#include <time.h> size_t strftime(char *restrict s, size_t maxsize, const char *restrict format, const struct tm *restrict timeptr); size_t strftime_l(char *restrict s, size_t maxsize, const char *restrict format, const struct tm *restrict timeptr, locale_t locale);
The strftime() function shall place bytes into the array pointed to by s as controlled by the string pointed to by format. The format is a character string, beginning and ending in its initial shift state, if any. The format string consists of zero or more conversion specifications and ordinary characters.
Each conversion specification is introduced by the '%' character after which the following appear in sequence:
The default padding character is unspecified.
The results are unspecified if more than one flag character is specified, a flag character is specified without a minimum field width; a minimum field width is specified without a flag character; a modifier is specified with a flag or with a minimum field width; or if a minimum field width is specified for any conversion specifier other than C, F, G, or Y.
All ordinary characters (including the terminating NUL character) are copied unchanged into the array. If copying takes place between objects that overlap, the behavior is undefined. No more than maxsize bytes are placed into the array. Each conversion specifier is replaced by appropriate characters as described in the following list. The appropriate characters are determined using the LC_TIME category of the current locale and by the values of zero or more members of the broken-down time structure pointed to by timeptr, as specified in brackets in the description. If any of the specified values are outside the normal range, the characters stored are unspecified.
The strftime_l() function shall be equivalent to the strftime() function, except that the locale data used is from the locale represented by locale.
Local timezone information is used as though strftime() called tzset().
The following conversion specifiers shall be supported:
If a minimum field width is not specified, the number of characters placed into the array pointed to by s will be the number of digits in the year divided by 100 or two, whichever is greater. If a minimum field width is specified, the number of characters placed into the array pointed to by s will be the number of digits in the year divided by 100 or the minimum field width, whichever is greater.
If a minimum field width of x is specified, the year shall be output as if by the Y specifier (described below) with whatever flag was given and a minimum field width of x-6. If x is less than 6, the behavior shall be as if x equalled 6.
If the minimum field width is specified to be 10, and the year is four digits long, then the output string produced will match the ISO 8601:2004 standard subclause 4.1.2.2 complete representation, extended format date representation of a specific day. If a + flag is specified, a minimum field width of x is specified, and x-7 bytes are sufficient to hold the digits of the year (not including any needed sign character), then the output will match the ISO 8601:2004 standard subclause 4.1.2.4 complete representation, expanded format date representation of a specific day.
If a minimum field width is specified, the number of characters placed into the array pointed to by s will be the number of digits and leading sign characters (if any) in the year, or the minimum field width, whichever is greater.
If a minimum field width is specified, the number of characters placed into the array pointed to by s will be the number of digits and leading sign characters (if any) in the year, or the minimum field width, whichever is greater.
If a conversion specification does not correspond to any of the above, the behavior is undefined.
If a struct tm broken-down time structure is created by localtime() or localtime_r(), or modified by mktime(), and the value of TZ is subsequently modified, the results of the %Z and %z strftime() conversion specifiers are undefined, when strftime() is called with such a broken-down time structure.
If a struct tm broken-down time structure is created or modified by gmtime() or gmtime_r(), it is unspecified whether the result of the %Z and %z conversion specifiers shall refer to UTC or the current local timezone, when strftime() is called with such a broken-down time structure.
Some conversion specifiers can be modified by the E or O modifier characters to indicate that an alternative format or specification should be used rather than the one normally used by the unmodified conversion specifier. If the alternative format or specification does not exist for the current locale (see ERA in the Base Definitions volume of POSIX.1-2017, Section 7.3.5, LC_TIME), the behavior shall be as if the unmodified conversion specification were used.
%g, %G, and %V give values according to the ISO 8601:2004 standard week-based year. In this system, weeks begin on a Monday and week 1 of the year is the week that includes January 4th, which is also the week that includes the first Thursday of the year, and is also the first week that contains at least four days in the year. If the first Monday of January is the 2nd, 3rd, or 4th, the preceding days are part of the last week of the preceding year; thus, for Saturday 2nd January 1999, %G is replaced by 1998 and %V is replaced by 53. If December 29th, 30th, or 31st is a Monday, it and any following days are part of week 1 of the following year. Thus, for Tuesday 30th December 1997, %G is replaced by 1998 and %V is replaced by 01.
If a conversion specifier is not one of the above, the behavior is undefined.
The behavior is undefined if the locale argument to strftime_l() is the special locale object LC_GLOBAL_LOCALE or is not a valid locale object handle.
The following sections are informative.
The following example first sets the locale to the user's default. The locale information will be used in the nl_langinfo() and strftime() functions. The nl_langinfo() function returns the localized date string which specifies how the date is laid out. The strftime() function takes this information and, using the tm structure for values, places the date and time information into datestring.
#include <time.h> #include <locale.h> #include <langinfo.h> ... struct tm *tm; char datestring[256]; ... setlocale (LC_ALL, ""); ... strftime (datestring, sizeof(datestring), nl_langinfo (D_T_FMT), tm); ...
Some of the conversion specifications are duplicates of others. They are included for compatibility with nl_cxtime() and nl_ascxtime(), which were published in Issue 2.
The %C, %F, %G, and %Y format specifiers in strftime() always print full values, but the strptime() %C, %F, and %Y format specifiers only scan two digits (assumed to be the first two digits of a four-digit year) for %C and four digits (assumed to be the entire (four-digit) year) for %F and %Y. This mimics the behavior of printf() and scanf(); that is:
printf("%2d", x = 1000);
prints "1000", but:
scanf(%2d", &x);
when given "1000" as input will only store 10 in x). Applications using extended ranges of years must be sure that the number of digits specified for scanning years with strptime() matches the number of digits that will actually be present in the input stream. Historic implementations of the %Y conversion specification (with no flags and no minimum field width) produced different output formats. Some always produced at least four digits (with 0 fill for years from 0 through 999) while others only produced the number of digits present in the year (with no fill and no padding). These two forms can be produced with the '0' flag and a minimum field width options using the conversions specifications %04Y and %01Y, respectively.
In the past, the C and POSIX standards specified that %F produced an ISO 8601:2004 standard date format, but didn't specify which one. For years in the range [0001,9999], POSIX.1-2008 requires that the output produced match the ISO 8601:2004 standard complete representation extended format (YYYY-MM-DD) and for years outside of this range produce output that matches the ISO 8601:2004 standard expanded representation extended format (<+/-><Underline>Y</Underline>YYYY-MM-DD). To fully meet ISO 8601:2004 standard requirements, the producer and consumer must agree on a date format that has a specific number of bytes reserved to hold the characters used to represent the years that is sufficiently large to hold all values that will be shared. For example, the %+13F conversion specification will produce output matching the format "<+/->YYYYYY-MM-DD" (a leading '+' or '-' sign; a six-digit, 0-filled year; a '-'; a two-digit, leading 0-filled month; another '-'; and the two-digit, leading 0-filled day within the month).
Note that if the year being printed is greater than 9999, the resulting string from the unadorned %F conversion specifications will not conform to the ISO 8601:2004 standard extended format, complete representation for a date and will instead be an extended format, expanded representation (presumably without the required agreement between the date's producer and consumer).
In the C or POSIX locale, the E and O modifiers are ignored and the replacement strings for the following specifiers are:
POSIX.1-2008 now allows the format specifications %0xC, %0xF, %0xG, and %0xY (where 'x' is a string of decimal digits used to specify printing and scanning of a string of x decimal digits) with leading zero fill characters. Allowing applications to set the field width enables them to agree on the number of digits to be printed and scanned in the ISO 8601:2004 standard expanded representation of a year (for %F, %G, and %Y) or all but the last two digits of the year (for %C). This is based on a feature in some versions of GNU libc's strftime(). The GNU version allows specifying space, zero, or no-fill characters in strftime() format strings, but does not allow any flags to be specified in strptime() format strings. These implementations also allow these flags to be specified for any numeric field. POSIX.1-2008 only requires the zero fill flag ('0') and only requires that it be recognized when processing %C, %F, %G, and %Y specifications when a minimum field width is also specified. The '0' flag is the only flag needed to produce and scan the ISO 8601:2004 standard year fields using the extended format forms. POSIX.1-2008 also allows applications to specify the same flag and field width specifiers to be used in both strftime() and strptime() format strings for symmetry. Systems may provide other flag characters and may accept flags in conjunction with conversion specifiers other than %C, %F, %G, and %Y; but portable applications cannot depend on such extensions.
POSIX.1-2008 now also allows the format specifications %+xC, %+xF, %+xG, and %+xY (where 'x' is a string of decimal digits used to specify printing and scanning of a string of 'x' decimal digits) with leading zero fill characters and a leading '+' sign character if the year being converted is more than four digits or a minimum field width is specified that allows room for more than four digits for the year. This allows date providers and consumers to agree on a specific number of digits to represent a year as required by the ISO 8601:2004 standard expanded representation formats. The expanded representation formats all require the year to begin with a leading '+' or '-' sign. (All of these specifiers can also provide a leading '-' sign for negative years. Since negative years and the year 0 don't fit well with the Gregorian or Julian calendars, the normal ranges of dates start with year 1. The ISO C standard allows tm_year to assume values corresponding to years before year 1, but the use of such years provided unspecified results.)
Some earlier version of this standard specified that applications wanting to use strptime() to scan dates and times printed by strftime() should provide non-digit characters between fields to separate years from months and days. It also supported %F to print and scan the ISO 8601:2004 standard extended format, complete representation date for years 1 through 9999 (i.e., YYYY-MM-DD). However, many applications were written to print (using strftime()) and scan (using strptime()) dates written using the basic format complete representation (four-digit years) and truncated representation (two-digit years) specified by the ISO 8601:2004 standard representation of dates and times which do not have any separation characters between fields. The ISO 8601:2004 standard also specifies basic format expanded representation where the creator and consumer of these fields agree beforehand to represent years as leading zero-filled strings of an agreed length of more than four digits to represent a year (again with no separation characters when year, month, and day are all displayed). Applications producing and consuming expanded representations are encouraged to use the '+' flag and an appropriate maximum field width to scan the year including the leading sign. Note that even without the '+' flag, years less than zero may be represented with a leading <hyphen-minus> for %F, %G, and %Y conversion specifications. Using negative years results in unspecified behavior.
If a format specification %+xF with the field width x greater than 11 is specified and the width is large enough to display the full year, the output string produced will match the ISO 8601:2004 standard subclause 4.1.2.4 expanded representation, extended format date representation for a specific day. (For years in the range [1,99999], %+12F is sufficient for an agreed five-digit year with a leading sign using the ISO 8601:2004 standard expanded representation, extended format for a specific day "<+/->YYYYY-MM-DD".) Note also that years less than 0 may produce a leading <hyphen-minus> character ('-') when using %Y or %C whether or not the '0' or '+' flags are used.
The difference between the '0' flag and the '+' flag is whether the leading '+' character will be provided for years >9999 as required for the ISO 8601:2004 standard extended representation format containing a year. For example:
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In the cases above marked with a * in the strptime() scan back field, the implied or specified number of characters scanned by strptime() was less than the number of characters output by strftime() using the same format; so the remaining digits of the year were dropped when the output date produced by strftime() was scanned back in by strptime().
The Base Definitions volume of POSIX.1-2017, Section 7.3.5, LC_TIME, <time.h>
Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .