Add functions to read and write string data

.gitignore

@@ -10,3 +10,6 @@
 
 # Output of the go coverage tool, specifically when used with LiteIDE
 *.out
+
+# IDE files
+.idea/*

.travis.yml

@@ -2,12 +2,14 @@ language: go
 
 go:
   - 1.12
-  - 1.11
 
 os:
   - osx
   - linux
 
+env:
+  - GO111MODULE=on
+
 script:
   - go test -race -coverprofile=coverage.txt -covermode=atomic
 

mmap.go

@@ -2,7 +2,9 @@ package mmap
 
 import (
 	"errors"
+	"io"
 	"os"
+	"strings"
 	"syscall"
 )
 
@@ -13,8 +15,25 @@ var (
 	ErrIndexOutOfBound = errors.New("offset out of mapped region")
 )
 
-// Mmap provides abstraction around a memory mapped file
-type Mmap struct {
+// File provides an interface to a memory mapped file
+type File interface {
+	io.ReaderAt
+	io.WriterAt
+
+	ReadStringAt(dest *strings.Builder, offset int64) int
+	WriteStringAt(src string, offset int64) int
+	ReadUint64At(offset int64) uint64
+	WriteUint64At(num uint64, offset int64)
+
+	Lock() error
+	Unlock() error
+	Advise(advice int) error
+	Flush(flags int) error
+	Unmap() error
+}
+
+// mmapFile provides abstraction around a memory mapped file
+type mmapFile struct {
 	data   []byte
 	length int64
 }
@@ -26,21 +45,21 @@ type Mmap struct {
 //              then all the mapped memory is accessible
 //    case 2 => if   file size <= memory region (offset + length)
 //              then from offset to file size memory region is accessible
-func NewSharedFileMmap(f *os.File, offset int64, length int, prot int) (IMmap, error) {
+func NewSharedFileMmap(f *os.File, offset int64, length int, prot int) (File, error) {
 	data, err := syscall.Mmap(int(f.Fd()), offset, length, prot, syscall.MAP_SHARED)
 	if err != nil {
 		return nil, err
 	}
 
-	return &Mmap{
+	return &mmapFile{
 		data:   data,
 		length: int64(length),
 	}, nil
 }
 
 // Unmap unmaps the memory mapped file. An error will be returned
 // if any of the functions are called on Mmap after calling Unmap
-func (m *Mmap) Unmap() error {
+func (m *mmapFile) Unmap() error {
 	err := syscall.Munmap(m.data)
 	m.data = nil
 	return err

mmap_data.go

@@ -2,72 +2,86 @@ package mmap
 
 import (
 	"encoding/binary"
+	"strings"
 	"syscall"
 	"unsafe"
 )
 
-// ReadAt copies data to dest slice from mapped region starting at
-// given offset and returns number of bytes copied to the dest slice.
-// There are two possibilities -
-//   Case 1: len(dest) >= (len(m.data) - offset)
-//        => copies (len(m.data) - offset) bytes to dest from mapped region
-//   Case 2: len(dest) < (len(m.data) - offset)
-//        => copies len(dest) bytes to dest from mapped region
-// err is always nil, hence, can be ignored
-func (m *Mmap) ReadAt(dest []byte, offset int64) (int, error) {
+func (m *mmapFile) boundaryChecks(offset, numBytes int64) {
 	if m.data == nil {
 		panic(ErrUnmappedMemory)
-	} else if offset >= m.length || offset < 0 {
+	} else if offset+numBytes > m.length || offset < 0 {
 		panic(ErrIndexOutOfBound)
 	}
+}
 
+// ReadAt copies data to dest slice from mapped region starting at
+// given offset and returns number of bytes copied to the dest slice.
+// There are two possibilities -
+//   Case 1: len(dest) >= (m.length - offset)
+//        => copies (m.length - offset) bytes to dest from mapped region
+//   Case 2: len(dest) < (m.length - offset)
+//        => copies len(dest) bytes to dest from mapped region
+// err is always nil, hence, can be ignored
+func (m *mmapFile) ReadAt(dest []byte, offset int64) (int, error) {
+	m.boundaryChecks(offset, 1)
 	return copy(dest, m.data[offset:]), nil
 }
 
 // WriteAt copies data to mapped region from the src slice starting at
 // given offset and returns number of bytes copied to the mapped region.
 // There are two possibilities -
-//  Case 1: len(src) >= (len(m.data) - offset)
-//      => copies (len(m.data) - offset) bytes to the mapped region from src
-//  Case 2: len(src) < (len(m.data) - offset)
+//  Case 1: len(src) >= (m.length - offset)
+//      => copies (m.length - offset) bytes to the mapped region from src
+//  Case 2: len(src) < (m.length - offset)
 //      => copies len(src) bytes to the mapped region from src
 // err is always nil, hence, can be ignored
-func (m *Mmap) WriteAt(src []byte, offset int64) (int, error) {
-	if m.data == nil {
-		panic(ErrUnmappedMemory)
-	} else if offset >= m.length || offset < 0 {
-		panic(ErrIndexOutOfBound)
-	}
-
+func (m *mmapFile) WriteAt(src []byte, offset int64) (int, error) {
+	m.boundaryChecks(offset, 1)
 	return copy(m.data[offset:], src), nil
 }
 
-// ReadUint64At reads uint64 from offset
-func (m *Mmap) ReadUint64At(offset int64) uint64 {
-	if m.data == nil {
-		panic(ErrUnmappedMemory)
-	} else if offset+8 > m.length || offset < 0 {
-		panic(ErrIndexOutOfBound)
+// ReadStringAt copies data to dest string builder from mapped region starting at
+// given offset until the min value of (length - offset) or (dest.Cap() - dest.Len())
+// and returns number of bytes copied to the dest slice.
+func (m *mmapFile) ReadStringAt(dest *strings.Builder, offset int64) int {
+	m.boundaryChecks(offset, 1)
+
+	dataLength := m.length - offset
+	emptySpace := int64(dest.Cap() - dest.Len())
+	end := m.length
+	if dataLength > emptySpace {
+		end = offset + emptySpace
 	}
 
+	n, _ := dest.Write(m.data[offset:end])
+	return n
+}
+
+// WriteStringAt copies data to mapped region from the src string starting at
+// given offset and returns number of bytes copied to the mapped region.
+// See github.com/grandecola/mmap/#Mmap.WriteAt for more details.
+func (m *mmapFile) WriteStringAt(src string, offset int64) int {
+	m.boundaryChecks(offset, 1)
+	return copy(m.data[offset:], src)
+}
+
+// ReadUint64At reads uint64 from offset
+func (m *mmapFile) ReadUint64At(offset int64) uint64 {
+	m.boundaryChecks(offset, 8)
 	return binary.LittleEndian.Uint64(m.data[offset : offset+8])
 }
 
 // WriteUint64At writes num at offset
-func (m *Mmap) WriteUint64At(num uint64, offset int64) {
-	if m.data == nil {
-		panic(ErrUnmappedMemory)
-	} else if offset+8 > m.length || offset < 0 {
-		panic(ErrIndexOutOfBound)
-	}
-
+func (m *mmapFile) WriteUint64At(num uint64, offset int64) {
+	m.boundaryChecks(offset, 8)
 	binary.LittleEndian.PutUint64(m.data[offset:offset+8], num)
 }
 
 // Flush flushes the memory mapped region to disk
-func (m *Mmap) Flush(flags int) error {
+func (m *mmapFile) Flush(flags int) error {
 	_, _, err := syscall.Syscall(syscall.SYS_MSYNC,
-		uintptr(unsafe.Pointer(&m.data[0])), uintptr(len(m.data)), uintptr(flags))
+		uintptr(unsafe.Pointer(&m.data[0])), uintptr(m.length), uintptr(flags))
 	if err != 0 {
 		return err
 	}

mmap_page.go

@@ -6,9 +6,9 @@ import (
 )
 
 // Advise provides hints to kernel regarding the use of memory mapped region
-func (m *Mmap) Advise(advice int) error {
+func (m *mmapFile) Advise(advice int) error {
 	_, _, err := syscall.Syscall(syscall.SYS_MADVISE,
-		uintptr(unsafe.Pointer(&m.data[0])), uintptr(len(m.data)), uintptr(advice))
+		uintptr(unsafe.Pointer(&m.data[0])), uintptr(m.length), uintptr(advice))
 	if err != 0 {
 		return err
 	}
@@ -17,9 +17,9 @@ func (m *Mmap) Advise(advice int) error {
 }
 
 // Lock locks all the mapped memory to RAM, preventing the pages from swapping out
-func (m *Mmap) Lock() error {
+func (m *mmapFile) Lock() error {
 	_, _, err := syscall.Syscall(syscall.SYS_MLOCK,
-		uintptr(unsafe.Pointer(&m.data[0])), uintptr(len(m.data)), 0)
+		uintptr(unsafe.Pointer(&m.data[0])), uintptr(m.length), 0)
 	if err != 0 {
 		return err
 	}
@@ -28,9 +28,9 @@ func (m *Mmap) Lock() error {
 }
 
 // Unlock unlocks the mapped memory from RAM, enabling swapping out of RAM if required
-func (m *Mmap) Unlock() error {
+func (m *mmapFile) Unlock() error {
 	_, _, err := syscall.Syscall(syscall.SYS_MUNLOCK,
-		uintptr(unsafe.Pointer(&m.data[0])), uintptr(len(m.data)), 0)
+		uintptr(unsafe.Pointer(&m.data[0])), uintptr(m.length), 0)
 	if err != 0 {
 		return err
 	}