Source file src/pkg/os/file.go
1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
4
5 // Package os provides a platform-independent interface to operating system
6 // functionality. The design is Unix-like, although the error handling is
7 // Go-like; failing calls return values of type error rather than error numbers.
8 // Often, more information is available within the error. For example,
9 // if a call that takes a file name fails, such as Open or Stat, the error
10 // will include the failing file name when printed and will be of type
11 // *PathError, which may be unpacked for more information.
12 //
13 // The os interface is intended to be uniform across all operating systems.
14 // Features not generally available appear in the system-specific package syscall.
15 //
16 // Here is a simple example, opening a file and reading some of it.
17 //
18 // file, err := os.Open("file.go") // For read access.
19 // if err != nil {
20 // log.Fatal(err)
21 // }
22 //
23 // If the open fails, the error string will be self-explanatory, like
24 //
25 // open file.go: no such file or directory
26 //
27 // The file's data can then be read into a slice of bytes. Read and
28 // Write take their byte counts from the length of the argument slice.
29 //
30 // data := make([]byte, 100)
31 // count, err := file.Read(data)
32 // if err != nil {
33 // log.Fatal(err)
34 // }
35 // fmt.Printf("read %d bytes: %q\n", count, data[:count])
36 //
37 package os
38
39 import (
40 "io"
41 "syscall"
42 )
43
44 // Name returns the name of the file as presented to Open.
45 func (f *File) Name() string { return f.name }
46
47 // Stdin, Stdout, and Stderr are open Files pointing to the standard input,
48 // standard output, and standard error file descriptors.
49 var (
50 Stdin = NewFile(uintptr(syscall.Stdin), "/dev/stdin")
51 Stdout = NewFile(uintptr(syscall.Stdout), "/dev/stdout")
52 Stderr = NewFile(uintptr(syscall.Stderr), "/dev/stderr")
53 )
54
55 // Flags to Open wrapping those of the underlying system. Not all flags
56 // may be implemented on a given system.
57 const (
58 O_RDONLY int = syscall.O_RDONLY // open the file read-only.
59 O_WRONLY int = syscall.O_WRONLY // open the file write-only.
60 O_RDWR int = syscall.O_RDWR // open the file read-write.
61 O_APPEND int = syscall.O_APPEND // append data to the file when writing.
62 O_CREATE int = syscall.O_CREAT // create a new file if none exists.
63 O_EXCL int = syscall.O_EXCL // used with O_CREATE, file must not exist
64 O_SYNC int = syscall.O_SYNC // open for synchronous I/O.
65 O_TRUNC int = syscall.O_TRUNC // if possible, truncate file when opened.
66 )
67
68 // Seek whence values.
69 const (
70 SEEK_SET int = 0 // seek relative to the origin of the file
71 SEEK_CUR int = 1 // seek relative to the current offset
72 SEEK_END int = 2 // seek relative to the end
73 )
74
75 // LinkError records an error during a link or symlink or rename
76 // system call and the paths that caused it.
77 type LinkError struct {
78 Op string
79 Old string
80 New string
81 Err error
82 }
83
84 func (e *LinkError) Error() string {
85 return e.Op + " " + e.Old + " " + e.New + ": " + e.Err.Error()
86 }
87
88 // Read reads up to len(b) bytes from the File.
89 // It returns the number of bytes read and an error, if any.
90 // EOF is signaled by a zero count with err set to io.EOF.
91 func (f *File) Read(b []byte) (n int, err error) {
92 if f == nil {
93 return 0, ErrInvalid
94 }
95 n, e := f.read(b)
96 if n < 0 {
97 n = 0
98 }
99 if n == 0 && len(b) > 0 && e == nil {
100 return 0, io.EOF
101 }
102 if e != nil {
103 err = &PathError{"read", f.name, e}
104 }
105 return n, err
106 }
107
108 // ReadAt reads len(b) bytes from the File starting at byte offset off.
109 // It returns the number of bytes read and the error, if any.
110 // ReadAt always returns a non-nil error when n < len(b).
111 // At end of file, that error is io.EOF.
112 func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
113 if f == nil {
114 return 0, ErrInvalid
115 }
116 for len(b) > 0 {
117 m, e := f.pread(b, off)
118 if m == 0 && e == nil {
119 return n, io.EOF
120 }
121 if e != nil {
122 err = &PathError{"read", f.name, e}
123 break
124 }
125 n += m
126 b = b[m:]
127 off += int64(m)
128 }
129 return
130 }
131
132 // Write writes len(b) bytes to the File.
133 // It returns the number of bytes written and an error, if any.
134 // Write returns a non-nil error when n != len(b).
135 func (f *File) Write(b []byte) (n int, err error) {
136 if f == nil {
137 return 0, ErrInvalid
138 }
139 n, e := f.write(b)
140 if n < 0 {
141 n = 0
142 }
143
144 epipecheck(f, e)
145
146 if e != nil {
147 err = &PathError{"write", f.name, e}
148 }
149 return n, err
150 }
151
152 // WriteAt writes len(b) bytes to the File starting at byte offset off.
153 // It returns the number of bytes written and an error, if any.
154 // WriteAt returns a non-nil error when n != len(b).
155 func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
156 if f == nil {
157 return 0, ErrInvalid
158 }
159 for len(b) > 0 {
160 m, e := f.pwrite(b, off)
161 if e != nil {
162 err = &PathError{"write", f.name, e}
163 break
164 }
165 n += m
166 b = b[m:]
167 off += int64(m)
168 }
169 return
170 }
171
172 // Seek sets the offset for the next Read or Write on file to offset, interpreted
173 // according to whence: 0 means relative to the origin of the file, 1 means
174 // relative to the current offset, and 2 means relative to the end.
175 // It returns the new offset and an error, if any.
176 func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
177 r, e := f.seek(offset, whence)
178 if e == nil && f.dirinfo != nil && r != 0 {
179 e = syscall.EISDIR
180 }
181 if e != nil {
182 return 0, &PathError{"seek", f.name, e}
183 }
184 return r, nil
185 }
186
187 // WriteString is like Write, but writes the contents of string s rather than
188 // an array of bytes.
189 func (f *File) WriteString(s string) (ret int, err error) {
190 if f == nil {
191 return 0, ErrInvalid
192 }
193 return f.Write([]byte(s))
194 }
195
196 // Mkdir creates a new directory with the specified name and permission bits.
197 // If there is an error, it will be of type *PathError.
198 func Mkdir(name string, perm FileMode) error {
199 e := syscall.Mkdir(name, syscallMode(perm))
200 if e != nil {
201 return &PathError{"mkdir", name, e}
202 }
203 return nil
204 }
205
206 // Chdir changes the current working directory to the named directory.
207 // If there is an error, it will be of type *PathError.
208 func Chdir(dir string) error {
209 if e := syscall.Chdir(dir); e != nil {
210 return &PathError{"chdir", dir, e}
211 }
212 return nil
213 }
214
215 // Chdir changes the current working directory to the file,
216 // which must be a directory.
217 // If there is an error, it will be of type *PathError.
218 func (f *File) Chdir() error {
219 if e := syscall.Fchdir(f.fd); e != nil {
220 return &PathError{"chdir", f.name, e}
221 }
222 return nil
223 }
224
225 // Open opens the named file for reading. If successful, methods on
226 // the returned file can be used for reading; the associated file
227 // descriptor has mode O_RDONLY.
228 // If there is an error, it will be of type *PathError.
229 func Open(name string) (file *File, err error) {
230 return OpenFile(name, O_RDONLY, 0)
231 }
232
233 // Create creates the named file mode 0666 (before umask), truncating
234 // it if it already exists. If successful, methods on the returned
235 // File can be used for I/O; the associated file descriptor has mode
236 // O_RDWR.
237 // If there is an error, it will be of type *PathError.
238 func Create(name string) (file *File, err error) {
239 return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
240 }