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 *restrict format, ...);
13 int fscanf(FILE *restrict stream,
14 const char *restrict format, ...);
15 int sscanf(const char *restrict str,
16 const char *restrict format, ...);
17 #include <stdarg.h>
19 int vscanf(const char *restrict format, va_list ap);
21 int vfscanf(FILE *restrict stream,
22 const char *restrict format, va_list ap);
23 int vsscanf(const char *restrict str,
24 const char *restrict format, va_list ap);
25 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
27 vscanf(), vsscanf(), vfscanf():
29 _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
30
32 The scanf() family of functions scans input according to format as de‐
33 scribed below. This format may contain conversion specifications; the
34 results from such conversions, if any, are stored in the locations
35 pointed to by the pointer arguments that follow format. Each pointer
36 argument must be of a type that is appropriate for the value returned
37 by the corresponding conversion specification.
38 If the number of conversion specifications in format exceeds the number
40 of pointer arguments, the results are undefined. If the number of
41 pointer arguments exceeds the number of conversion specifications, then
42 the excess pointer arguments are evaluated, but are otherwise ignored.
43 The scanf() function reads input from the standard input stream stdin,
45 fscanf() reads input from the stream pointer stream, and sscanf() reads
46 its input from the character string pointed to by str.
47 The vfscanf() function is analogous to vfprintf(3) and reads input from
49 the stream pointer stream using a variable argument list of pointers
50 (see stdarg(3). The vscanf() function scans a variable argument list
51 from the standard input and the vsscanf() function scans it from a
52 string; these are analogous to the vprintf(3) and vsprintf(3) functions
53 respectively.
54 The format string consists of a sequence of directives which describe
56 how to process the sequence of input characters. If processing of a
57 directive fails, no further input is read, and scanf() returns. A
58 "failure" can be either of the following: input failure, meaning that
59 input characters were unavailable, or matching failure, meaning that
60 the input was inappropriate (see below).
61 A directive is one of the following:
63 • A sequence of white-space characters (space, tab, newline, etc.;
65 see isspace(3)). This directive matches any amount of white
66 space, including none, in the input.
67 • An ordinary character (i.e., one other than white space or '%').
69 This character must exactly match the next character of input.
70 • A conversion specification, which commences with a '%' (percent)
72 character. A sequence of characters from the input is converted
73 according to this specification, and the result is placed in the
74 corresponding pointer argument. If the next item of input does
75 not match the conversion specification, the conversion fails—
76 this is a matching failure.
77 Each conversion specification in format begins with either the charac‐
79 ter '%' or the character sequence "%n$" (see below for the distinction)
80 followed by:
81 • An optional '*' assignment-suppression character: scanf() reads
83 input as directed by the conversion specification, but discards
84 the input. No corresponding pointer argument is required, and
85 this specification is not included in the count of successful
86 assignments returned by scanf().
87 • For decimal conversions, an optional quote character ('). This
89 specifies that the input number may include thousands' separa‐
90 tors as defined by the LC_NUMERIC category of the current lo‐
91 cale. (See setlocale(3).) The quote character may precede or
92 follow the '*' assignment-suppression character.
93 • An optional 'm' character. This is used with string conversions
95 (%s, %c, %[), and relieves the caller of the need to allocate a
96 corresponding buffer to hold the input: instead, scanf() allo‐
97 cates a buffer of sufficient size, and assigns the address of
98 this buffer to the corresponding pointer argument, which should
99 be a pointer to a char * variable (this variable does not need
100 to be initialized before the call). The caller should subse‐
101 quently free(3) this buffer when it is no longer required.
102 • An optional decimal integer which specifies the maximum field
104 width. Reading of characters stops either when this maximum is
105 reached or when a nonmatching character is found, whichever hap‐
106 pens first. Most conversions discard initial white space char‐
107 acters (the exceptions are noted below), and these discarded
108 characters don't count toward the maximum field width. String
109 input conversions store a terminating null byte ('\0') to mark
110 the end of the input; the maximum field width does not include
111 this terminator.
112 • An optional type modifier character. For example, the l type
114 modifier is used with integer conversions such as %d to specify
115 that the corresponding pointer argument refers to a long rather
116 than a pointer to an int.
117 • A conversion specifier that specifies the type of input conver‐
119 sion to be performed.
120 The conversion specifications in format are of two forms, either begin‐
122 ning with '%' or beginning with "%n$". The two forms should not be
123 mixed in the same format string, except that a string containing "%n$"
124 specifications can include %% and %*. If format contains '%' specifi‐
125 cations, then these correspond in order with successive pointer argu‐
126 ments. In the "%n$" form (which is specified in POSIX.1-2001, but not
127 C99), n is a decimal integer that specifies that the converted input
128 should be placed in the location referred to by the n-th pointer argu‐
129 ment following format.
130 Conversions
132 The following type modifier characters can appear in a conversion spec‐
133 ification:
134 h Indicates that the conversion will be one of d, i, o, u, x, X,
136 or n and the next pointer is a pointer to a short or unsigned
137 short (rather than int).
138 hh As for h, but the next pointer is a pointer to a signed char or
140 unsigned char.
141 j As for h, but the next pointer is a pointer to an intmax_t or a
143 uintmax_t. This modifier was introduced in C99.
144 l Indicates either that the conversion will be one of d, i, o, u,
146 x, X, or n and the next pointer is a pointer to a long or un‐
147 signed long (rather than int), or that the conversion will be
148 one of e, f, or g and the next pointer is a pointer to double
149 (rather than float). Specifying two l characters is equivalent
150 to L. If used with %c or %s, the corresponding parameter is
151 considered as a pointer to a wide character or wide-character
152 string respectively.
153 L Indicates that the conversion will be either e, f, or g and the
155 next pointer is a pointer to long double or the conversion will
156 be d, i, o, u, or x and the next pointer is a pointer to long
157 long.
158 q equivalent to L. This specifier does not exist in ANSI C.
160 t As for h, but the next pointer is a pointer to a ptrdiff_t.
162 This modifier was introduced in C99.
163 z As for h, but the next pointer is a pointer to a size_t. This
165 modifier was introduced in C99.
166 The following conversion specifiers are available:
168 % Matches a literal '%'. That is, %% in the format string matches
170 a single input '%' character. No conversion is done (but ini‐
171 tial white space characters are discarded), and assignment does
172 not occur.
173 d Matches an optionally signed decimal integer; the next pointer
175 must be a pointer to int.
176 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 o Matches an unsigned octal integer; the next pointer must be a
184 pointer to unsigned int.
185 u Matches an unsigned decimal integer; the next pointer must be a
187 pointer to unsigned int.
188 x Matches an unsigned hexadecimal integer (that may optionally be‐
190 gin with a prefix of 0x or 0X, which is discarded); the next
191 pointer must be a pointer to unsigned int.
192 X Equivalent to x.
194 f Matches an optionally signed floating-point number; the next
196 pointer must be a pointer to float.
197 e Equivalent to f.
199 g Equivalent to f.
201 E Equivalent to f.
203 a (C99) Equivalent to f.
205 s Matches a sequence of non-white-space characters; the next
207 pointer must be a pointer to the initial element of a character
208 array that is long enough to hold the input sequence and the
209 terminating null byte ('\0'), which is added automatically. The
210 input string stops at white space or at the maximum field width,
211 whichever occurs first.
212 c Matches a sequence of characters whose length is specified by
214 the maximum field width (default 1); the next pointer must be a
215 pointer to char, and there must be enough room for all the char‐
216 acters (no terminating null byte is added). The usual skip of
217 leading white space is suppressed. To skip white space first,
218 use an explicit space in the format.
219 [ Matches a nonempty sequence of characters from the specified set
221 of accepted characters; the next pointer must be a pointer to
222 char, and there must be enough room for all the characters in
223 the string, plus a terminating null byte. The usual skip of
224 leading white space is suppressed. The string is to be made up
225 of characters in (or not in) a particular set; the set is de‐
226 fined by the characters between the open bracket [ character and
227 a close bracket ] character. The set excludes those characters
228 if the first character after the open bracket is a circumflex
229 (^). To include a close bracket in the set, make it the first
230 character after the open bracket or the circumflex; any other
231 position will end the set. The hyphen character - is also spe‐
232 cial; when placed between two other characters, it adds all in‐
233 tervening characters to the set. To include a hyphen, make it
234 the last character before the final close bracket. For in‐
235 stance, [^]0-9-] means the set "everything except close bracket,
236 zero through nine, and hyphen". The string ends with the ap‐
237 pearance of a character not in the (or, with a circumflex, in)
238 set or when the field width runs out.
239 p Matches a pointer value (as printed by %p in printf(3)); the
241 next pointer must be a pointer to a pointer to void.
242 n Nothing is expected; instead, the number of characters consumed
244 thus far from the input is stored through the next pointer,
245 which must be a pointer to int, or variant whose size matches
246 the (optionally) supplied integer length modifier. This is not
247 a conversion and does not increase the count returned by the
248 function. The assignment can be suppressed with the * assign‐
249 ment-suppression character, but the effect on the return value
250 is undefined. Therefore %*n conversions should not be used.
251
253 On success, these functions return the number of input items success‐
254 fully matched and assigned; this can be fewer than provided for, or
255 even zero, in the event of an early matching failure.
256 The value EOF is returned if the end of input is reached before either
258 the first successful conversion or a matching failure occurs. EOF is
259 also returned if a read error occurs, in which case the error indicator
260 for the stream (see ferror(3)) is set, and errno is set to indicate the
261 error.
262
264 EAGAIN The file descriptor underlying stream is marked nonblocking, and
265 the read operation would block.
266 EBADF The file descriptor underlying stream is invalid, or not open
268 for reading.
269 EILSEQ Input byte sequence does not form a valid character.
271 EINTR The read operation was interrupted by a signal; see signal(7).
273 EINVAL Not enough arguments; or format is NULL.
275 ENOMEM Out of memory.
277 ERANGE The result of an integer conversion would exceed the size that
279 can be stored in the corresponding integer type.
280
282 For an explanation of the terms used in this section, see at‐
283 tributes(7).
284 ┌─────────────────────────────────────┬───────────────┬────────────────┐
286 │Interface │ Attribute │ Value │
287 ├─────────────────────────────────────┼───────────────┼────────────────┤
288 │scanf(), fscanf(), sscanf(), │ Thread safety │ MT-Safe locale │
289 │vscanf(), vsscanf(), vfscanf() │ │ │
290 └─────────────────────────────────────┴───────────────┴────────────────┘
291
293 The functions fscanf(), scanf(), and sscanf() conform to C89 and C99
294 and POSIX.1-2001. These standards do not specify the ERANGE error.
295 The q specifier is the 4.4BSD notation for long long, while ll or the
297 usage of L in integer conversions is the GNU notation.
298 The Linux version of these functions is based on the GNU libio library.
300 Take a look at the info documentation of GNU libc (glibc-1.08) for a
301 more concise description.
302
304 The 'a' assignment-allocation modifier
305 Originally, the GNU C library supported dynamic allocation for string
306 inputs (as a nonstandard extension) via the a character. (This feature
307 is present at least as far back as glibc 2.0.) Thus, one could write
308 the following to have scanf() allocate a buffer for an input string,
309 with a pointer to that buffer being returned in *buf:
310 char *buf;
312 scanf("%as", &buf);
313 The use of the letter a for this purpose was problematic, since a is
315 also specified by the ISO C standard as a synonym for f (floating-point
316 input). POSIX.1-2008 instead specifies the m modifier for assignment
317 allocation (as documented in DESCRIPTION, above).
318 Note that the a modifier is not available if the program is compiled
320 with gcc -std=c99 or gcc -D_ISOC99_SOURCE (unless _GNU_SOURCE is also
321 specified), in which case the a is interpreted as a specifier for
322 floating-point numbers (see above).
323 Support for the m modifier was added to glibc starting with version
325 2.7, and new programs should use that modifier instead of a.
326 As well as being standardized by POSIX, the m modifier has the follow‐
328 ing further advantages over the use of a:
329 * It may also be applied to %c conversion specifiers (e.g., %3mc).
331 * It avoids ambiguity with respect to the %a floating-point conversion
333 specifier (and is unaffected by gcc -std=c99 etc.).
334
336 All functions are fully C89 conformant, but provide the additional
337 specifiers q and a as well as an additional behavior of the L and l
338 specifiers. The latter may be considered to be a bug, as it changes
339 the behavior of specifiers defined in C89.
340 Some combinations of the type modifiers and conversion specifiers de‐
342 fined by ANSI C do not make sense (e.g., %Ld). While they may have a
343 well-defined behavior on Linux, this need not to be so on other archi‐
344 tectures. Therefore it usually is better to use modifiers that are not
345 defined by ANSI C at all, that is, use q instead of L in combination
346 with d, i, o, u, x, and X conversions or ll.
347 The usage of q is not the same as on 4.4BSD, as it may be used in float
349 conversions equivalently to L.
350
352 To use the dynamic allocation conversion specifier, specify m as a
353 length modifier (thus %ms or %m[range]). The caller must free(3) the
354 returned string, as in the following example:
355 char *p;
357 int n;
358 errno = 0;
360 n = scanf("%m[a-z]", &p);
361 if (n == 1) {
362 printf("read: %s\n", p);
363 free(p);
364 } else if (errno != 0) {
365 perror("scanf");
366 } else {
367 fprintf(stderr, "No matching characters\n");
368 }
369 As shown in the above example, it is necessary to call free(3) only if
371 the scanf() call successfully read a string.
372
374 getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)
375
377 This page is part of release 5.12 of the Linux man-pages project. A
378 description of the project, information about reporting bugs, and the
379 latest version of this page, can be found at
380 https://www.kernel.org/doc/man-pages/.
381GNU 2021-03-22 SCANF(3)