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 _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
26
28 The scanf() family of functions scans input according to format as de‐
29 scribed below. This format may contain conversion specifications; the
30 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 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 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 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 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 A directive is one of the following:
59 • 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 • An ordinary character (i.e., one other than white space or '%').
65 This character must exactly match the next character of input.
66 • 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 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 • 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 • 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 lo‐
87 cale. (See setlocale(3).) The quote character may precede or
88 follow the '*' assignment-suppression character.
89 • 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 • 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 • 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 rather
112 than a pointer to an int.
113 • A conversion specifier that specifies the type of input conver‐
115 sion to be performed.
116 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 Conversions
128 The following type modifier characters can appear in a conversion spec‐
129 ification:
130 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 or unsigned
133 short (rather than int).
134 hh As for h, but the next pointer is a pointer to a signed char or
136 unsigned char.
137 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 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 or un‐
143 signed long (rather than int), or that the conversion will be
144 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 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 q equivalent to L. This specifier does not exist in ANSI C.
156 t As for h, but the next pointer is a pointer to a ptrdiff_t.
158 This modifier was introduced in C99.
159 z As for h, but the next pointer is a pointer to a size_t. This
161 modifier was introduced in C99.
162 The following conversion specifiers are available:
164 % 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 d Matches an optionally signed decimal integer; the next pointer
171 must be a pointer to int.
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 (that may optionally be‐
186 gin with a prefix of 0x or 0X, which is discarded); the next
187 pointer must be a pointer to unsigned int.
188 X Equivalent to x.
190 f Matches an optionally signed floating-point number; the next
192 pointer must be a pointer to float.
193 e Equivalent to f.
195 g Equivalent to f.
197 E Equivalent to f.
199 a (C99) Equivalent to f.
201 s Matches a sequence of non-white-space characters; the next
203 pointer must be a pointer to the initial element of a character
204 array that is long enough to hold the input sequence and the
205 terminating null byte ('\0'), which is added automatically. The
206 input string stops at white space or at the maximum field width,
207 whichever occurs first.
208 c Matches a sequence of characters whose length is specified by
210 the maximum field width (default 1); the next pointer must be a
211 pointer to char, and there must be enough room for all the char‐
212 acters (no terminating null byte is added). The usual skip of
213 leading white space is suppressed. To skip white space first,
214 use an explicit space in the format.
215 [ Matches a nonempty sequence of characters from the specified set
217 of accepted characters; the next pointer must be a pointer to
218 char, and there must be enough room for all the characters in
219 the string, plus a terminating null byte. The usual skip of
220 leading white space is suppressed. The string is to be made up
221 of characters in (or not in) a particular set; the set is de‐
222 fined by the characters between the open bracket [ character and
223 a close bracket ] character. The set excludes those characters
224 if the first character after the open bracket is a circumflex
225 (^). To include a close bracket in the set, make it the first
226 character after the open bracket or the circumflex; any other
227 position will end the set. The hyphen character - is also spe‐
228 cial; when placed between two other characters, it adds all in‐
229 tervening characters to the set. To include a hyphen, make it
230 the last character before the final close bracket. For in‐
231 stance, [^]0-9-] means the set "everything except close bracket,
232 zero through nine, and hyphen". The string ends with the ap‐
233 pearance of a character not in the (or, with a circumflex, in)
234 set or when the field width runs out.
235 p Matches a pointer value (as printed by %p in printf(3)); the
237 next pointer must be a pointer to a pointer to void.
238 n Nothing is expected; instead, the number of characters consumed
240 thus far from the input is stored through the next pointer,
241 which must be a pointer to int. This is not a conversion and
242 does not increase the count returned by the function. The as‐
243 signment can be suppressed with the * assignment-suppression
244 character, but the effect on the return value is undefined.
245 Therefore %*n conversions should not be used.
246
248 On success, these functions return the number of input items success‐
249 fully matched and assigned; this can be fewer than provided for, or
250 even zero, in the 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 to 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 For an explanation of the terms used in this section, see at‐
278 tributes(7).
279 ┌─────────────────────┬───────────────┬────────────────┐
281 │Interface │ Attribute │ Value │
282 ├─────────────────────┼───────────────┼────────────────┤
283 │scanf(), fscanf(), │ Thread safety │ MT-Safe locale │
284 │sscanf(), vscanf(), │ │ │
285 │vsscanf(), vfscanf() │ │ │
286 └─────────────────────┴───────────────┴────────────────┘
287
289 The functions fscanf(), scanf(), and sscanf() conform to C89 and C99
290 and POSIX.1-2001. These standards do not specify the ERANGE error.
291 The q specifier is the 4.4BSD notation for long long, while ll or the
293 usage of L in integer conversions is the GNU notation.
294 The Linux version of these functions is based on the GNU libio library.
296 Take a look at the info documentation of GNU libc (glibc-1.08) for a
297 more concise description.
298
300 The 'a' assignment-allocation modifier
301 Originally, the GNU C library supported dynamic allocation for string
302 inputs (as a nonstandard extension) via the a character. (This feature
303 is present at least as far back as glibc 2.0.) Thus, one could write
304 the following to have scanf() allocate a buffer for an input string,
305 with a pointer to that buffer being returned in *buf:
306 char *buf;
308 scanf("%as", &buf);
309 The use of the letter a for this purpose was problematic, since a is
311 also specified by the ISO C standard as a synonym for f (floating-point
312 input). POSIX.1-2008 instead specifies the m modifier for assignment
313 allocation (as documented in DESCRIPTION, above).
314 Note that the a modifier is not available if the program is compiled
316 with gcc -std=c99 or gcc -D_ISOC99_SOURCE (unless _GNU_SOURCE is also
317 specified), in which case the a is interpreted as a specifier for
318 floating-point numbers (see above).
319 Support for the m modifier was added to glibc starting with version
321 2.7, and new programs should use that modifier instead of a.
322 As well as being standardized by POSIX, the m modifier has the follow‐
324 ing further advantages over the use of a:
325 * It may also be applied to %c conversion specifiers (e.g., %3mc).
327 * It avoids ambiguity with respect to the %a floating-point conversion
329 specifier (and is unaffected by gcc -std=c99 etc.).
330
332 All functions are fully C89 conformant, but provide the additional
333 specifiers q and a as well as an additional behavior of the L and l
334 specifiers. The latter may be considered to be a bug, as it changes
335 the behavior of specifiers defined in C89.
336 Some combinations of the type modifiers and conversion specifiers de‐
338 fined by ANSI C do not make sense (e.g., %Ld). While they may have a
339 well-defined behavior on Linux, this need not to be so on other archi‐
340 tectures. Therefore it usually is better to use modifiers that are not
341 defined by ANSI C at all, that is, use q instead of L in combination
342 with d, i, o, u, x, and X conversions or ll.
343 The usage of q is not the same as on 4.4BSD, as it may be used in float
345 conversions equivalently to L.
346
348 To use the dynamic allocation conversion specifier, specify m as a
349 length modifier (thus %ms or %m[range]). The caller must free(3) the
350 returned string, as in the following example:
351 char *p;
353 int n;
354 errno = 0;
356 n = scanf("%m[a-z]", &p);
357 if (n == 1) {
358 printf("read: %s\n", p);
359 free(p);
360 } else if (errno != 0) {
361 perror("scanf");
362 } else {
363 fprintf(stderr, "No matching characters\n");
364 }
365 As shown in the above example, it is necessary to call free(3) only if
367 the scanf() call successfully read a string.
368
370 getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)
371
373 This page is part of release 5.10 of the Linux man-pages project. A
374 description of the project, information about reporting bugs, and the
375 latest version of this page, can be found at
376 https://www.kernel.org/doc/man-pages/.
377GNU 2020-08-13 SCANF(3)