man3/fopen.3

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  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  2017-09-15 "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 Output is always appended to the end of the file.
 104 POSIX is silent on what the initial read position is when using this mode.
 105 For glibc, the initial file position for reading is at
 106 the beginning of the file, but for Android/BSD/MacOS, the
 107 initial file position for reading is at the end of the file.
 108 .PP
 109 The
 110 .I mode
 111 string can also include the letter \(aqb\(aq either as a last character or as
 112 a character between the characters in any of the two-character strings
 113 described above.
 114 This is strictly for compatibility with C89
 115 and has no effect; the \(aqb\(aq is ignored on all POSIX
 116 conforming systems, including Linux.
 117 (Other systems may treat text files and binary files differently,
 118 and adding the \(aqb\(aq may be a good idea if you do I/O to a binary
 119 file and expect that your program may be ported to non-UNIX
 120 environments.)
 121 .PP
 122 See NOTES below for details of glibc extensions for
 123 .IR mode .
 124 .PP
 125 Any created file will have the mode
 126 .BR S_IRUSR " | " S_IWUSR " | "  S_IRGRP " | "  S_IWGRP " | " S_IROTH " | " S_IWOTH
 127 (0666), as modified by the process's umask value (see
 128 .BR umask (2)).
 129 .PP
 130 Reads and writes may be intermixed on read/write streams in any order.
 131 Note that ANSI C requires that a file positioning function intervene
 132 between output and input, unless an input operation encounters end-of-file.
 133 (If this condition is not met, then a read is allowed to return the
 134 result of writes other than the most recent.)
 135 Therefore it is good practice (and indeed sometimes necessary
 136 under Linux) to put an
 137 .BR fseek (3)
 138 or
 139 .BR fgetpos (3)
 140 operation between write and read operations on such a stream.
 141 This operation may be an apparent no-op
 142 (as in \fIfseek(..., 0L, SEEK_CUR)\fP
 143 called for its synchronizing side effect).
 144 .PP
 145 Opening a file in append mode (\fBa\fP as the first character of
 146 .IR mode )
 147 causes all subsequent write operations to this stream to occur
 148 at end-of-file, as if preceded the call:
 149 .PP
 150 .in +4n
 151 .EX
 152 fseek(stream, 0, SEEK_END);
 153 .EE
 154 .in
 155 .PP
 156 The file descriptor associated with the stream is opened as if by a call to
 157 .BR open (2)
 158 with the following flags:
 159 .RS
 160 .TS
 161 allbox;
 162 lb lb
 163 c l.
 164 fopen() mode    open() flags
 165 \fIr\fP O_RDONLY
 166 \fIw\fP O_WRONLY | O_CREAT | O_TRUNC
 167 \fIa\fP O_WRONLY | O_CREAT | O_APPEND
 168 \fIr+\fP        O_RDWR
 169 \fIw+\fP        O_RDWR | O_CREAT | O_TRUNC
 170 \fIa+\fP        O_RDWR | O_CREAT | O_APPEND
 171 .TE
 172 .RE
 173 .\"
 174 .SS fdopen()
 175 The
 176 .BR fdopen ()
 177 function associates a stream with the existing file descriptor,
 178 .IR fd .
 179 The
 180 .I mode
 181 of the stream (one of the values "r", "r+", "w", "w+", "a", "a+")
 182 must be compatible with the mode of the file descriptor.
 183 The file position indicator of the new stream is set to that
 184 belonging to
 185 .IR fd ,
 186 and the error and end-of-file indicators are cleared.
 187 Modes "w" or "w+" do not cause truncation of the file.
 188 The file descriptor is not dup'ed, and will be closed when
 189 the stream created by
 190 .BR fdopen ()
 191 is closed.
 192 The result of applying
 193 .BR fdopen ()
 194 to a shared memory object is undefined.
 195 .\"
 196 .SS freopen()
 197 The
 198 .BR freopen ()
 199 function opens the file whose name is the string pointed to by
 200 .I pathname
 201 and associates the stream pointed to by
 202 .I stream
 203 with it.
 204 The original stream (if it exists) is closed.
 205 The
 206 .I mode
 207 argument is used just as in the
 208 .BR fopen ()
 209 function.
 210 .PP
 211 If the
 212 .I pathname
 213 argument is a null pointer,
 214 .BR freopen ()
 215 changes the mode of the stream to that specified in
 216 .IR mode ;
 217 that is,
 218 .BR freopen ()
 219 reopens the pathname that is associated with the stream.
 220 The specification for this behavior was added in the C99 standard, which says:
 221 .PP
 222 .RS
 223 In this case,
 224 the file descriptor associated with the stream need not be closed
 225 if the call to
 226 .BR freopen ()
 227 succeeds.
 228 It is implementation-defined which changes of mode are permitted (if any),
 229 and under what circumstances.
 230 .RE
 231 .PP
 232 The primary use of the
 233 .BR freopen ()
 234 function is to change the file associated with a standard text stream
 235 .RI ( stderr ", " stdin ", or " stdout ).
 236 .SH RETURN VALUE
 237 Upon successful completion
 238 .BR fopen (),
 239 .BR fdopen ()
 240 and
 241 .BR freopen ()
 242 return a
 243 .I FILE
 244 pointer.
 245 Otherwise, NULL is returned and
 246 .I errno
 247 is set to indicate the error.
 248 .SH ERRORS
 249 .TP
 250 .B EINVAL
 251 The
 252 .I mode
 253 provided to
 254 .BR fopen (),
 255 .BR fdopen (),
 256 or
 257 .BR freopen ()
 258 was invalid.
 259 .PP
 260 The
 261 .BR fopen (),
 262 .BR fdopen ()
 263 and
 264 .BR freopen ()
 265 functions may also fail and set
 266 .I errno
 267 for any of the errors specified for the routine
 268 .BR malloc (3).
 269 .PP
 270 The
 271 .BR fopen ()
 272 function may also fail and set
 273 .I errno
 274 for any of the errors specified for the routine
 275 .BR open (2).
 276 .PP
 277 The
 278 .BR fdopen ()
 279 function may also fail and set
 280 .I errno
 281 for any of the errors specified for the routine
 282 .BR fcntl (2).
 283 .PP
 284 The
 285 .BR freopen ()
 286 function may also fail and set
 287 .I errno
 288 for any of the errors specified for the routines
 289 .BR open (2),
 290 .BR fclose (3),
 291 and
 292 .BR fflush (3).
 293 .SH ATTRIBUTES
 294 For an explanation of the terms used in this section, see
 295 .BR attributes (7).
 296 .TS
 297 allbox;
 298 lbw28 lb lb
 299 l l l.
 300 Interface       Attribute       Value
 301 T{
 302 .BR fopen (),
 303 .BR fdopen (),
 304 .BR freopen ()
 305 T}      Thread safety   MT-Safe
 306 .TE
 307 .SH CONFORMING TO
 308 .BR fopen (),
 309 .BR freopen ():
 310 POSIX.1-2001, POSIX.1-2008, C89, C99.
 311 .PP
 312 .BR fdopen ():
 313 POSIX.1-2001, POSIX.1-2008.
 314 .SH NOTES
 315 .SS Glibc notes
 316 The GNU C library allows the following extensions for the string specified in
 317 .IR mode :
 318 .TP
 319 .BR c " (since glibc 2.3.3)"
 320 Do not make the open operation,
 321 or subsequent read and write operations,
 322 thread cancellation points.
 323 This flag is ignored for
 324 .BR fdopen ().
 325 .TP
 326 .BR e " (since glibc 2.7)"
 327 Open the file with the
 328 .B O_CLOEXEC
 329 flag.
 330 See
 331 .BR open (2)
 332 for more information.
 333 This flag is ignored for
 334 .BR fdopen ().
 335 .TP
 336 .BR m " (since glibc 2.3)"
 337 Attempt to access the file using
 338 .BR mmap (2),
 339 rather than I/O system calls
 340 .RB ( read (2),
 341 .BR write (2)).
 342 Currently,
 343 .\" As at glibc 2.4:
 344 use of
 345 .BR mmap (2)
 346 is attempted only for a file opened for reading.
 347 .TP
 348 .B x
 349 .\" Since glibc 2.0?
 350 .\" FIXME . C11 specifies this flag
 351 Open the file exclusively
 352 (like the
 353 .B O_EXCL
 354 flag of
 355 .BR open (2)).
 356 If the file already exists,
 357 .BR fopen ()
 358 fails, and sets
 359 .I errno
 360 to
 361 .BR EEXIST .
 362 This flag is ignored for
 363 .BR fdopen ().
 364 .PP
 365 In addition to the above characters,
 366 .BR fopen ()
 367 and
 368 .BR freopen ()
 369 support the following syntax
 370 in
 371 .IR mode :
 372 .PP
 373 .BI "    ,ccs=" string
 374 .PP
 375 The given
 376 .I string
 377 is taken as the name of a coded character set and
 378 the stream is marked as wide-oriented.
 379 Thereafter, internal conversion functions convert I/O
 380 to and from the character set
 381 .IR string .
 382 If the
 383 .BI ,ccs= string
 384 syntax is not specified,
 385 then the wide-orientation of the stream is
 386 determined by the first file operation.
 387 If that operation is a wide-character operation,
 388 the stream is marked wide-oriented,
 389 and functions to convert to the coded character set are loaded.
 390 .SH BUGS
 391 When parsing for individual flag characters in
 392 .IR mode
 393 (i.e., the characters preceding the "ccs" specification),
 394 the glibc implementation of
 395 .\" FIXME . http://sourceware.org/bugzilla/show_bug.cgi?id=12685
 396 .BR fopen ()
 397 and
 398 .BR freopen ()
 399 limits the number of characters examined in
 400 .I mode
 401 to 7 (or, in glibc versions before 2.14, to 6,
 402 which was not enough to include possible specifications such as "rb+cmxe").
 403 The current implementation of
 404 .BR fdopen ()
 405 parses at most 5 characters in
 406 .IR mode .
 407 .SH SEE ALSO
 408 .BR open (2),
 409 .BR fclose (3),
 410 .BR fileno (3),
 411 .BR fmemopen (3),
 412 .BR fopencookie (3),
 413 .BR open_memstream (3)