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 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 .PP
 147 .in +4n
 148 .EX
 149 fseek(stream, 0, SEEK_END);
 150 .EE
 151 .in
 152 .PP
 153 The file descriptor associated with the stream is opened as if by a call to
 154 .BR open (2)
 155 with the following flags:
 156 .RS
 157 .TS
 158 allbox;
 159 lb lb
 160 c l.
 161 fopen() mode    open() flags
 162 \fIr\fP O_RDONLY
 163 \fIw\fP O_WRONLY | O_CREAT | O_TRUNC
 164 \fIa\fP O_WRONLY | O_CREAT | O_APPEND
 165 \fIr+\fP        O_RDWR
 166 \fIw+\fP        O_RDWR | O_CREAT | O_TRUNC
 167 \fIa+\fP        O_RDWR | O_CREAT | O_APPEND
 168 .TE
 169 .RE
 170 .\"
 171 .SS fdopen()
 172 The
 173 .BR fdopen ()
 174 function associates a stream with the existing file descriptor,
 175 .IR fd .
 176 The
 177 .I mode
 178 of the stream (one of the values "r", "r+", "w", "w+", "a", "a+")
 179 must be compatible with the mode of the file descriptor.
 180 The file position indicator of the new stream is set to that
 181 belonging to
 182 .IR fd ,
 183 and the error and end-of-file indicators are cleared.
 184 Modes "w" or "w+" do not cause truncation of the file.
 185 The file descriptor is not dup'ed, and will be closed when
 186 the stream created by
 187 .BR fdopen ()
 188 is closed.
 189 The result of applying
 190 .BR fdopen ()
 191 to a shared memory object is undefined.
 192 .\"
 193 .SS freopen()
 194 The
 195 .BR freopen ()
 196 function opens the file whose name is the string pointed to by
 197 .I pathname
 198 and associates the stream pointed to by
 199 .I stream
 200 with it.
 201 The original stream (if it exists) is closed.
 202 The
 203 .I mode
 204 argument is used just as in the
 205 .BR fopen ()
 206 function.
 207 .PP
 208 If the
 209 .I pathname
 210 argument is a null pointer,
 211 .BR freopen ()
 212 changes the mode of the stream to that specified in
 213 .IR mode ;
 214 that is,
 215 .BR freopen ()
 216 reopens the pathname that is associated with the stream.
 217 The specification for this behavior was added in the C99 standard, which says:
 218 .PP
 219 .RS
 220 In this case,
 221 the file descriptor associated with the stream need not be closed
 222 if the call to
 223 .BR freopen ()
 224 succeeds.
 225 It is implementation-defined which changes of mode are permitted (if any),
 226 and under what circumstances.
 227 .RE
 228 .PP
 229 The primary use of the
 230 .BR freopen ()
 231 function is to change the file associated with a standard text stream
 232 .RI ( stderr ", " stdin ", or " stdout ).
 233 .SH RETURN VALUE
 234 Upon successful completion
 235 .BR fopen (),
 236 .BR fdopen ()
 237 and
 238 .BR freopen ()
 239 return a
 240 .I FILE
 241 pointer.
 242 Otherwise, NULL is returned and
 243 .I errno
 244 is set to indicate the error.
 245 .SH ERRORS
 246 .TP
 247 .B EINVAL
 248 The
 249 .I mode
 250 provided to
 251 .BR fopen (),
 252 .BR fdopen (),
 253 or
 254 .BR freopen ()
 255 was invalid.
 256 .PP
 257 The
 258 .BR fopen (),
 259 .BR fdopen ()
 260 and
 261 .BR freopen ()
 262 functions may also fail and set
 263 .I errno
 264 for any of the errors specified for the routine
 265 .BR malloc (3).
 266 .PP
 267 The
 268 .BR fopen ()
 269 function may also fail and set
 270 .I errno
 271 for any of the errors specified for the routine
 272 .BR open (2).
 273 .PP
 274 The
 275 .BR fdopen ()
 276 function may also fail and set
 277 .I errno
 278 for any of the errors specified for the routine
 279 .BR fcntl (2).
 280 .PP
 281 The
 282 .BR freopen ()
 283 function may also fail and set
 284 .I errno
 285 for any of the errors specified for the routines
 286 .BR open (2),
 287 .BR fclose (3),
 288 and
 289 .BR fflush (3).
 290 .SH ATTRIBUTES
 291 For an explanation of the terms used in this section, see
 292 .BR attributes (7).
 293 .TS
 294 allbox;
 295 lbw28 lb lb
 296 l l l.
 297 Interface       Attribute       Value
 298 T{
 299 .BR fopen (),
 300 .BR fdopen (),
 301 .BR freopen ()
 302 T}      Thread safety   MT-Safe
 303 .TE
 304 .SH CONFORMING TO
 305 .BR fopen (),
 306 .BR freopen ():
 307 POSIX.1-2001, POSIX.1-2008, C89, C99.
 308 .PP
 309 .BR fdopen ():
 310 POSIX.1-2001, POSIX.1-2008.
 311 .SH NOTES
 312 .SS Glibc notes
 313 The GNU C library allows the following extensions for the string specified in
 314 .IR mode :
 315 .TP
 316 .BR c " (since glibc 2.3.3)"
 317 Do not make the open operation,
 318 or subsequent read and write operations,
 319 thread cancellation points.
 320 This flag is ignored for
 321 .BR fdopen ().
 322 .TP
 323 .BR e " (since glibc 2.7)"
 324 Open the file with the
 325 .B O_CLOEXEC
 326 flag.
 327 See
 328 .BR open (2)
 329 for more information.
 330 This flag is ignored for
 331 .BR fdopen ().
 332 .TP
 333 .BR m " (since glibc 2.3)"
 334 Attempt to access the file using
 335 .BR mmap (2),
 336 rather than I/O system calls
 337 .RB ( read (2),
 338 .BR write (2)).
 339 Currently,
 340 .\" As at glibc 2.4:
 341 use of
 342 .BR mmap (2)
 343 is attempted only for a file opened for reading.
 344 .TP
 345 .B x
 346 .\" Since glibc 2.0?
 347 .\" FIXME . C11 specifies this flag
 348 Open the file exclusively
 349 (like the
 350 .B O_EXCL
 351 flag of
 352 .BR open (2)).
 353 If the file already exists,
 354 .BR fopen ()
 355 fails, and sets
 356 .I errno
 357 to
 358 .BR EEXIST .
 359 This flag is ignored for
 360 .BR fdopen ().
 361 .PP
 362 In addition to the above characters,
 363 .BR fopen ()
 364 and
 365 .BR freopen ()
 366 support the following syntax
 367 in
 368 .IR mode :
 369 .PP
 370 .BI "    ,ccs=" string
 371 .PP
 372 The given
 373 .I string
 374 is taken as the name of a coded character set and
 375 the stream is marked as wide-oriented.
 376 Thereafter, internal conversion functions convert I/O
 377 to and from the character set
 378 .IR string .
 379 If the
 380 .BI ,ccs= string
 381 syntax is not specified,
 382 then the wide-orientation of the stream is
 383 determined by the first file operation.
 384 If that operation is a wide-character operation,
 385 the stream is marked wide-oriented,
 386 and functions to convert to the coded character set are loaded.
 387 .SH BUGS
 388 When parsing for individual flag characters in
 389 .IR mode
 390 (i.e., the characters preceding the "ccs" specification),
 391 the glibc implementation of
 392 .\" FIXME . http://sourceware.org/bugzilla/show_bug.cgi?id=12685
 393 .BR fopen ()
 394 and
 395 .BR freopen ()
 396 limits the number of characters examined in
 397 .I mode
 398 to 7 (or, in glibc versions before 2.14, to 6,
 399 which was not enough to include possible specifications such as "rb+cmxe").
 400 The current implementation of
 401 .BR fdopen ()
 402 parses at most 5 characters in
 403 .IR mode .
 404 .SH SEE ALSO
 405 .BR open (2),
 406 .BR fclose (3),
 407 .BR fileno (3),
 408 .BR fmemopen (3),
 409 .BR fopencookie (3),
 410 .BR open_memstream (3)