man3/scanf.3

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  37 .\"
  38 .\"     @(#)scanf.3     6.14 (Berkeley) 1/8/93
  39 .\"
  40 .\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
  41 .\" modified to resemble the GNU libio setup used in the Linux libc
  42 .\" used in versions 4.x (x>4) and 5   Helmut.Geyer@iwr.uni-heidelberg.de
  43 .\" Modified, aeb, 970121
  44 .\" 2005-07-14, mtk, added description of %n$ form; various text
  45 .\"     incorporated from the GNU C library documentation ((C) The
  46 .\"     Free Software Foundation); other parts substantially rewritten.
  47 .\"
  48 .\" 2008-06-23, mtk
  49 .\"     Add ERRORS section.
  50 .\"     Document the 'a' and 'm' modifiers for dynamic string allocation.
  51 .\"
  52 .TH SCANF 3  2016-03-15 "GNU" "Linux Programmer's Manual"
  53 .SH NAME
  54 scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf \- input format conversion
  55 .SH SYNOPSIS
  56 .nf
  57 .B #include <stdio.h>
  58
  59 .BI "int scanf(const char *" format ", ...);"
  60 .BI "int fscanf(FILE *" stream ", const char *" format ", ...);"
  61 .BI "int sscanf(const char *" str ", const char *" format ", ...);"
  62 .sp
  63 .B #include <stdarg.h>
  64
  65 .BI "int vscanf(const char *" format ", va_list " ap );
  66 .BI "int vsscanf(const char *" str ", const char *" format ", va_list " ap );
  67 .BI "int vfscanf(FILE *" stream ", const char *" format ", va_list " ap );
  68 .fi
  69 .sp
  70 .in -4n
  71 Feature Test Macro Requirements for glibc (see
  72 .BR feature_test_macros (7)):
  73 .in
  74 .ad l
  75 .sp
  76 .BR vscanf (),
  77 .BR vsscanf (),
  78 .BR vfscanf ():
  79 .RS 4
  80 _ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
  81 .ad
  82 .RE
  83 .SH DESCRIPTION
  84 The
  85 .BR scanf ()
  86 family of functions scans input according to
  87 .I format
  88 as described below.
  89 This format may contain
  90 .IR "conversion specifications" ;
  91 the results from such conversions, if any,
  92 are stored in the locations pointed to by the
  93 .I pointer
  94 arguments that follow
  95 .IR format .
  96 Each
  97 .I pointer
  98 argument must be of a type that is appropriate for the value returned
  99 by the corresponding conversion specification.
 100
 101 If the number of conversion specifications in
 102 .I format
 103 exceeds the number of
 104 .I pointer
 105 arguments, the results are undefined.
 106 If the number of
 107 .I pointer
 108 arguments exceeds the number of conversion specifications, then the excess
 109 .I pointer
 110 arguments are evaluated, but are otherwise ignored.
 111
 112 The
 113 .BR scanf ()
 114 function reads input from the standard input stream
 115 .IR stdin ,
 116 .BR fscanf ()
 117 reads input from the stream pointer
 118 .IR stream ,
 119 and
 120 .BR sscanf ()
 121 reads its input from the character string pointed to by
 122 .IR str .
 123 .PP
 124 The
 125 .BR vfscanf ()
 126 function is analogous to
 127 .BR vfprintf (3)
 128 and reads input from the stream pointer
 129 .I stream
 130 using a variable argument list of pointers (see
 131 .BR stdarg (3).
 132 The
 133 .BR vscanf ()
 134 function scans a variable argument list from the standard input and the
 135 .BR vsscanf ()
 136 function scans it from a string; these are analogous to the
 137 .BR vprintf (3)
 138 and
 139 .BR vsprintf (3)
 140 functions respectively.
 141 .PP
 142 The
 143 .I format
 144 string consists of a sequence of
 145 .I directives
 146 which describe how to process the sequence of input characters.
 147 If processing of a directive fails, no further input is read, and
 148 .BR scanf ()
 149 returns.
 150 A "failure" can be either of the following:
 151 .IR "input failure" ,
 152 meaning that input characters were unavailable, or
 153 .IR "matching failure" ,
 154 meaning that the input was inappropriate (see below).
 155
 156 A directive is one of the following:
 157 .TP
 158 \(bu
 159 A sequence of white-space characters (space, tab, newline, etc.; see
 160 .BR isspace (3)).
 161 This directive matches any amount of white space,
 162 including none, in the input.
 163 .TP
 164 \(bu
 165 An ordinary character (i.e., one other than white space or \(aq%\(aq).
 166 This character must exactly match the next character of input.
 167 .TP
 168 \(bu
 169 A conversion specification,
 170 which commences with a \(aq%\(aq (percent) character.
 171 A sequence of characters from the input is converted according to
 172 this specification, and the result is placed in the corresponding
 173 .I pointer
 174 argument.
 175 If the next item of input does not match the conversion specification,
 176 the conversion fails\(emthis is a
 177 .IR "matching failure" .
 178 .PP
 179 Each
 180 .I conversion specification
 181 in
 182 .I format
 183 begins with either the character \(aq%\(aq or the character sequence
 184 "\fB%\fP\fIn\fP\fB$\fP"
 185 (see below for the distinction) followed by:
 186 .TP
 187 \(bu
 188 An optional \(aq*\(aq assignment-suppression character:
 189 .BR scanf ()
 190 reads input as directed by the conversion specification,
 191 but discards the input.
 192 No corresponding
 193 .I pointer
 194 argument is required, and this specification is not
 195 included in the count of successful assignments returned by
 196 .BR scanf ().
 197 .TP
 198 \(bu
 199 An optional \(aqm\(aq character.
 200 This is used with string conversions
 201 .RI ( %s ,
 202 .IR %c ,
 203 .IR %[ ),
 204 and relieves the caller of the
 205 need to allocate a corresponding buffer to hold the input: instead,
 206 .BR scanf ()
 207 allocates a buffer of sufficient size,
 208 and assigns the address of this buffer to the corresponding
 209 .I pointer
 210 argument, which should be a pointer to a
 211 .I "char\ *"
 212 variable (this variable does not need to be initialized before the call).
 213 The caller should subsequently
 214 .BR free (3)
 215 this buffer when it is no longer required.
 216 .TP
 217 \(bu
 218 An optional decimal integer which specifies the
 219 .IR "maximum field width" .
 220 Reading of characters stops either when this maximum is reached or
 221 when a nonmatching character is found, whichever happens first.
 222 Most conversions discard initial white space characters (the exceptions
 223 are noted below),
 224 and these discarded characters don't count toward the maximum field width.
 225 String input conversions store a terminating null byte (\(aq\\0\(aq)
 226 to mark the end of the input;
 227 the maximum field width does not include this terminator.
 228 .TP
 229 \(bu
 230 An optional
 231 .IR "type modifier character" .
 232 For example, the
 233 .B l
 234 type modifier is used with integer conversions such as
 235 .B %d
 236 to specify that the corresponding
 237 .I pointer
 238 argument refers to a
 239 .I "long int"
 240 rather than a pointer to an
 241 .IR int .
 242 .TP
 243 \(bu
 244 A
 245 .I "conversion specifier"
 246 that specifies the type of input conversion to be performed.
 247 .PP
 248 The conversion specifications in
 249 .I format
 250 are of two forms, either beginning with \(aq%\(aq or beginning with
 251 "\fB%\fP\fIn\fP\fB$\fP".
 252 The two forms should not be mixed in the same
 253 .I format
 254 string, except that a string containing
 255 "\fB%\fP\fIn\fP\fB$\fP"
 256 specifications can include
 257 .B %%
 258 and
 259 .BR %* .
 260 If
 261 .I format
 262 contains \(aq%\(aq
 263 specifications, then these correspond in order with successive
 264 .I pointer
 265 arguments.
 266 In the
 267 "\fB%\fP\fIn\fP\fB$\fP"
 268 form (which is specified in POSIX.1-2001, but not C99),
 269 .I n
 270 is a decimal integer that specifies that the converted input should
 271 be placed in the location referred to by the
 272 .IR n -th
 273 .I pointer
 274 argument following
 275 .IR format .
 276 .SS Conversions
 277 The following
 278 .I "type modifier characters"
 279 can appear in a conversion specification:
 280 .TP
 281 .B h
 282 Indicates that the conversion will be one of
 283 \fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, \fBx\fP, \fBX\fP, or \fBn\fP
 284 and the next pointer is a pointer to a
 285 .I short int
 286 or
 287 .I unsigned short int
 288 (rather than
 289 .IR int ).
 290 .TP
 291 .B hh
 292 As for
 293 .BR h ,
 294 but the next pointer is a pointer to a
 295 .I signed char
 296 or
 297 .IR "unsigned char" .
 298 .TP
 299 .B j
 300 As for
 301 .BR h ,
 302 but the next pointer is a pointer to an
 303 .I intmax_t
 304 or a
 305 .IR uintmax_t .
 306 This modifier was introduced in C99.
 307 .TP
 308 .B l
 309 Indicates either that the conversion will be one of
 310 \fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, \fBx\fP, \fBX\fP, or \fBn\fP
 311 and the next pointer is a pointer to a
 312 .I long int
 313 or
 314 .I unsigned long int
 315 (rather than
 316 .IR int ),
 317 or that the conversion will be one of
 318 \fBe\fP, \fBf\fP, or \fBg\fP
 319 and the next pointer is a pointer to
 320 .I double
 321 (rather than
 322 .IR float ).
 323 Specifying two
 324 .B l
 325 characters is equivalent to
 326 .BR L .
 327 If used with
 328 .B %c
 329 or
 330 .BR %s ,
 331 the corresponding parameter is considered
 332 as a pointer to a wide character or wide-character string respectively.
 333 .\" This use of l was introduced in Amendment 1 to ISO C90.
 334 .TP
 335 .B L
 336 Indicates that the conversion will be either
 337 \fBe\fP, \fBf\fP, or \fBg\fP
 338 and the next pointer is a pointer to
 339 .I "long double"
 340 or the conversion will be
 341 \fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, or \fBx\fP
 342 and the next pointer is a pointer to
 343 .IR "long long" .
 344 .\" MTK, Jul 05: The following is no longer true for modern
 345 .\" ANSI C (i.e., C99):
 346 .\" (Note that long long is not an
 347 .\" ANSI C
 348 .\" type. Any program using this will not be portable to all
 349 .\" architectures).
 350 .TP
 351 .B q
 352 equivalent to
 353 .BR L .
 354 This specifier does not exist in ANSI C.
 355 .TP
 356 .B t
 357 As for
 358 .BR h ,
 359 but the next pointer is a pointer to a
 360 .IR ptrdiff_t .
 361 This modifier was introduced in C99.
 362 .TP
 363 .B z
 364 As for
 365 .BR h ,
 366 but the next pointer is a pointer to a
 367 .IR size_t .
 368 This modifier was introduced in C99.
 369 .PP
 370 The following
 371 .I "conversion specifiers"
 372 are available:
 373 .TP
 374 .B %
 375 Matches a literal \(aq%\(aq.
 376 That is,
 377 .B %\&%
 378 in the format string matches a
 379 single input \(aq%\(aq character.
 380 No conversion is done (but initial white space characters are discarded),
 381 and assignment does not occur.
 382 .TP
 383 .B d
 384 Matches an optionally signed decimal integer;
 385 the next pointer must be a pointer to
 386 .IR int .
 387 .TP
 388 .B D
 389 Equivalent to
 390 .IR ld ;
 391 this exists only for backward compatibility.
 392 (Note: thus only in libc4.
 393 In libc5 and glibc the
 394 .B %D
 395 is silently ignored, causing old programs to fail mysteriously.)
 396 .TP
 397 .B i
 398 Matches an optionally signed integer; the next pointer must be a pointer to
 399 .IR int .
 400 The integer is read in base 16 if it begins with
 401 .I 0x
 402 or
 403 .IR 0X ,
 404 in base 8 if it begins with
 405 .IR 0 ,
 406 and in base 10 otherwise.
 407 Only characters that correspond to the base are used.
 408 .TP
 409 .B o
 410 Matches an unsigned octal integer; the next pointer must be a pointer to
 411 .IR "unsigned int" .
 412 .TP
 413 .B u
 414 Matches an unsigned decimal integer; the next pointer must be a
 415 pointer to
 416 .IR "unsigned int" .
 417 .TP
 418 .B x
 419 Matches an unsigned hexadecimal integer; the next pointer must
 420 be a pointer to
 421 .IR "unsigned int" .
 422 .TP
 423 .B X
 424 Equivalent to
 425 .BR x .
 426 .TP
 427 .B f
 428 Matches an optionally signed floating-point number; the next pointer must
 429 be a pointer to
 430 .IR float .
 431 .TP
 432 .B e
 433 Equivalent to
 434 .BR f .
 435 .TP
 436 .B g
 437 Equivalent to
 438 .BR f .
 439 .TP
 440 .B E
 441 Equivalent to
 442 .BR f .
 443 .TP
 444 .B a
 445 (C99) Equivalent to
 446 .BR f .
 447 .TP
 448 .B s
 449 Matches a sequence of non-white-space characters;
 450 the next pointer must be a pointer to the initial element of a
 451 character array that is long enough to hold the input sequence and
 452 the terminating null byte (\(aq\\0\(aq), which is added automatically.
 453 The input string stops at white space or at the maximum field
 454 width, whichever occurs first.
 455 .TP
 456 .B c
 457 Matches a sequence of characters whose length is specified by the
 458 .I maximum field width
 459 (default 1); the next pointer must be a pointer to
 460 .IR char ,
 461 and there must be enough room for all the characters
 462 (no terminating null byte is added).
 463 The usual skip of leading white space is suppressed.
 464 To skip white space first, use an explicit space in the format.
 465 .TP
 466 .B \&[
 467 Matches a nonempty sequence of characters from the specified set of
 468 accepted characters; the next pointer must be a pointer to
 469 .IR char ,
 470 and there must be enough room for all the characters in the string, plus a
 471 terminating null byte.
 472 The usual skip of leading white space is suppressed.
 473 The string is to be made up of characters in (or not in) a particular set;
 474 the set is defined by the characters between the open bracket
 475 .B [
 476 character and a close bracket
 477 .B ]
 478 character.
 479 The set
 480 .I excludes
 481 those characters if the first character after the open bracket is a
 482 circumflex
 483 .RB ( ^ ).
 484 To include a close bracket in the set, make it the first character after
 485 the open bracket or the circumflex; any other position will end the set.
 486 The hyphen character
 487 .B \-
 488 is also special; when placed between two other characters, it adds all
 489 intervening characters to the set.
 490 To include a hyphen, make it the last
 491 character before the final close bracket.
 492 For instance,
 493 .B [^]0\-9\-]
 494 means
 495 the set "everything except close bracket, zero through nine, and hyphen".
 496 The string ends with the appearance of a character not in the (or, with a
 497 circumflex, in) set or when the field width runs out.
 498 .TP
 499 .B p
 500 Matches a pointer value (as printed by
 501 .B %p
 502 in
 503 .BR printf (3);
 504 the next pointer must be a pointer to a pointer to
 505 .IR void .
 506 .TP
 507 .B n
 508 Nothing is expected; instead, the number of characters consumed thus far
 509 from the input is stored through the next pointer, which must be a pointer
 510 to
 511 .IR int .
 512 This is
 513 .I not
 514 a conversion and does
 515 .I not
 516 increase the count returned by the function.
 517 The assignment can be suppressed with the
 518 .B *
 519 assignment-suppression character, but the effect on the
 520 return value is undefined.
 521 Therefore
 522 .B %*n
 523 conversions should not be used.
 524 .SH RETURN VALUE
 525 On success, these functions return the number of input items
 526 successfully matched and assigned;
 527 this can be fewer than provided for,
 528 or even zero, in the event of an early matching failure.
 529
 530 The value
 531 .B EOF
 532 is returned if the end of input is reached before either the first
 533 successful conversion or a matching failure occurs.
 534 .B EOF
 535 is also returned if a read error occurs,
 536 in which case the error indicator for the stream (see
 537 .BR ferror (3))
 538 is set, and
 539 .I errno
 540 is set to indicate the error.
 541 .SH ERRORS
 542 .TP
 543 .B EAGAIN
 544 The file descriptor underlying
 545 .I stream
 546 is marked nonblocking, and the read operation would block.
 547 .TP
 548 .B EBADF
 549 The file descriptor underlying
 550 .I stream
 551 is invalid, or not open for reading.
 552 .TP
 553 .B EILSEQ
 554 Input byte sequence does not form a valid character.
 555 .TP
 556 .B EINTR
 557 The read operation was interrupted by a signal; see
 558 .BR signal (7).
 559 .TP
 560 .B EINVAL
 561 Not enough arguments; or
 562 .I format
 563 is NULL.
 564 .TP
 565 .B ENOMEM
 566 Out of memory.
 567 .TP
 568 .B ERANGE
 569 The result of an integer conversion would exceed the size
 570 that can be stored in the corresponding integer type.
 571 .SH ATTRIBUTES
 572 For an explanation of the terms used in this section, see
 573 .BR attributes (7).
 574 .TS
 575 allbox;
 576 lbw20 lb lb
 577 l l l.
 578 Interface       Attribute       Value
 579 T{
 580 .BR scanf (),
 581 .BR fscanf (),
 582 .br
 583 .BR sscanf (),
 584 .BR vscanf (),
 585 .br
 586 .BR vsscanf (),
 587 .BR vfscanf ()
 588 T}      Thread safety   MT-Safe locale
 589 .TE
 590
 591 .SH CONFORMING TO
 592 The functions
 593 .BR fscanf (),
 594 .BR scanf (),
 595 and
 596 .BR sscanf ()
 597 conform to C89 and C99 and POSIX.1-2001.
 598 These standards do not specify the
 599 .B ERANGE
 600 error.
 601 .PP
 602 The
 603 .B q
 604 specifier is the 4.4BSD notation for
 605 .IR "long long" ,
 606 while
 607 .B ll
 608 or the usage of
 609 .B L
 610 in integer conversions is the GNU notation.
 611 .PP
 612 The Linux version of these functions is based on the
 613 .I GNU
 614 .I libio
 615 library.
 616 Take a look at the
 617 .I info
 618 documentation of
 619 .I GNU
 620 .I libc (glibc-1.08)
 621 for a more concise description.
 622 .SH NOTES
 623 .SS The 'a' assignment-allocation modifier
 624 Originally, the GNU C library supported dynamic allocation for string inputs
 625 (as a nonstandard extension) via the
 626 .B a
 627 character.
 628 (This feature is present at least as far back as glibc 2.0.)
 629 Thus, one could write the following to have
 630 .BR scanf ()
 631 allocate a buffer for an input string,
 632 with a pointer to that buffer being returned in
 633 .IR *buf :
 634
 635     char *buf;
 636     scanf("%as", &buf);
 637 .PP
 638 The use of the letter
 639 .B a
 640 for this purpose was problematic, since
 641 .B a
 642 is also specified by the ISO C standard as a synonym for
 643 .B f
 644 (floating-point input).
 645 POSIX.1-2008 instead specifies the
 646 .B m
 647 modifier for assignment allocation (as documented in DESCRIPTION, above).
 648 .PP
 649 Note that the
 650 .B a
 651 modifier is not available if the program is compiled with
 652 .I "gcc -std=c99"
 653 or
 654 .IR "gcc -D_ISOC99_SOURCE"
 655 (unless
 656 .B _GNU_SOURCE
 657 is also specified), in which case the
 658 .B a
 659 is interpreted as a specifier for floating-point numbers (see above).
 660
 661 Support for the
 662 .B m
 663 modifier was added to glibc starting with version 2.7,
 664 and new programs should use that modifier instead of
 665 .BR a .
 666
 667 As well as being standardized by POSIX, the
 668 .B m
 669 modifier has the following further advantages over
 670 the use of
 671 .BR a:
 672 .IP * 2
 673 It may also be applied to
 674 .B %c
 675 conversion specifiers (e.g.,
 676 .BR %3mc ).
 677 .IP *
 678 It avoids ambiguity with respect to the
 679 .B %a
 680 floating-point conversion specifier (and is unaffected by
 681 .IR "gcc -std=c99"
 682 etc.).
 683 .SH BUGS
 684 All functions are fully C89 conformant, but provide the
 685 additional specifiers
 686 .B q
 687 and
 688 .B a
 689 as well as an additional behavior of the
 690 .B L
 691 and
 692 .B l
 693 specifiers.
 694 The latter may be considered to be a bug, as it changes the
 695 behavior of specifiers defined in C89.
 696 .PP
 697 Some combinations of the type modifiers and conversion
 698 specifiers defined by ANSI C do not make sense
 699 (e.g.,
 700 .BR "%Ld" ).
 701 While they may have a well-defined behavior on Linux, this need not
 702 to be so on other architectures.
 703 Therefore it usually is better to use
 704 modifiers that are not defined by ANSI C at all, that is, use
 705 .B q
 706 instead of
 707 .B L
 708 in combination with
 709 \fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, \fBx\fP, and \fBX\fP
 710 conversions or
 711 .BR ll .
 712 .PP
 713 The usage of
 714 .B q
 715 is not the same as on 4.4BSD,
 716 as it may be used in float conversions equivalently to
 717 .BR L .
 718 .SH EXAMPLE
 719 To use the dynamic allocation conversion specifier, specify
 720 .B m
 721 as a length modifier (thus
 722 .B %ms
 723 or
 724 \fB%m[\fP\fIrange\fP\fB]\fP).
 725 The caller must
 726 .BR free (3)
 727 the returned string, as in the following example:
 728 .in +4n
 729 .nf
 730
 731 char *p;
 732 int n;
 733
 734 errno = 0;
 735 n = scanf("%m[a-z]", &p);
 736 if (n == 1) {
 737     printf("read: %s\\n", p);
 738     free(p);
 739 } else if (errno != 0) {
 740     perror("scanf");
 741 } else {
 742     fprintf(stderr, "No matching characters\\n");
 743 }
 744 .fi
 745 .in
 746 .PP
 747 As shown in the above example, it is necessary to call
 748 .BR free (3)
 749 only if the
 750 .BR scanf ()
 751 call successfully read a string.
 752 .SH SEE ALSO
 753 .BR getc (3),
 754 .BR printf (3),
 755 .BR setlocale (3),
 756 .BR strtod (3),
 757 .BR strtol (3),
 758 .BR strtoul (3)