytes from the File starting at byte offset off.
+// It returns the number of bytes read and the Error, if any.
+// EOF is signaled by a zero count with err set to EOF.
+// ReadAt always returns a non-nil Error when n != len(b).
+func (file *File) ReadAt(b []byte, off int64) (n int, err Error) {
+	if file == nil {
+		return 0, EINVAL;
+	}
+	for len(b) > 0 {
+		m, e := syscall.Pread(file.fd, b, off);
+		n += m;
+		if e != 0 {
+			err = &PathError{"read", file.name, Errno(e)};
+			break;
+		}
+		b = b[m:len(b)];
+		off += int64(m);
+	}
+	return;
+}
+
 // Write writes len(b) bytes to the File.
 // It returns the number of bytes written and an Error, if any.
-// If the byte count differs from len(b), it usually implies an error occurred.
-func (file *File) Write(b []byte) (ret int, err Error) {
+// Write returns a non-nil Error when n != len(b).
+func (file *File) Write(b []byte) (n int, err Error) {
 	if file == nil {
 		return 0, EINVAL
 	}
@@ -157,6 +178,26 @@ func (file *File) Write(b []byte) (ret int, err Error) {
 	return n, err
 }
 
+// WriteAt writes len(b) bytes to the File starting at byte offset off.
+// It returns the number of bytes written and an Error, if any.
+// WriteAt returns a non-nil Error when n != len(b).
+func (file *File) WriteAt(b []byte, off int64) (n int, err Error) {
+	if file == nil {
+		return 0, EINVAL;
+	}
+	for len(b) > 0 {
+		m, e := syscall.Pwrite(file.fd, b, off);
+		n += m;
+		if e != 0 {
+			err = &PathError{"write", file.name, Errno(e)};
+			break;
+		}
+		b = b[m:len(b)];
+		off += int64(m);
+	}
+	return;
+}
+
 // Seek sets the offset for the next Read or Write on file to offset, interpreted
 // according to whence: 0 means relative to the origin of the file, 1 means
 // relative to the current offset, and 2 means relative to the end.
diff --git a/src/pkg/os/os_test.go b/src/pkg/os/os_test.go
index 84c4d15a01..9f7df2ac58 100644
--- a/src/pkg/os/os_test.go
+++ b/src/pkg/os/os_test.go
@@ -628,3 +628,43 @@ func TestHostname(t *testing.T) {
 		t.Errorf("Hostname() = %q, want %q", hostname, want);
 	}
 }
+
+func TestReadAt(t *testing.T) {
+	f, err := Open("_obj/readtest", O_CREAT|O_RDWR|O_TRUNC, 0666);
+	if err != nil {
+		t.Fatalf("open _obj/readtest: %s", err);
+	}
+	const data = "hello, world\n";
+	io.WriteString(f, data);
+
+	b := make([]byte, 5);
+	n, err := f.ReadAt(b, 7);
+	if err != nil || n != len(b) {
+		t.Fatalf("ReadAt 7: %d, %r", n, err);
+	}
+	if string(b) != "world" {
+		t.Fatalf("ReadAt 7: have %q want %q", string(b), "world");
+	}
+}
+
+func TestWriteAt(t *testing.T) {
+	f, err := Open("_obj/writetest", O_CREAT|O_RDWR|O_TRUNC, 0666);
+	if err != nil {
+		t.Fatalf("open _obj/writetest: %s", err);
+	}
+	const data = "hello, world\n";
+	io.WriteString(f, data);
+
+	n, err := f.WriteAt(strings.Bytes("WORLD"), 7);
+	if err != nil || n != 5 {
+		t.Fatalf("WriteAt 7: %d, %v", n, err);
+	}
+
+	b, err := io.ReadFile("_obj/writetest");
+	if err != nil {
+		t.Fatalf("ReadFile _obj/writetest: %v", err);
+	}
+	if string(b) != "hello, WORLD\n" {
+		t.Fatalf("after write: have %q want %q", string(b), "hello, WORLD\n");
+	}
+}
-- 
2.48.1