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1 .\" $NetBSD: fopen.3,v 1.31 2015/07/15 19:08:43 christos Exp $
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6 .\" This code is derived from software contributed to Berkeley by
7 .\" Chris Torek and the American National Standards Committee X3,
8 .\" on Information Processing Systems.
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34 .\" @(#)fopen.3 8.1 (Berkeley) 6/4/93
35 .\"
36 .Dd November 14, 2012
37 .Dt FOPEN 3
38 .Os
39 .Sh NAME
40 .Nm fopen ,
41 .Nm fdopen ,
42 .Nm freopen
43 .Nd stream open functions
44 .Sh LIBRARY
45 .Lb libc
46 .Sh SYNOPSIS
47 .In stdio.h
48 .Ft FILE *
49 .Fn fopen "const char * restrict path" "const char * restrict mode"
50 .Ft FILE *
51 .Fn fdopen "int fildes" "const char *mode"
52 .Ft FILE *
53 .Fn freopen "const char * restrict path" "const char * restrict mode" "FILE * restrict stream"
54 .Sh DESCRIPTION
55 The
56 .Fn fopen
57 function
58 opens the file whose name is the string pointed to by
59 .Fa path
60 and associates a stream with it.
61 .Pp
62 The argument
63 .Fa mode
64 points to a string beginning with one of the following
65 sequences (Additional characters may follow these sequences.):
66 .Bl -tag -width 4n
67 .It Dq Li a
68 Append; open for writing.
69 The file is created if it does not exist.
70 .It Dq Li a+
71 Append; open for reading and writing.
72 The file is created if it does not exist.
73 .It Dq Li r
74 Open for reading.
75 .It Dq Li r+
76 Open for reading and writing.
77 .It Dq Li w
78 Open for writing.
79 Truncate file to zero length or create file.
80 .It Dq Li w+
81 Open for reading and writing.
82 Truncate file to zero length or create file.
83 .El
84 .Pp
85 Additionally, the
86 .Fa mode
87 string can also include one of the following letters:
88 .Bl -tag -width 4n
89 .It Sq b
90 The letter
91 .Sq b
92 may appear either as a last character or as a character between the
93 characters in any of the two-character strings described above.
94 This is strictly for compatibility with
95 .St -ansiC
96 and has no effect; the
97 .Sq b
98 is ignored.
99 .It Sq e
100 The letter
101 .Sq e
102 in the mode string sets the close-on-exec flag in the file descriptors of
103 the newly opened file files; if the operation fails,
104 .Fn fopen
105 will fail.
106 This is a non
107 .St -ansiC
108 extension.
109 .It Sq f
110 The letter
111 .Sq f
112 in the mode string restricts
113 .Fn fopen
114 to regular files; if the file opened is not a regular file,
115 .Fn fopen
116 will fail.
117 This is a non
118 .St -ansiC
119 extension.
120 .It Sq x
121 The letter
122 .Sq x
123 in the mode turns on exclusive open mode to the file
124 .Pq Dv O_EXCL
125 which means that the file will not be created if it already exists.
126 .El
127 .Pp
128 Any created files will have mode
129 .Pf \*q Dv S_IRUSR
130 \&|
131 .Dv S_IWUSR
132 \&|
133 .Dv S_IRGRP
134 \&|
135 .Dv S_IWGRP
136 \&|
137 .Dv S_IROTH
138 \&|
139 .Dv S_IWOTH Ns \*q
140 .Pq Li 0666 ,
141 as modified by the process'
142 .Xr umask 2
143 value.
144 .Pp
145 Opening a file with append mode causes all subsequent writes to it
146 to be forced to the then current end of file, regardless of intervening
147 repositioning of the stream.
148 .Pp
149 The
150 .Fn fopen
151 and
152 .Fn freopen
153 functions initially position the stream at the start of the file
154 unless the file is opened with append mode,
155 in which case the stream is initially positioned at the end of the file.
156 .\" PR 6072 claims this paragraph is not correct.
157 .\" .Pp
158 .\" Reads and writes may be intermixed on read/write streams in any order,
159 .\" and do not require an intermediate seek as in previous versions of
160 .\" .Em stdio .
161 .\" This is not portable to other systems, however;
162 .\" .Tn ANSI C
163 .\" requires that
164 .\" a file positioning function intervene between output and input, unless
165 .\" an input operation encounters end-of-file.
166 .Pp
167 The
168 .Fn fdopen
169 function associates a stream with the existing file descriptor,
170 .Fa fildes .
171 The
172 .Fa mode
173 of the stream must be compatible with the mode of the file descriptor.
174 The stream is positioned at the file offset of the file descriptor.
175 .Pp
176 The
177 .Fn freopen
178 function
179 opens the file whose name is the string pointed to by
180 .Fa path
181 and associates the stream pointed to by
182 .Fa stream
183 with it.
184 The original stream (if it exists) is closed.
185 The
186 .Fa mode
187 argument is used just as in the
188 .Fn fopen
189 function.
190 The primary use of the
191 .Fn freopen
192 function
193 is to change the file associated with a
194 standard text stream
195 .Pf ( Em stderr ,
196 .Em stdin ,
197 or
198 .Em stdout ) .
199 .Pp
200 Input and output against the opened stream will be fully buffered, unless
201 it refers to an interactive terminal device, or a different kind of buffering
202 is specified in the environment.
203 See
204 .Xr setvbuf 3
205 for additional details.
206 .Sh RETURN VALUES
207 Upon successful completion
208 .Fn fopen ,
209 .Fn fdopen
210 and
211 .Fn freopen
212 return a
213 .Tn FILE
214 pointer.
215 Otherwise,
216 .Dv NULL
217 is returned and the global variable
218 .Va errno
219 is set to indicate the error.
220 .Sh ERRORS
221 The functions may fail if:
222 .Bl -tag -width Er
223 .It Bq Er EFTYPE
224 The file is not a regular file and the character ``f'' is specified
225 in the mode.
226 .It Bq Er EINVAL
227 The specified
228 .Fa mode
229 was invalid.
230 .El
231 .Pp
232 The
233 .Fn fopen ,
234 .Fn fdopen
235 and
236 .Fn freopen
237 functions
238 may also fail and set
239 .Va errno
240 for any of the errors specified for the routine
241 .Xr malloc 3 .
242 .Pp
243 The
244 .Fn fopen
245 function
246 may also fail and set
247 .Va errno
248 for any of the errors specified for the routine
249 .Xr open 2 .
250 .Pp
251 The
252 .Fn fdopen
253 function
254 may also fail and set
255 .Va errno
256 for any of the errors specified for the routine
257 .Xr fcntl 2 .
258 .Pp
259 The
260 .Fn freopen
261 function
262 may also fail and set
263 .Va errno
264 for any of the errors specified for the routines
265 .Xr open 2 ,
266 .Xr fclose 3
267 and
268 .Xr fflush 3 .
269 .Sh SEE ALSO
270 .Xr open 2 ,
271 .Xr fclose 3 ,
272 .Xr fileno 3 ,
273 .Xr fseek 3 ,
274 .Xr funopen 3
275 .Sh STANDARDS
276 The
277 .Fn fopen
278 and
279 .Fn freopen
280 functions conform to
281 .St -ansiC .
282 All three functions are specified in
283 .St -p1003.1-2008 .
284 .Sh CAVEATS
285 Proper code using
286 .Fn fdopen
287 with error checking should
288 .Xr close 2
289 .Fa fildes
290 in case of failure, and
291 .Xr fclose 3
292 the resulting FILE * in case of success.
293 .Bd -literal
294 FILE *file;
295 int fd;
297 if ((file = fdopen(fd, "r")) != NULL) {
298 /* perform operations on the FILE * */
299 fclose(file);
300 } else {
301 /* failure, report the error */
302 close(fd);
303 }
304 .Ed