1STRFTIME(3) Linux Programmer's Manual STRFTIME(3)
2
6 strftime - format date and time
7
9 #include <time.h>
10 size_t strftime(char *s, size_t max, const char *format,
12 const struct tm *tm);
13
15 The strftime() function formats the broken-down time tm according to
16 the format specification format and places the result in the character
17 array s of size max.
18 Ordinary characters placed in the format string are copied to s without
20 conversion. Conversion specifications are introduced by a `%' charac‐
21 ter, and terminated by a conversion specifier character, and are
22 replaced in s as follows:
23 %a The abbreviated weekday name according to the current locale.
25 %A The full weekday name according to the current locale.
27 %b The abbreviated month name according to the current locale.
29 %B The full month name according to the current locale.
31 %c The preferred date and time representation for the current
33 locale.
34 %C The century number (year/100) as a 2-digit integer. (SU)
36 %d The day of the month as a decimal number (range 01 to 31).
38 %D Equivalent to %m/%d/%y. (Yecch — for Americans only. Americans
40 should note that in other countries %d/%m/%y is rather common.
41 This means that in international context this format is ambigu‐
42 ous and should not be used.) (SU)
43 %e Like %d, the day of the month as a decimal number, but a leading
45 zero is replaced by a space. (SU)
46 %E Modifier: use alternative format, see below. (SU)
48 %F Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)
50 %G The ISO 8601 year with century as a decimal number. The 4-digit
52 year corresponding to the ISO week number (see %V). This has
53 the same format and value as %y, except that if the ISO week
54 number belongs to the previous or next year, that year is used
55 instead. (TZ)
56 %g Like %G, but without century, i.e., with a 2-digit year (00-99).
58 (TZ)
59 %h Equivalent to %b. (SU)
61 %H The hour as a decimal number using a 24-hour clock (range 00 to
63 23).
64 %I The hour as a decimal number using a 12-hour clock (range 01 to
66 12).
67 %j The day of the year as a decimal number (range 001 to 366).
69 %k The hour (24-hour clock) as a decimal number (range 0 to 23);
71 single digits are preceded by a blank. (See also %H.) (TZ)
72 %l The hour (12-hour clock) as a decimal number (range 1 to 12);
74 single digits are preceded by a blank. (See also %I.) (TZ)
75 %m The month as a decimal number (range 01 to 12).
77 %M The minute as a decimal number (range 00 to 59).
79 %n A newline character. (SU)
81 %O Modifier: use alternative format, see below. (SU)
83 %p Either `AM' or `PM' according to the given time value, or the
85 corresponding strings for the current locale. Noon is treated
86 as `pm' and midnight as `am'.
87 %P Like %p but in lowercase: `am' or `pm' or a corresponding string
89 for the current locale. (GNU)
90 %r The time in a.m. or p.m. notation. In the POSIX locale this is
92 equivalent to `%I:%M:%S %p'. (SU)
93 %R The time in 24-hour notation (%H:%M). (SU) For a version includ‐
95 ing the seconds, see %T below.
96 %s The number of seconds since the Epoch, i.e., since 1970-01-01
98 00:00:00 UTC. (TZ)
99 %S The second as a decimal number (range 00 to 60). (The range is
101 up to 60 to allow for occasional leap seconds.)
102 %t A tab character. (SU)
104 %T The time in 24-hour notation (%H:%M:%S). (SU)
106 %u The day of the week as a decimal, range 1 to 7, Monday being 1.
108 See also %w. (SU)
109 %U The week number of the current year as a decimal number, range
111 00 to 53, starting with the first Sunday as the first day of
112 week 01. See also %V and %W.
113 %V The ISO 8601:1988 week number of the current year as a decimal
115 number, range 01 to 53, where week 1 is the first week that has
116 at least 4 days in the current year, and with Monday as the
117 first day of the week. See also %U and %W. (SU)
118 %w The day of the week as a decimal, range 0 to 6, Sunday being 0.
120 See also %u.
121 %W The week number of the current year as a decimal number, range
123 00 to 53, starting with the first Monday as the first day of
124 week 01.
125 %x The preferred date representation for the current locale without
127 the time.
128 %X The preferred time representation for the current locale without
130 the date.
131 %y The year as a decimal number without a century (range 00 to 99).
133 %Y The year as a decimal number including the century.
135 %z The time-zone as hour offset from GMT. Required to emit
137 RFC 822-conformant dates (using "%a, %d %b %Y %H:%M:%S %z").
138 (GNU)
139 %Z The time zone or name or abbreviation.
141 %+ The date and time in date(1) format. (TZ) (Not supported in
143 glibc2.)
144 %% A literal `%' character.
146 Some conversion specifications can be modified by preceding the conver‐
148 sion specifier character by the E or O modifier to indicate that an
149 alternative format should be used. If the alternative format or speci‐
150 fication does not exist for the current locale, the behaviour will be
151 as if the unmodified conversion specification were used. (SU) The Sin‐
152 gle Unix Specification mentions %Ec, %EC, %Ex, %EX, %Ey, %EY, %Od, %Oe,
153 %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, %Oy, where the effect
154 of the O modifier is to use alternative numeric symbols (say, roman
155 numerals), and that of the E modifier is to use a locale-dependent
156 alternative representation.
157 The broken-down time structure tm is defined in <time.h>. See also
159 ctime(3).
160
162 The strftime() function returns the number of characters placed in the
163 array s, not including the terminating null byte, provided the string,
164 including the terminating null byte, fits. Otherwise, it returns 0,
165 and the contents of the array is undefined. (Thus at least since libc
166 4.4.4; very old versions of libc, such as libc 4.4.1, would return max
167 if the array was too small.)
168 Note that the return value 0 does not necessarily indicate an error;
170 for example, in many locales %p yields an empty string.
171
173 The environment variables TZ and LC_TIME are used.
174
176 SVr4, C89, C99. There are strict inclusions between the set of conver‐
177 sions given in ANSI C (unmarked), those given in the Single Unix Speci‐
178 fication (marked SU), those given in Olson's timezone package (marked
179 TZ), and those given in glibc (marked GNU), except that %+ is not sup‐
180 ported in glibc2. On the other hand glibc2 has several more extensions.
181 POSIX.1 only refers to ANSI C; POSIX.2 describes under date(1) several
182 extensions that could apply to strftime() as well. The %F conversion
183 is in C99 and POSIX.1-2001.
184 In SUSv2, the %S specified allowed a range of 00 to 61, to allow for
186 the theoretical possibility of a minute that included a double leap
187 second (there never has been such a minute).
188
190 Glibc provides some extensions for conversion specifications. (These
191 extensions are not specified in POSIX.1-2001, but a few other systems
192 provide similar features.) Between the % character and the conversion
193 specifier character, an optional flag and field width may be specified.
194 (These precede the E or O modifiers, if present.)
195 The following flag characters are permitted:
197 _ (underscore) Pad a numeric result string with spaces.
199 - (dash) Do not pad a numeric result string.
201 0 Pad a numeric result string with zeros even if the conversion
203 specifier character uses space-padding by default.
204 ^ Convert alphabetic characters in result string to upper case.
206 # Swap the case of the result string. (This flag only works with
208 certain conversion specifier characters, and of these, it is
209 only really useful with %Z).
210 An optional decimal width specifier may follow the (possibly absent)
212 flag. If the natural size of the field is smaller than this width,
213 then the result string is padded (on the left) to the specified width.
214
216 Some buggy versions of gcc complain about the use of %c: warning: `%c'
217 yields only last 2 digits of year in some locales. Of course program‐
218 mers are encouraged to use %c, it gives the preferred date and time
219 representation. One meets all kinds of strange obfuscations to circum‐
220 vent this gcc problem. A relatively clean one is to add an intermediate
221 function
222 size_t my_strftime(char *s, size_t max, const char *fmt, const
223 struct tm *tm) {
224 return strftime(s, max, fmt, tm);
225 }
226
228 The program below can be used to experiment with strftime().
229 #include <time.h>
231 #include <stdio.h>
232 #include <stdlib.h>
233 int
235 main(int argc, char *argv[])
236 {
237 char outstr[200];
238 time_t t;
239 struct tm *tmp;
240 t = time(NULL);
242 tmp = localtime(&t);
243 if (tmp == NULL) {
244 perror("localtime");
245 exit(EXIT_FAILURE);
246 }
247 if (strftime(outstr, sizeof(outstr), argv[1], tmp) == 0) {
249 fprintf(stderr, "strftime returned 0");
250 exit(EXIT_FAILURE);
251 }
252 printf("Result string is \"%s\"\n", outstr);
254 exit(EXIT_SUCCESS);
255 } /* main */
256 Some examples of the result string produced by the glibc implementation
258 of strftime() are as follows:
259 $ ./a.out "%m"
261 Result string is "11"
262 $ ./a.out "%5m"
263 Result string is "00011"
264 $ ./a.out "%_5m"
265 Result string is " 11"
266
268 date(1), time(2), ctime(3), setlocale(3), sprintf(3), strptime(3)
269GNU 2005-11-23 STRFTIME(3)