| strftime, strftime_l(3) | format date and time |
| strftime, strftime_l(3P, 3p) | convert date and time to a string |
| strftime(3) | Library Functions Manual | strftime(3) |
strftime - format date and time
Standard C library (libc, -lc)
#include <time.h>
size_t strftime(char s[restrict .max], size_t max,
const char *restrict format,
const struct tm *restrict tm);
size_t strftime_l(char s[restrict .max], size_t max,
const char *restrict format,
const struct tm *restrict tm,
locale_t locale);
The strftime() function formats the broken-down time tm according to the format specification format and places the result in the character array s of size max. The broken-down time structure tm is defined in <time.h>. See also ctime(3).
The format specification is a null-terminated string and may contain special character sequences called conversion specifications, each of which is introduced by a '%' character and terminated by some other character known as a conversion specifier character. All other character sequences are ordinary character sequences.
The characters of ordinary character sequences (including the null byte) are copied verbatim from format to s. However, the characters of conversion specifications are replaced as shown in the list below. In this list, the field(s) employed from the tm structure are also shown.
Some conversion specifications can be modified by preceding the conversion specifier character by the E or O modifier to indicate that an alternative format should be used. If the alternative format or specification does not exist for the current locale, the behavior will be as if the unmodified conversion specification were used. (SU) The Single UNIX Specification mentions %Ec, %EC, %Ex, %EX, %Ey, %EY, %Od, %Oe, %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, %Oy, where the effect of the O modifier is to use alternative numeric symbols (say, roman numerals), and that of the E modifier is to use a locale-dependent alternative representation. The rules governing date representation with the E modifier can be obtained by supplying ERA as an argument to a nl_langinfo(3). One example of such alternative forms is the Japanese era calendar scheme in the ja_JP glibc locale.
strftime_l() is equivalent to strftime(), except it uses the specified locale instead of the current locale. The behaviour is undefined if locale is invalid or LC_GLOBAL_LOCALE.
Provided that the result string, including the terminating null byte, does not exceed max bytes, strftime() returns the number of bytes (excluding the terminating null byte) placed in the array s. If the length of the result string (including the terminating null byte) would exceed max bytes, then strftime() returns 0, and the contents of the array are undefined.
Note that the return value 0 does not necessarily indicate an error. For example, in many locales %p yields an empty string. An empty format string will likewise yield an empty string.
The environment variables TZ and LC_TIME are used.
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
| strftime (), strftime_l () | Thread safety | MT-Safe env locale |
There are strict inclusions between the set of conversions given in ANSI C (unmarked), those given in the Single UNIX Specification (marked SU), those given in Olson's timezone package (marked TZ), and those given in glibc (marked GNU), except that %+ is not supported in glibc2. On the other hand glibc2 has several more extensions. POSIX.1 only refers to ANSI C; POSIX.2 describes under date(1) several extensions that could apply to strftime() as well. The %F conversion is in C99 and POSIX.1-2001.
In SUSv2, the %S specifier allowed a range of 00 to 61, to allow for the theoretical possibility of a minute that included a double leap second (there never has been such a minute).
%G, %g, and %V yield values calculated from the week-based year defined by the ISO 8601 standard. In this system, weeks start on a Monday, and are numbered from 01, for the first week, up to 52 or 53, for the last week. Week 1 is the first week where four or more days fall within the new year (or, synonymously, week 01 is: the first week of the year that contains a Thursday; or, the week that has 4 January in it). When three or fewer days of the first calendar week of the new year fall within that year, then the ISO 8601 week-based system counts those days as part of week 52 or 53 of the preceding year. For example, 1 January 2010 is a Friday, meaning that just three days of that calendar week fall in 2010. Thus, the ISO 8601 week-based system considers these days to be part of week 53 (%V) of the year 2009 (%G); week 01 of ISO 8601 year 2010 starts on Monday, 4 January 2010. Similarly, the first two days of January 2011 are considered to be part of week 52 of the year 2010.
glibc provides some extensions for conversion specifications. (These extensions are not specified in POSIX.1-2001, but a few other systems provide similar features.) Between the '%' character and the conversion specifier character, an optional flag and field width may be specified. (These precede the E or O modifiers, if present.)
The following flag characters are permitted:
An optional decimal width specifier may follow the (possibly absent) flag. If the natural size of the field is smaller than this width, then the result string is padded (on the left) to the specified width.
If the output string would exceed max bytes, errno is not set. This makes it impossible to distinguish this error case from cases where the format string legitimately produces a zero-length output string. POSIX.1-2001 does not specify any errno settings for strftime().
Some buggy versions of gcc(1) complain about the use of %c: warning: `%c' yields only last 2 digits of year in some locales. Of course programmers are encouraged to use %c, as it gives the preferred date and time representation. One meets all kinds of strange obfuscations to circumvent this gcc(1) problem. A relatively clean one is to add an intermediate function
size_t my_strftime(char *s, size_t max, const char *fmt,
const struct tm *tm) {
return strftime(s, max, fmt, tm); }
Nowadays, gcc(1) provides the -Wno-format-y2k option to prevent the warning, so that the above workaround is no longer required.
RFC 2822-compliant date format (with an English locale for %a and %b)
"%a, %d %b %Y %T %z"
RFC 822-compliant date format (with an English locale for %a and %b)
"%a, %d %b %y %T %z"
The program below can be used to experiment with strftime().
Some examples of the result string produced by the glibc implementation of strftime() are as follows:
$ ./a.out '%m' Result string is "11" $ ./a.out '%5m' Result string is "00011" $ ./a.out '%_5m' Result string is " 11"
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int
main(int argc, char *argv[])
{
char outstr[200];
time_t t;
struct tm *tmp;
t = time(NULL);
tmp = localtime(&t);
if (tmp == NULL) {
perror("localtime");
exit(EXIT_FAILURE);
}
if (strftime(outstr, sizeof(outstr), argv[1], tmp) == 0) {
fprintf(stderr, "strftime returned 0");
exit(EXIT_FAILURE);
}
printf("Result string is \"%s\"\n", outstr);
exit(EXIT_SUCCESS);
}
date(1), time(2), ctime(3), nl_langinfo(3), setlocale(3), sprintf(3), strptime(3)
| 2023-07-20 | Linux man-pages 6.05.01 |