1SCANF(3) Linux Programmer's Manual SCANF(3)
2
6 scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf - input format conver‐
7 sion
8
10 #include <stdio.h>
11 int scanf(const char *format, ...);
13 int fscanf(FILE *stream, const char *format, ...);
14 int sscanf(const char *str, const char *format, ...);
15 #include <stdarg.h>
17 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 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
23 vscanf(), vsscanf(), vfscanf():
25 _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE ||
26 _POSIX_C_SOURCE >= 200112L;
27 or cc -std=c99
28
30 The scanf() family of functions scans input according to format as
31 described below. This format may contain conversion specifications;
32 the results from such conversions, if any, are stored in the locations
33 pointed to by the pointer arguments that follow format. Each pointer
34 argument must be of a type that is appropriate for the value returned
35 by the corresponding conversion specification.
36 If the number of conversion specifications in format exceeds the number
38 of pointer arguments, the results are undefined. If the number of
39 pointer arguments exceeds the number of conversion specifications, then
40 the excess pointer arguments are evaluated, but are otherwise ignored.
41 The scanf() function reads input from the standard input stream stdin,
43 fscanf() reads input from the stream pointer stream, and sscanf() reads
44 its input from the character string pointed to by str.
45 The vfscanf() function is analogous to vfprintf(3) and reads input from
47 the stream pointer stream using a variable argument list of pointers
48 (see stdarg(3). The vscanf() function scans a variable argument list
49 from the standard input and the vsscanf() function scans it from a
50 string; these are analogous to the vprintf(3) and vsprintf(3) functions
51 respectively.
52 The format string consists of a sequence of directives which describe
54 how to process the sequence of input characters. If processing of a
55 directive fails, no further input is read, and scanf() returns. A
56 "failure" can be either of the following: input failure, meaning that
57 input characters were unavailable, or matching failure, meaning that
58 the input was inappropriate (see below).
59 A directive is one of the following:
61 · A sequence of white-space characters (space, tab, newline, etc.;
63 see isspace(3)). This directive matches any amount of white
64 space, including none, in the input.
65 · An ordinary character (i.e., one other than white space or '%').
67 This character must exactly match the next character of input.
68 · A conversion specification, which commences with a '%' (percent)
70 character. A sequence of characters from the input is converted
71 according to this specification, and the result is placed in the
72 corresponding pointer argument. If the next item of input does
73 not match the conversion specification, the conversion fails—
74 this is a matching failure.
75 Each conversion specification in format begins with either the charac‐
77 ter '%' or the character sequence "%n$" (see below for the distinction)
78 followed by:
79 · An optional '*' assignment-suppression character: scanf() reads
81 input as directed by the conversion specification, but discards
82 the input. No corresponding pointer argument is required, and
83 this specification is not included in the count of successful
84 assignments returned by scanf().
85 · An optional 'm' character. This is used with string conversions
87 (%s, %c, %[), and relieves the caller of the need to allocate a
88 corresponding buffer to hold the input: instead, scanf() allo‐
89 cates a buffer of sufficient size, and assigns the address of
90 this buffer to the corresponding pointer argument, which should
91 be a pointer to a char * variable (this variable does not need
92 to be initialized before the call). The caller should subse‐
93 quently free(3) this buffer when it is no longer required.
94 · An optional decimal integer which specifies the maximum field
96 width. Reading of characters stops either when this maximum is
97 reached or when a nonmatching character is found, whichever hap‐
98 pens first. Most conversions discard initial white space char‐
99 acters (the exceptions are noted below), and these discarded
100 characters don't count toward the maximum field width. String
101 input conversions store a terminating null byte ('\0') to mark
102 the end of the input; the maximum field width does not include
103 this terminator.
104 · An optional type modifier character. For example, the l type
106 modifier is used with integer conversions such as %d to specify
107 that the corresponding pointer argument refers to a long int
108 rather than a pointer to an int.
109 · A conversion specifier that specifies the type of input conver‐
111 sion to be performed.
112 The conversion specifications in format are of two forms, either begin‐
114 ning with '%' or beginning with "%n$". The two forms should not be
115 mixed in the same format string, except that a string containing "%n$"
116 specifications can include %% and %*. If format contains '%' specifi‐
117 cations then these correspond in order with successive pointer argu‐
118 ments. In the "%n$" form (which is specified in POSIX.1-2001, but not
119 C99), n is a decimal integer that specifies that the converted input
120 should be placed in the location referred to by the n-th pointer argu‐
121 ment following format.
122 Conversions
124 The following type modifier characters can appear in a conversion spec‐
125 ification:
126 h Indicates that the conversion will be one of d, i, o, u, x, X,
128 or n and the next pointer is a pointer to a short int or
129 unsigned short int (rather than int).
130 hh As for h, but the next pointer is a pointer to a signed char or
132 unsigned char.
133 j As for h, but the next pointer is a pointer to an intmax_t or a
135 uintmax_t. This modifier was introduced in C99.
136 l Indicates either that the conversion will be one of d, i, o, u,
138 x, X, or n and the next pointer is a pointer to a long int or
139 unsigned long int (rather than int), or that the conversion will
140 be one of e, f, or g and the next pointer is a pointer to double
141 (rather than float). Specifying two l characters is equivalent
142 to L. If used with %c or %s the corresponding parameter is con‐
143 sidered as a pointer to a wide character or wide-character
144 string respectively.
145 L Indicates that the conversion will be either e, f, or g and the
147 next pointer is a pointer to long double or the conversion will
148 be d, i, o, u, or x and the next pointer is a pointer to long
149 long.
150 q equivalent to L. This specifier does not exist in ANSI C.
152 t As for h, but the next pointer is a pointer to a ptrdiff_t.
154 This modifier was introduced in C99.
155 z As for h, but the next pointer is a pointer to a size_t. This
157 modifier was introduced in C99.
158 The following conversion specifiers are available:
160 % Matches a literal '%'. That is, %% in the format string matches
162 a single input '%' character. No conversion is done (but ini‐
163 tial white space characters are discarded), and assignment does
164 not occur.
165 d Matches an optionally signed decimal integer; the next pointer
167 must be a pointer to int.
168 D Equivalent to ld; this exists only for backward compatibility.
170 (Note: thus only in libc4. In libc5 and glibc the %D is
171 silently ignored, causing old programs to fail mysteriously.)
172 i Matches an optionally signed integer; the next pointer must be a
174 pointer to int. The integer is read in base 16 if it begins
175 with 0x or 0X, in base 8 if it begins with 0, and in base 10
176 otherwise. Only characters that correspond to the base are
177 used.
178 o Matches an unsigned octal integer; the next pointer must be a
180 pointer to unsigned int.
181 u Matches an unsigned decimal integer; the next pointer must be a
183 pointer to unsigned int.
184 x Matches an unsigned hexadecimal integer; the next pointer must
186 be a pointer to unsigned int.
187 X Equivalent to x.
189 f Matches an optionally signed floating-point number; the next
191 pointer must be a pointer to float.
192 e Equivalent to f.
194 g Equivalent to f.
196 E Equivalent to f.
198 a (C99) Equivalent to f.
200 s Matches a sequence of non-white-space characters; the next
202 pointer must be a pointer to character array that is long enough
203 to hold the input sequence and the terminating null byte ('\0'),
204 which is added automatically. The input string stops at white
205 space or at the maximum field width, whichever occurs first.
206 c Matches a sequence of characters whose length is specified by
208 the maximum field width (default 1); the next pointer must be a
209 pointer to char, and there must be enough room for all the char‐
210 acters (no terminating null byte is added). The usual skip of
211 leading white space is suppressed. To skip white space first,
212 use an explicit space in the format.
213 [ Matches a nonempty sequence of characters from the specified set
215 of accepted characters; the next pointer must be a pointer to
216 char, and there must be enough room for all the characters in
217 the string, plus a terminating null byte. The usual skip of
218 leading white space is suppressed. The string is to be made up
219 of characters in (or not in) a particular set; the set is
220 defined by the characters between the open bracket [ character
221 and a close bracket ] character. The set excludes those charac‐
222 ters if the first character after the open bracket is a circum‐
223 flex (^). To include a close bracket in the set, make it the
224 first character after the open bracket or the circumflex; any
225 other position will end the set. The hyphen character - is also
226 special; when placed between two other characters, it adds all
227 intervening characters to the set. To include a hyphen, make it
228 the last character before the final close bracket. For
229 instance, [^]0-9-] means the set "everything except close
230 bracket, zero through nine, and hyphen". The string ends with
231 the appearance of a character not in the (or, with a circumflex,
232 in) set or when the field width runs out.
233 p Matches a pointer value (as printed by %p in printf(3); the next
235 pointer must be a pointer to a pointer to void.
236 n Nothing is expected; instead, the number of characters consumed
238 thus far from the input is stored through the next pointer,
239 which must be a pointer to int. This is not a conversion,
240 although it can be suppressed with the * assignment-suppression
241 character. The C standard says: "Execution of a %n directive
242 does not increment the assignment count returned at the comple‐
243 tion of execution" but the Corrigendum seems to contradict this.
244 Probably it is wise not to make any assumptions on the effect of
245 %n conversions on the return value.
246
248 These functions return the number of input items successfully matched
249 and assigned, which can be fewer than provided for, or even zero in the
250 event of an early matching failure.
251 The value EOF is returned if the end of input is reached before either
253 the first successful conversion or a matching failure occurs. EOF is
254 also returned if a read error occurs, in which case the error indicator
255 for the stream (see ferror(3)) is set, and errno is set indicate the
256 error.
257
259 EAGAIN The file descriptor underlying stream is marked nonblocking, and
260 the read operation would block.
261 EBADF The file descriptor underlying stream is invalid, or not open
263 for reading.
264 EILSEQ Input byte sequence does not form a valid character.
266 EINTR The read operation was interrupted by a signal; see signal(7).
268 EINVAL Not enough arguments; or format is NULL.
270 ENOMEM Out of memory.
272 ERANGE The result of an integer conversion would exceed the size that
274 can be stored in the corresponding integer type.
275
277 The functions fscanf(), scanf(), and sscanf() conform to C89 and C99
278 and POSIX.1-2001. These standards do not specify the ERANGE error.
279 The q specifier is the 4.4BSD notation for long long, while ll or the
281 usage of L in integer conversions is the GNU notation.
282 The Linux version of these functions is based on the GNU libio library.
284 Take a look at the info documentation of GNU libc (glibc-1.08) for a
285 more concise description.
286
288 The GNU C library supported the dynamic allocation conversion specifier
289 (as a nonstandard extension) via the a character. This feature seems
290 to be present at least as far back as glibc 2.0.
291 It is not available if the program is compiled with gcc -std=c99 or gcc
293 -D_ISOC99_SOURCE (unless _GNU_SOURCE is also specified), in which case
294 the a is interpreted as a specifier for floating-point numbers (see
295 above).
296 Since version 2.7, glibc also provides the m modifier for the same pur‐
298 pose as the a modifier. The m modifier has the following advantages:
299 * It may also be applied to %c conversion specifiers (e.g., %3mc).
301 * It avoids ambiguity with respect to the %a floating-point conversion
303 specifier (and is unaffected by gcc -std=c99 etc.)
304 * It is specified in the POSIX.1-2008 standard.
306
308 All functions are fully C89 conformant, but provide the additional
309 specifiers q and a as well as an additional behavior of the L and l
310 specifiers. The latter may be considered to be a bug, as it changes
311 the behavior of specifiers defined in C89.
312 Some combinations of the type modifiers and conversion specifiers
314 defined by ANSI C do not make sense (e.g., %Ld). While they may have a
315 well-defined behavior on Linux, this need not to be so on other archi‐
316 tectures. Therefore it usually is better to use modifiers that are not
317 defined by ANSI C at all, that is, use q instead of L in combination
318 with d, i, o, u, x, and X conversions or ll.
319 The usage of q is not the same as on 4.4BSD, as it may be used in float
321 conversions equivalently to L.
322
324 To use the dynamic allocation conversion specifier, specify m as a
325 length modifier (thus %ms or %m[range]). The caller must free(3) the
326 returned string, as in the following example:
327 char *p;
329 int n;
330 errno = 0;
332 n = scanf("%m[a-z]", &p);
333 if (n == 1) {
334 printf("read: %s\n", p);
335 free(p);
336 } else if (errno != 0) {
337 perror("scanf");
338 } else {
339 fprintf(stderr, "No matching characters\n");
340 }
341 As shown in the above example, it is necessary to call free(3) only if
343 the scanf() call successfully read a string.
344
346 getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)
347
349 This page is part of release 3.53 of the Linux man-pages project. A
350 description of the project, and information about reporting bugs, can
351 be found at http://www.kernel.org/doc/man-pages/.
352GNU 2013-01-30 SCANF(3)