1SCANF(3)                   Linux Programmer's Manual                  SCANF(3)
2
3
4

NAME

6       scanf,  fscanf, sscanf, vscanf, vsscanf, vfscanf - input format conver‐
7       sion
8

SYNOPSIS

10       #include <stdio.h>
11
12       int scanf(const char *format, ...);
13       int fscanf(FILE *stream, const char *format, ...);
14       int sscanf(const char *str, const char *format, ...);
15
16       #include <stdarg.h>
17
18       int vscanf(const char *format, va_list ap);
19       int vsscanf(const char *str, const char *format, va_list ap);
20       int vfscanf(FILE *stream, const char *format, va_list ap);
21
22   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
23
24       vscanf(), vsscanf(), vfscanf():
25           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
26

DESCRIPTION

28       The scanf() family of functions scans  input  according  to  format  as
29       described  below.   This  format may contain conversion specifications;
30       the results from such conversions, if any, are stored in the  locations
31       pointed  to  by the pointer arguments that follow format.  Each pointer
32       argument must be of a type that is appropriate for the  value  returned
33       by the corresponding conversion specification.
34
35       If the number of conversion specifications in format exceeds the number
36       of pointer arguments, the results are  undefined.   If  the  number  of
37       pointer arguments exceeds the number of conversion specifications, then
38       the excess pointer arguments are evaluated, but are otherwise ignored.
39
40       The scanf() function reads input from the standard input stream  stdin,
41       fscanf() reads input from the stream pointer stream, and sscanf() reads
42       its input from the character string pointed to by str.
43
44       The vfscanf() function is analogous to vfprintf(3) and reads input from
45       the  stream  pointer  stream using a variable argument list of pointers
46       (see stdarg(3).  The vscanf() function scans a variable  argument  list
47       from  the  standard  input  and  the vsscanf() function scans it from a
48       string; these are analogous to the vprintf(3) and vsprintf(3) functions
49       respectively.
50
51       The  format  string consists of a sequence of directives which describe
52       how to process the sequence of input characters.  If  processing  of  a
53       directive  fails,  no  further  input  is read, and scanf() returns.  A
54       "failure" can be either of the following: input failure,  meaning  that
55       input  characters  were  unavailable, or matching failure, meaning that
56       the input was inappropriate (see below).
57
58       A directive is one of the following:
59
60       ·      A sequence of white-space characters (space, tab, newline, etc.;
61              see  isspace(3)).   This  directive  matches any amount of white
62              space, including none, in the input.
63
64       ·      An ordinary character (i.e., one other than white space or '%').
65              This character must exactly match the next character of input.
66
67       ·      A conversion specification, which commences with a '%' (percent)
68              character.  A sequence of characters from the input is converted
69              according to this specification, and the result is placed in the
70              corresponding pointer argument.  If the next item of input  does
71              not  match  the  conversion specification, the conversion fails—
72              this is a matching failure.
73
74       Each conversion specification in format begins with either the  charac‐
75       ter '%' or the character sequence "%n$" (see below for the distinction)
76       followed by:
77
78       ·      An optional '*' assignment-suppression character: scanf()  reads
79              input  as directed by the conversion specification, but discards
80              the input.  No corresponding pointer argument is  required,  and
81              this  specification  is  not included in the count of successful
82              assignments returned by scanf().
83
84       ·      For decimal conversions, an optional quote character (').   This
85              specifies  that  the input number may include thousands' separa‐
86              tors as defined  by  the  LC_NUMERIC  category  of  the  current
87              locale.  (See setlocale(3).)  The quote character may precede or
88              follow the '*' assignment-suppression character.
89
90       ·      An optional 'm' character.  This is used with string conversions
91              (%s,  %c, %[), and relieves the caller of the need to allocate a
92              corresponding buffer to hold the input: instead,  scanf()  allo‐
93              cates  a  buffer  of sufficient size, and assigns the address of
94              this buffer to the corresponding pointer argument, which  should
95              be  a  pointer to a char * variable (this variable does not need
96              to be initialized before the call).  The  caller  should  subse‐
97              quently free(3) this buffer when it is no longer required.
98
99       ·      An  optional  decimal  integer which specifies the maximum field
100              width.  Reading of characters stops either when this maximum  is
101              reached or when a nonmatching character is found, whichever hap‐
102              pens first.  Most conversions discard initial white space  char‐
103              acters  (the  exceptions  are  noted below), and these discarded
104              characters don't count toward the maximum field  width.   String
105              input  conversions  store a terminating null byte ('\0') to mark
106              the end of the input; the maximum field width does  not  include
107              this terminator.
108
109       ·      An  optional  type  modifier character.  For example, the l type
110              modifier is used with integer conversions such as %d to  specify
111              that  the  corresponding  pointer  argument refers to a long int
112              rather than a pointer to an int.
113
114       ·      A conversion specifier that specifies the type of input  conver‐
115              sion to be performed.
116
117       The conversion specifications in format are of two forms, either begin‐
118       ning with '%' or beginning with "%n$".  The two  forms  should  not  be
119       mixed  in the same format string, except that a string containing "%n$"
120       specifications can include %% and %*.  If format contains '%'  specifi‐
121       cations,  then  these correspond in order with successive pointer argu‐
122       ments.  In the "%n$" form (which is specified in POSIX.1-2001, but  not
123       C99),  n  is  a decimal integer that specifies that the converted input
124       should be placed in the location referred to by the n-th pointer  argu‐
125       ment following format.
126
127   Conversions
128       The following type modifier characters can appear in a conversion spec‐
129       ification:
130
131       h      Indicates that the conversion will be one of d, i, o, u,  x,  X,
132              or  n  and  the  next  pointer  is  a  pointer to a short int or
133              unsigned short int (rather than int).
134
135       hh     As for h, but the next pointer is a pointer to a signed char  or
136              unsigned char.
137
138       j      As  for h, but the next pointer is a pointer to an intmax_t or a
139              uintmax_t.  This modifier was introduced in C99.
140
141       l      Indicates either that the conversion will be one of d, i, o,  u,
142              x,  X,  or  n and the next pointer is a pointer to a long int or
143              unsigned long int (rather than int), or that the conversion will
144              be one of e, f, or g and the next pointer is a pointer to double
145              (rather than float).  Specifying two l characters is  equivalent
146              to  L.   If  used  with %c or %s, the corresponding parameter is
147              considered as a pointer to a wide  character  or  wide-character
148              string respectively.
149
150       L      Indicates  that the conversion will be either e, f, or g and the
151              next pointer is a pointer to long double or the conversion  will
152              be  d,  i,  o, u, or x and the next pointer is a pointer to long
153              long.
154
155       q      equivalent to L.  This specifier does not exist in ANSI C.
156
157       t      As for h, but the next pointer is  a  pointer  to  a  ptrdiff_t.
158              This modifier was introduced in C99.
159
160       z      As  for  h, but the next pointer is a pointer to a size_t.  This
161              modifier was introduced in C99.
162
163       The following conversion specifiers are available:
164
165       %      Matches a literal '%'.  That is, %% in the format string matches
166              a  single  input '%' character.  No conversion is done (but ini‐
167              tial white space characters are discarded), and assignment  does
168              not occur.
169
170       d      Matches  an  optionally signed decimal integer; the next pointer
171              must be a pointer to int.
172
173       D      Equivalent to ld; this exists only for  backward  compatibility.
174              (Note:  thus  only  in  libc4.   In  libc5  and  glibc the %D is
175              silently ignored, causing old programs to fail mysteriously.)
176
177       i      Matches an optionally signed integer; the next pointer must be a
178              pointer  to  int.   The  integer is read in base 16 if it begins
179              with 0x or 0X, in base 8 if it begins with 0,  and  in  base  10
180              otherwise.   Only  characters  that  correspond  to the base are
181              used.
182
183       o      Matches an unsigned octal integer; the next pointer  must  be  a
184              pointer to unsigned int.
185
186       u      Matches  an unsigned decimal integer; the next pointer must be a
187              pointer to unsigned int.
188
189       x      Matches an unsigned hexadecimal integer; the next  pointer  must
190              be a pointer to unsigned int.
191
192       X      Equivalent to x.
193
194       f      Matches  an  optionally  signed  floating-point number; the next
195              pointer must be a pointer to float.
196
197       e      Equivalent to f.
198
199       g      Equivalent to f.
200
201       E      Equivalent to f.
202
203       a      (C99) Equivalent to f.
204
205       s      Matches a  sequence  of  non-white-space  characters;  the  next
206              pointer  must be a pointer to the initial element of a character
207              array that is long enough to hold the  input  sequence  and  the
208              terminating null byte ('\0'), which is added automatically.  The
209              input string stops at white space or at the maximum field width,
210              whichever occurs first.
211
212       c      Matches  a  sequence  of characters whose length is specified by
213              the maximum field width (default 1); the next pointer must be  a
214              pointer to char, and there must be enough room for all the char‐
215              acters (no terminating null byte is added).  The usual  skip  of
216              leading  white  space is suppressed.  To skip white space first,
217              use an explicit space in the format.
218
219       [      Matches a nonempty sequence of characters from the specified set
220              of  accepted  characters;  the next pointer must be a pointer to
221              char, and there must be enough room for all  the  characters  in
222              the  string,  plus  a  terminating null byte.  The usual skip of
223              leading white space is suppressed.  The string is to be made  up
224              of  characters  in  (or  not  in)  a  particular set; the set is
225              defined by the characters between the open bracket  [  character
226              and a close bracket ] character.  The set excludes those charac‐
227              ters if the first character after the open bracket is a  circum‐
228              flex  (^).   To  include a close bracket in the set, make it the
229              first character after the open bracket or  the  circumflex;  any
230              other position will end the set.  The hyphen character - is also
231              special; when placed between two other characters, it  adds  all
232              intervening characters to the set.  To include a hyphen, make it
233              the  last  character  before  the  final  close  bracket.    For
234              instance,  [^]0-9-]  means  the  set  "everything  except  close
235              bracket, zero through nine, and hyphen".  The string  ends  with
236              the appearance of a character not in the (or, with a circumflex,
237              in) set or when the field width runs out.
238
239       p      Matches a pointer value (as printed by %p in printf(3); the next
240              pointer must be a pointer to a pointer to void.
241
242       n      Nothing  is expected; instead, the number of characters consumed
243              thus far from the input is  stored  through  the  next  pointer,
244              which  must  be  a pointer to int.  This is not a conversion and
245              does not increase the  count  returned  by  the  function.   The
246              assignment  can  be suppressed with the * assignment-suppression
247              character, but the effect on  the  return  value  is  undefined.
248              Therefore %*n conversions should not be used.
249

RETURN VALUE

251       On  success,  these functions return the number of input items success‐
252       fully matched and assigned; this can be fewer  than  provided  for,  or
253       even zero, in the event of an early matching failure.
254
255       The  value EOF is returned if the end of input is reached before either
256       the first successful conversion or a matching failure occurs.   EOF  is
257       also returned if a read error occurs, in which case the error indicator
258       for the stream (see ferror(3)) is set, and errno is set to indicate the
259       error.
260

ERRORS

262       EAGAIN The file descriptor underlying stream is marked nonblocking, and
263              the read operation would block.
264
265       EBADF  The file descriptor underlying stream is invalid,  or  not  open
266              for reading.
267
268       EILSEQ Input byte sequence does not form a valid character.
269
270       EINTR  The read operation was interrupted by a signal; see signal(7).
271
272       EINVAL Not enough arguments; or format is NULL.
273
274       ENOMEM Out of memory.
275
276       ERANGE The  result  of an integer conversion would exceed the size that
277              can be stored in the corresponding integer type.
278

ATTRIBUTES

280       For  an  explanation  of  the  terms  used   in   this   section,   see
281       attributes(7).
282
283       ┌─────────────────────┬───────────────┬────────────────┐
284Interface            Attribute     Value          
285       ├─────────────────────┼───────────────┼────────────────┤
286scanf(), fscanf(),   │ Thread safety │ MT-Safe locale │
287sscanf(), vscanf(),  │               │                │
288vsscanf(), vfscanf() │               │                │
289       └─────────────────────┴───────────────┴────────────────┘
290

CONFORMING TO

292       The  functions  fscanf(),  scanf(), and sscanf() conform to C89 and C99
293       and POSIX.1-2001.  These standards do not specify the ERANGE error.
294
295       The q specifier is the 4.4BSD notation for long long, while ll  or  the
296       usage of L in integer conversions is the GNU notation.
297
298       The Linux version of these functions is based on the GNU libio library.
299       Take a look at the info documentation of GNU libc  (glibc-1.08)  for  a
300       more concise description.
301

NOTES

303   The 'a' assignment-allocation modifier
304       Originally,  the  GNU C library supported dynamic allocation for string
305       inputs (as a nonstandard extension) via the a character.  (This feature
306       is  present  at least as far back as glibc 2.0.)  Thus, one could write
307       the following to have scanf() allocate a buffer for  an  input  string,
308       with a pointer to that buffer being returned in *buf:
309
310           char *buf;
311           scanf("%as", &buf);
312
313       The  use  of  the letter a for this purpose was problematic, since a is
314       also specified by the ISO C standard as a synonym for f (floating-point
315       input).   POSIX.1-2008  instead specifies the m modifier for assignment
316       allocation (as documented in DESCRIPTION, above).
317
318       Note that the a modifier is not available if the  program  is  compiled
319       with  gcc  -std=c99 or gcc -D_ISOC99_SOURCE (unless _GNU_SOURCE is also
320       specified), in which case the a  is  interpreted  as  a  specifier  for
321       floating-point numbers (see above).
322
323       Support  for  the  m  modifier was added to glibc starting with version
324       2.7, and new programs should use that modifier instead of a.
325
326       As well as being standardized by POSIX, the m modifier has the  follow‐
327       ing further advantages over the use of a:
328
329       * It may also be applied to %c conversion specifiers (e.g., %3mc).
330
331       * It  avoids ambiguity with respect to the %a floating-point conversion
332         specifier (and is unaffected by gcc -std=c99 etc.).
333

BUGS

335       All functions are fully C89  conformant,  but  provide  the  additional
336       specifiers  q  and  a  as well as an additional behavior of the L and l
337       specifiers.  The latter may be considered to be a bug,  as  it  changes
338       the behavior of specifiers defined in C89.
339
340       Some  combinations  of  the  type  modifiers  and conversion specifiers
341       defined by ANSI C do not make sense (e.g., %Ld).  While they may have a
342       well-defined  behavior on Linux, this need not to be so on other archi‐
343       tectures.  Therefore it usually is better to use modifiers that are not
344       defined  by  ANSI  C at all, that is, use q instead of L in combination
345       with d, i, o, u, x, and X conversions or ll.
346
347       The usage of q is not the same as on 4.4BSD, as it may be used in float
348       conversions equivalently to L.
349

EXAMPLE

351       To  use  the  dynamic  allocation  conversion specifier, specify m as a
352       length modifier (thus %ms or %m[range]).  The caller must  free(3)  the
353       returned string, as in the following example:
354
355           char *p;
356           int n;
357
358           errno = 0;
359           n = scanf("%m[a-z]", &p);
360           if (n == 1) {
361               printf("read: %s\n", p);
362               free(p);
363           } else if (errno != 0) {
364               perror("scanf");
365           } else {
366               fprintf(stderr, "No matching characters\n");
367           }
368
369       As  shown in the above example, it is necessary to call free(3) only if
370       the scanf() call successfully read a string.
371

SEE ALSO

373       getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)
374

COLOPHON

376       This page is part of release 5.02 of the Linux  man-pages  project.   A
377       description  of  the project, information about reporting bugs, and the
378       latest    version    of    this    page,    can     be     found     at
379       https://www.kernel.org/doc/man-pages/.
380
381
382
383GNU                               2019-03-06                          SCANF(3)
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