man3/fopen.3

   1 .\" Copyright (c) 1990, 1991 The Regents of the University of California.
   2 .\" All rights reserved.
   3 .\"
   4 .\" This code is derived from software contributed to Berkeley by
   5 .\" Chris Torek and the American National Standards Committee X3,
   6 .\" on Information Processing Systems.
   7 .\"
   8 .\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
   9 .\" Redistribution and use in source and binary forms, with or without
  10 .\" modification, are permitted provided that the following conditions
  11 .\" are met:
  12 .\" 1. Redistributions of source code must retain the above copyright
  13 .\"    notice, this list of conditions and the following disclaimer.
  14 .\" 2. Redistributions in binary form must reproduce the above copyright
  15 .\"    notice, this list of conditions and the following disclaimer in the
  16 .\"    documentation and/or other materials provided with the distribution.
  17 .\" 3. All advertising materials mentioning features or use of this software
  18 .\"    must display the following acknowledgement:
  19 .\"     This product includes software developed by the University of
  20 .\"     California, Berkeley and its contributors.
  21 .\" 4. Neither the name of the University nor the names of its contributors
  22 .\"    may be used to endorse or promote products derived from this software
  23 .\"    without specific prior written permission.
  24 .\"
  25 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  26 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  27 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  28 .\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  29 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  30 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  31 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  32 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  33 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  34 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  35 .\" SUCH DAMAGE.
  36 .\" %%%LICENSE_END
  37 .\"
  38 .\"     @(#)fopen.3     6.8 (Berkeley) 6/29/91
  39 .\"
  40 .\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
  41 .\" Modified, aeb, 960421, 970806
  42 .\" Modified, joey, aeb, 2002-01-03
  43 .\"
  44 .TH FOPEN 3  2017-03-13 "GNU" "Linux Programmer's Manual"
  45 .SH NAME
  46 fopen, fdopen, freopen \- stream open functions
  47 .SH SYNOPSIS
  48 .nf
  49 .B #include <stdio.h>
  50 .PP
  51 .BI "FILE *fopen(const char *" pathname ", const char *" mode );
  52 .PP
  53 .BI "FILE *fdopen(int " fd ", const char *" mode );
  54 .PP
  55 .BI "FILE *freopen(const char *" pathname ", const char *" mode ", FILE *" stream );
  56 .fi
  57 .PP
  58 .in -4n
  59 Feature Test Macro Requirements for glibc (see
  60 .BR feature_test_macros (7)):
  61 .in
  62 .PP
  63 .BR fdopen ():
  64 _POSIX_C_SOURCE
  65 .SH DESCRIPTION
  66 The
  67 .BR fopen ()
  68 function opens the file whose name is the string pointed to by
  69 .I pathname
  70 and associates a stream with it.
  71 .PP
  72 The argument
  73 .I mode
  74 points to a string beginning with one of the following sequences
  75 (possibly followed by additional characters, as described below):
  76 .TP
  77 .B r
  78 Open text file for reading.
  79 The stream is positioned at the beginning of the file.
  80 .TP
  81 .B r+
  82 Open for reading and writing.
  83 The stream is positioned at the beginning of the file.
  84 .TP
  85 .B w
  86 Truncate file to zero length or create text file for writing.
  87 The stream is positioned at the beginning of the file.
  88 .TP
  89 .B w+
  90 Open for reading and writing.
  91 The file is created if it does not exist, otherwise it is truncated.
  92 The stream is positioned at the beginning of
  93 the file.
  94 .TP
  95 .B a
  96 Open for appending (writing at end of file).
  97 The file is created if it does not exist.
  98 The stream is positioned at the end of the file.
  99 .TP
 100 .B a+
 101 Open for reading and appending (writing at end of file).
 102 The file is created if it does not exist.
 103 The initial file position for reading is at the beginning of the file,
 104 but output is always appended to the end of the file.
 105 .PP
 106 The
 107 .I mode
 108 string can also include the letter \(aqb\(aq either as a last character or as
 109 a character between the characters in any of the two-character strings
 110 described above.
 111 This is strictly for compatibility with C89
 112 and has no effect; the \(aqb\(aq is ignored on all POSIX
 113 conforming systems, including Linux.
 114 (Other systems may treat text files and binary files differently,
 115 and adding the \(aqb\(aq may be a good idea if you do I/O to a binary
 116 file and expect that your program may be ported to non-UNIX
 117 environments.)
 118 .PP
 119 See NOTES below for details of glibc extensions for
 120 .IR mode .
 121 .PP
 122 Any created file will have the mode
 123 .BR S_IRUSR " | " S_IWUSR " | "  S_IRGRP " | "  S_IWGRP " | " S_IROTH " | " S_IWOTH
 124 (0666), as modified by the process's umask value (see
 125 .BR umask (2)).
 126 .PP
 127 Reads and writes may be intermixed on read/write streams in any order.
 128 Note that ANSI C requires that a file positioning function intervene
 129 between output and input, unless an input operation encounters end-of-file.
 130 (If this condition is not met, then a read is allowed to return the
 131 result of writes other than the most recent.)
 132 Therefore it is good practice (and indeed sometimes necessary
 133 under Linux) to put an
 134 .BR fseek (3)
 135 or
 136 .BR fgetpos (3)
 137 operation between write and read operations on such a stream.
 138 This operation may be an apparent no-op
 139 (as in \fIfseek(..., 0L, SEEK_CUR)\fP
 140 called for its synchronizing side effect).
 141 .PP
 142 Opening a file in append mode (\fBa\fP as the first character of
 143 .IR mode )
 144 causes all subsequent write operations to this stream to occur
 145 at end-of-file, as if preceded the call:
 146 .nf
 147
 148     fseek(stream, 0, SEEK_END);
 149 .fi
 150 .PP
 151 The file descriptor associated with the stream is opened as if by a call to
 152 .BR open (2)
 153 with the following flags:
 154 .RS
 155 .TS
 156 allbox;
 157 lb lb
 158 c l.
 159 fopen() mode    open() flags
 160 \fIr\fP O_RDONLY
 161 \fIw\fP O_WRONLY | O_CREAT | O_TRUNC
 162 \fIa\fP O_WRONLY | O_CREAT | O_APPEND
 163 \fIr+\fP        O_RDWR
 164 \fIw+\fP        O_RDWR | O_CREAT | O_TRUNC
 165 \fIa+\fP        O_RDWR | O_CREAT | O_APPEND
 166 .TE
 167 .RE
 168 .\"
 169 .SS fdopen()
 170 The
 171 .BR fdopen ()
 172 function associates a stream with the existing file descriptor,
 173 .IR fd .
 174 The
 175 .I mode
 176 of the stream (one of the values "r", "r+", "w", "w+", "a", "a+")
 177 must be compatible with the mode of the file descriptor.
 178 The file position indicator of the new stream is set to that
 179 belonging to
 180 .IR fd ,
 181 and the error and end-of-file indicators are cleared.
 182 Modes "w" or "w+" do not cause truncation of the file.
 183 The file descriptor is not dup'ed, and will be closed when
 184 the stream created by
 185 .BR fdopen ()
 186 is closed.
 187 The result of applying
 188 .BR fdopen ()
 189 to a shared memory object is undefined.
 190 .\"
 191 .SS freopen()
 192 The
 193 .BR freopen ()
 194 function opens the file whose name is the string pointed to by
 195 .I pathname
 196 and associates the stream pointed to by
 197 .I stream
 198 with it.
 199 The original stream (if it exists) is closed.
 200 The
 201 .I mode
 202 argument is used just as in the
 203 .BR fopen ()
 204 function.
 205 .PP
 206 If the
 207 .I pathname
 208 argument is a null pointer,
 209 .BR freopen ()
 210 changes the mode of the stream to that specified in
 211 .IR mode ;
 212 that is,
 213 .BR freopen ()
 214 reopens the pathname that is associated with the stream.
 215 The specification for this behavior was added in the C99 standard, which says:
 216 .PP
 217 .RS
 218 In this case,
 219 the file descriptor associated with the stream need not be closed
 220 if the call to
 221 .BR freopen ()
 222 succeeds.
 223 It is implementation-defined which changes of mode are permitted (if any),
 224 and under what circumstances.
 225 .RE
 226 .PP
 227 The primary use of the
 228 .BR freopen ()
 229 function is to change the file associated with a standard text stream
 230 .RI ( stderr ", " stdin ", or " stdout ).
 231 .SH RETURN VALUE
 232 Upon successful completion
 233 .BR fopen (),
 234 .BR fdopen ()
 235 and
 236 .BR freopen ()
 237 return a
 238 .I FILE
 239 pointer.
 240 Otherwise, NULL is returned and
 241 .I errno
 242 is set to indicate the error.
 243 .SH ERRORS
 244 .TP
 245 .B EINVAL
 246 The
 247 .I mode
 248 provided to
 249 .BR fopen (),
 250 .BR fdopen (),
 251 or
 252 .BR freopen ()
 253 was invalid.
 254 .PP
 255 The
 256 .BR fopen (),
 257 .BR fdopen ()
 258 and
 259 .BR freopen ()
 260 functions may also fail and set
 261 .I errno
 262 for any of the errors specified for the routine
 263 .BR malloc (3).
 264 .PP
 265 The
 266 .BR fopen ()
 267 function may also fail and set
 268 .I errno
 269 for any of the errors specified for the routine
 270 .BR open (2).
 271 .PP
 272 The
 273 .BR fdopen ()
 274 function may also fail and set
 275 .I errno
 276 for any of the errors specified for the routine
 277 .BR fcntl (2).
 278 .PP
 279 The
 280 .BR freopen ()
 281 function may also fail and set
 282 .I errno
 283 for any of the errors specified for the routines
 284 .BR open (2),
 285 .BR fclose (3),
 286 and
 287 .BR fflush (3).
 288 .SH ATTRIBUTES
 289 For an explanation of the terms used in this section, see
 290 .BR attributes (7).
 291 .TS
 292 allbox;
 293 lbw28 lb lb
 294 l l l.
 295 Interface       Attribute       Value
 296 T{
 297 .BR fopen (),
 298 .BR fdopen (),
 299 .BR freopen ()
 300 T}      Thread safety   MT-Safe
 301 .TE
 302 .SH CONFORMING TO
 303 .BR fopen (),
 304 .BR freopen ():
 305 POSIX.1-2001, POSIX.1-2008, C89, C99.
 306 .PP
 307 .BR fdopen ():
 308 POSIX.1-2001, POSIX.1-2008.
 309 .SH NOTES
 310 .SS Glibc notes
 311 The GNU C library allows the following extensions for the string specified in
 312 .IR mode :
 313 .TP
 314 .BR c " (since glibc 2.3.3)"
 315 Do not make the open operation,
 316 or subsequent read and write operations,
 317 thread cancellation points.
 318 This flag is ignored for
 319 .BR fdopen ().
 320 .TP
 321 .BR e " (since glibc 2.7)"
 322 Open the file with the
 323 .B O_CLOEXEC
 324 flag.
 325 See
 326 .BR open (2)
 327 for more information.
 328 This flag is ignored for
 329 .BR fdopen ().
 330 .TP
 331 .BR m " (since glibc 2.3)"
 332 Attempt to access the file using
 333 .BR mmap (2),
 334 rather than I/O system calls
 335 .RB ( read (2),
 336 .BR write (2)).
 337 Currently,
 338 .\" As at glibc 2.4:
 339 use of
 340 .BR mmap (2)
 341 is attempted only for a file opened for reading.
 342 .TP
 343 .B x
 344 .\" Since glibc 2.0?
 345 .\" FIXME . C11 specifies this flag
 346 Open the file exclusively
 347 (like the
 348 .B O_EXCL
 349 flag of
 350 .BR open (2)).
 351 If the file already exists,
 352 .BR fopen ()
 353 fails, and sets
 354 .I errno
 355 to
 356 .BR EEXIST .
 357 This flag is ignored for
 358 .BR fdopen ().
 359 .PP
 360 In addition to the above characters,
 361 .BR fopen ()
 362 and
 363 .BR freopen ()
 364 support the following syntax
 365 in
 366 .IR mode :
 367 .PP
 368 .BI "    ,ccs=" string
 369 .PP
 370 The given
 371 .I string
 372 is taken as the name of a coded character set and
 373 the stream is marked as wide-oriented.
 374 Thereafter, internal conversion functions convert I/O
 375 to and from the character set
 376 .IR string .
 377 If the
 378 .BI ,ccs= string
 379 syntax is not specified,
 380 then the wide-orientation of the stream is
 381 determined by the first file operation.
 382 If that operation is a wide-character operation,
 383 the stream is marked wide-oriented,
 384 and functions to convert to the coded character set are loaded.
 385 .SH BUGS
 386 When parsing for individual flag characters in
 387 .IR mode
 388 (i.e., the characters preceding the "ccs" specification),
 389 the glibc implementation of
 390 .\" FIXME . http://sourceware.org/bugzilla/show_bug.cgi?id=12685
 391 .BR fopen ()
 392 and
 393 .BR freopen ()
 394 limits the number of characters examined in
 395 .I mode
 396 to 7 (or, in glibc versions before 2.14, to 6,
 397 which was not enough to include possible specifications such as "rb+cmxe").
 398 The current implementation of
 399 .BR fdopen ()
 400 parses at most 5 characters in
 401 .IR mode .
 402 .SH SEE ALSO
 403 .BR open (2),
 404 .BR fclose (3),
 405 .BR fileno (3),
 406 .BR fmemopen (3),
 407 .BR fopencookie (3),
 408 .BR open_memstream (3)