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  2016-03-15 "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 .sp
  51 .BI "FILE *fopen(const char *" path ", const char *" mode );
  52
  53 .BI "FILE *fdopen(int " fd ", const char *" mode );
  54
  55 .BI "FILE *freopen(const char *" path ", const char *" mode ", FILE *" stream );
  56 .fi
  57 .sp
  58 .in -4n
  59 Feature Test Macro Requirements for glibc (see
  60 .BR feature_test_macros (7)):
  61 .in
  62 .sp
  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 path
  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 files will have 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
 152 .BR fdopen ()
 153 function associates a stream with the existing file descriptor,
 154 .IR fd .
 155 The
 156 .I mode
 157 of the stream (one of the values "r", "r+", "w", "w+", "a", "a+")
 158 must be compatible with the mode of the file descriptor.
 159 The file position indicator of the new stream is set to that
 160 belonging to
 161 .IR fd ,
 162 and the error and end-of-file indicators are cleared.
 163 Modes "w" or "w+" do not cause truncation of the file.
 164 The file descriptor is not dup'ed, and will be closed when
 165 the stream created by
 166 .BR fdopen ()
 167 is closed.
 168 The result of applying
 169 .BR fdopen ()
 170 to a shared memory object is undefined.
 171 .PP
 172 The
 173 .BR freopen ()
 174 function opens the file whose name is the string pointed to by
 175 .I path
 176 and associates the stream pointed to by
 177 .I stream
 178 with it.
 179 The original stream (if it exists) is closed.
 180 The
 181 .I mode
 182 argument is used just as in the
 183 .BR fopen ()
 184 function.
 185 The primary use of the
 186 .BR freopen ()
 187 function is to change the file associated with a standard text stream
 188 .RI ( stderr ", " stdin ", or " stdout ).
 189 .SH RETURN VALUE
 190 Upon successful completion
 191 .BR fopen (),
 192 .BR fdopen ()
 193 and
 194 .BR freopen ()
 195 return a
 196 .I FILE
 197 pointer.
 198 Otherwise, NULL is returned and
 199 .I errno
 200 is set to indicate the error.
 201 .SH ERRORS
 202 .TP
 203 .B EINVAL
 204 The
 205 .I mode
 206 provided to
 207 .BR fopen (),
 208 .BR fdopen (),
 209 or
 210 .BR freopen ()
 211 was invalid.
 212 .PP
 213 The
 214 .BR fopen (),
 215 .BR fdopen ()
 216 and
 217 .BR freopen ()
 218 functions may also fail and set
 219 .I errno
 220 for any of the errors specified for the routine
 221 .BR malloc (3).
 222 .PP
 223 The
 224 .BR fopen ()
 225 function may also fail and set
 226 .I errno
 227 for any of the errors specified for the routine
 228 .BR open (2).
 229 .PP
 230 The
 231 .BR fdopen ()
 232 function may also fail and set
 233 .I errno
 234 for any of the errors specified for the routine
 235 .BR fcntl (2).
 236 .PP
 237 The
 238 .BR freopen ()
 239 function may also fail and set
 240 .I errno
 241 for any of the errors specified for the routines
 242 .BR open (2),
 243 .BR fclose (3),
 244 and
 245 .BR fflush (3).
 246 .SH ATTRIBUTES
 247 For an explanation of the terms used in this section, see
 248 .BR attributes (7).
 249 .TS
 250 allbox;
 251 lbw28 lb lb
 252 l l l.
 253 Interface       Attribute       Value
 254 T{
 255 .BR fopen (),
 256 .BR fdopen (),
 257 .BR freopen ()
 258 T}      Thread safety   MT-Safe
 259 .TE
 260 .SH CONFORMING TO
 261 .BR fopen (),
 262 .BR freopen ():
 263 POSIX.1-2001, POSIX.1-2008, C89, C99.
 264
 265 .BR fdopen ():
 266 POSIX.1-2001, POSIX.1-2008.
 267 .SH NOTES
 268 .SS Glibc notes
 269 The GNU C library allows the following extensions for the string specified in
 270 .IR mode :
 271 .TP
 272 .BR c " (since glibc 2.3.3)"
 273 Do not make the open operation,
 274 or subsequent read and write operations,
 275 thread cancellation points.
 276 This flag is ignored for
 277 .BR fdopen ().
 278 .TP
 279 .BR e " (since glibc 2.7)"
 280 Open the file with the
 281 .B O_CLOEXEC
 282 flag.
 283 See
 284 .BR open (2)
 285 for more information.
 286 This flag is ignored for
 287 .BR fdopen ().
 288 .TP
 289 .BR m " (since glibc 2.3)"
 290 Attempt to access the file using
 291 .BR mmap (2),
 292 rather than I/O system calls
 293 .RB ( read (2),
 294 .BR write (2)).
 295 Currently,
 296 .\" As at glibc 2.4:
 297 use of
 298 .BR mmap (2)
 299 is attempted only for a file opened for reading.
 300 .TP
 301 .B x
 302 .\" Since glibc 2.0?
 303 .\" FIXME . C11 specifies this flag
 304 Open the file exclusively
 305 (like the
 306 .B O_EXCL
 307 flag of
 308 .BR open (2)).
 309 If the file already exists,
 310 .BR fopen ()
 311 fails, and sets
 312 .I errno
 313 to
 314 .BR EEXIST .
 315 This flag is ignored for
 316 .BR fdopen ().
 317 .PP
 318 In addition to the above characters,
 319 .BR fopen ()
 320 and
 321 .BR freopen ()
 322 support the following syntax
 323 in
 324 .IR mode :
 325
 326 .BI "    ,ccs=" string
 327
 328 The given
 329 .I string
 330 is taken as the name of a coded character set and
 331 the stream is marked as wide-oriented.
 332 Thereafter, internal conversion functions convert I/O
 333 to and from the character set
 334 .IR string .
 335 If the
 336 .BI ,ccs= string
 337 syntax is not specified,
 338 then the wide-orientation of the stream is
 339 determined by the first file operation.
 340 If that operation is a wide-character operation,
 341 the stream is marked wide-oriented,
 342 and functions to convert to the coded character set are loaded.
 343 .SH BUGS
 344 When parsing for individual flag characters in
 345 .IR mode
 346 (i.e., the characters preceding the "ccs" specification),
 347 the glibc implementation of
 348 .\" FIXME . http://sourceware.org/bugzilla/show_bug.cgi?id=12685
 349 .BR fopen ()
 350 and
 351 .BR freopen ()
 352 limits the number of characters examined in
 353 .I mode
 354 to 7 (or, in glibc versions before 2.14, to 6,
 355 which was not enough to include possible specifications such as "rb+cmxe").
 356 The current implementation of
 357 .BR fdopen ()
 358 parses at most 5 characters in
 359 .IR mode .
 360 .SH SEE ALSO
 361 .BR open (2),
 362 .BR fclose (3),
 363 .BR fileno (3),
 364 .BR fmemopen (3),
 365 .BR fopencookie (3),
 366 .BR open_memstream (3)