diff options
Diffstat (limited to 'man/man3/strftime.3')
| -rw-r--r-- | man/man3/strftime.3 | 777 |
1 files changed, 777 insertions, 0 deletions
diff --git a/man/man3/strftime.3 b/man/man3/strftime.3 new file mode 100644 index 0000000..e94a3ef --- /dev/null +++ b/man/man3/strftime.3 @@ -0,0 +1,777 @@ +'\" t +.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk) +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.\" References consulted: +.\" Linux libc source code +.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991) +.\" 386BSD man pages +.\" GNU texinfo documentation on glibc date/time functions. +.\" Modified Sat Jul 24 18:03:44 1993 by Rik Faith (faith@cs.unc.edu) +.\" Applied fix by Wolfgang Franke, aeb, 961011 +.\" Corrected return value, aeb, 970307 +.\" Added Single UNIX Spec conversions and %z, aeb/esr, 990329. +.\" 2005-11-22 mtk, added glibc Notes covering optional 'flag' and +.\" 'width' components of conversion specifications. +.\" +.TH strftime 3 2024-05-02 "Linux man-pages (unreleased)" +.SH NAME +strftime \- format date and time +.SH LIBRARY +Standard C library +.RI ( libc ", " \-lc ) +.SH SYNOPSIS +.nf +.B #include <time.h> +.P +.BI "size_t strftime(char " s "[restrict ." max "], size_t " max , +.BI " const char *restrict " format , +.BI " const struct tm *restrict " tm ); +.P +.BI "size_t strftime_l(char " s "[restrict ." max "], size_t " max , +.BI " const char *restrict " format , +.BI " const struct tm *restrict " tm , +.BI " locale_t " locale ); +.fi +.SH DESCRIPTION +The +.BR strftime () +function formats the broken-down time +.I tm +according to the format specification +.I format +and places the +result in the character array +.I s +of size +.IR max . +The broken-down time structure +.I tm +is defined in +.IR <time.h> . +See also +.BR ctime (3). +.\" FIXME . POSIX says: Local timezone information is used as though +.\" strftime() called tzset(). But this doesn't appear to be the case +.P +The format specification is a null-terminated string and may contain +special character sequences called +.IR "conversion specifications", +each of which is introduced by a \[aq]%\[aq] character and terminated by +some other character known as a +.IR "conversion specifier character". +All other character sequences are +.IR "ordinary character sequences". +.P +The characters of ordinary character sequences (including the null byte) +are copied verbatim from +.I format +to +.IR s . +However, the characters +of conversion specifications are replaced as shown in the list below. +In this list, the field(s) employed from the +.I tm +structure are also shown. +.TP +.B %a +The abbreviated name of the day of the week according to the current locale. +(Calculated from +.IR tm_wday .) +(The specific names used in the current locale can be obtained by calling +.BR nl_langinfo (3) +with +.BR ABDAY_ { 1 \[en] 7 } +as an argument.) +.TP +.B %A +The full name of the day of the week according to the current locale. +(Calculated from +.IR tm_wday .) +(The specific names used in the current locale can be obtained by calling +.BR nl_langinfo (3) +with +.BR DAY_ { 1 \[en] 7 } +as an argument.) +.TP +.B %b +The abbreviated month name according to the current locale. +(Calculated from +.IR tm_mon .) +(The specific names used in the current locale can be obtained by calling +.BR nl_langinfo (3) +with +.BR ABMON_ { 1 \[en] 12 } +as an argument.) +.TP +.B %B +The full month name according to the current locale. +(Calculated from +.IR tm_mon .) +(The specific names used in the current locale can be obtained by calling +.BR nl_langinfo (3) +with +.BR MON_ { 1 \[en] 12 } +as an argument.) +.TP +.B %c +The preferred date and time representation for the current locale. +(The specific format used in the current locale can be obtained by calling +.BR nl_langinfo (3) +with +.B D_T_FMT +as an argument for the +.B %c +conversion specification, and with +.B ERA_D_T_FMT +for the +.B %Ec +conversion specification.) +(In the POSIX locale this is equivalent to +.BR "%a %b %e %H:%M:%S %Y" .) +.TP +.B %C +The century number (year/100) as a 2-digit integer. (SU) +(The +.B %EC +conversion specification corresponds to the name of the era.) +(Calculated from +.IR tm_year .) +.TP +.B %d +The day of the month as a decimal number (range 01 to 31). +(Calculated from +.IR tm_mday .) +.TP +.B %D +Equivalent to +.BR %m/%d/%y . +(Yecch\[em]for Americans only. +Americans should note that in other countries +.B %d/%m/%y +is rather common. +This means that in international context this format is +ambiguous and should not be used.) (SU) +.TP +.B %e +Like +.BR %d , +the day of the month as a decimal number, but a leading +zero is replaced by a space. (SU) +(Calculated from +.IR tm_mday .) +.TP +.B %E +Modifier: use alternative ("era-based") format, see below. (SU) +.TP +.B %F +Equivalent to +.B %Y\-%m\-%d +(the ISO\~8601 date format). (C99) +.TP +.B %G +The ISO\~8601 week-based year (see NOTES) with century as a decimal number. +The 4-digit year corresponding to the ISO week number (see +.BR %V ). +This has the same format and value as +.BR %Y , +except that if the ISO week number belongs to the previous or next year, +that year is used instead. (TZ) +(Calculated from +.IR tm_year , +.IR tm_yday , +and +.IR tm_wday .) +.TP +.B %g +Like +.BR %G , +but without century, that is, with a 2-digit year (00\[en]99). (TZ) +(Calculated from +.IR tm_year , +.IR tm_yday , +and +.IR tm_wday .) +.TP +.B %h +Equivalent to +.BR %b . +(SU) +.TP +.B %H +The hour as a decimal number using a 24-hour clock (range 00 to 23). +(Calculated from +.IR tm_hour .) +.TP +.B %I +The hour as a decimal number using a 12-hour clock (range 01 to 12). +(Calculated from +.IR tm_hour .) +.TP +.B %j +The day of the year as a decimal number (range 001 to 366). +(Calculated from +.IR tm_yday .) +.TP +.B %k +The hour (24-hour clock) as a decimal number (range 0 to 23); +single digits are preceded by a blank. +(See also +.BR %H .) +(Calculated from +.IR tm_hour .) +(TZ) +.TP +.B %l +The hour (12-hour clock) as a decimal number (range 1 to 12); +single digits are preceded by a blank. +(See also +.BR %I .) +(Calculated from +.IR tm_hour .) +(TZ) +.TP +.B %m +The month as a decimal number (range 01 to 12). +(Calculated from +.IR tm_mon .) +.TP +.B %M +The minute as a decimal number (range 00 to 59). +(Calculated from +.IR tm_min .) +.TP +.B %n +A newline character. (SU) +.TP +.B %O +Modifier: use alternative numeric symbols, see below. (SU) +.TP +.B %p +Either "AM" or "PM" according to the given time value, or the +corresponding strings for the current locale. +Noon is treated as "PM" and midnight as "AM". +(Calculated from +.IR tm_hour .) +(The specific string representations used for "AM" and "PM" +in the current locale can be obtained by calling +.BR nl_langinfo (3) +with +.BR AM_STR " and " PM_STR , +respectively.) +.TP +.B %P +Like +.B %p +but in lowercase: "am" or "pm" or a corresponding +string for the current locale. +(Calculated from +.IR tm_hour .) +(GNU) +.TP +.B %r +The time in a.m. or p.m. notation. +(SU) +(The specific format used in the current locale can be obtained by calling +.BR nl_langinfo (3) +with +.B T_FMT_AMPM +as an argument.) +(In the POSIX locale this is equivalent to +.BR "%I:%M:%S %p" .) +.TP +.B %R +The time in 24-hour notation +.RB ( %H:%M ). +(SU) +For a version including the seconds, see +.B %T +below. +.TP +.B %s +The number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). (TZ) +(Calculated from +.IR mktime(tm) .) +.TP +.B %S +The second as a decimal number (range 00 to 60). +(The range is up to 60 to allow for occasional leap seconds.) +(Calculated from +.IR tm_sec .) +.TP +.B %t +A tab character. (SU) +.TP +.B %T +The time in 24-hour notation +.RB ( %H:%M:%S ). +(SU) +.TP +.B %u +The day of the week as a decimal, range 1 to 7, Monday being 1. +See also +.BR %w . +(Calculated from +.IR tm_wday .) +(SU) +.TP +.B %U +The week number of the current year as a decimal number, +range 00 to 53, starting with the first Sunday as the first day +of week 01. +See also +.B %V +and +.BR %W . +(Calculated from +.I tm_yday +and +.IR tm_wday .) +.TP +.B %V +The ISO\~8601 week number (see NOTES) of the current year as a decimal number, +range 01 to 53, where week 1 is the first week that has at least +4 days in the new year. +See also +.B %U +and +.BR %W . +(Calculated from +.IR tm_year , +.IR tm_yday , +and +.IR tm_wday .) +(SU) +.TP +.B %w +The day of the week as a decimal, range 0 to 6, Sunday being 0. +See also +.BR %u . +(Calculated from +.IR tm_wday .) +.TP +.B %W +The week number of the current year as a decimal number, +range 00 to 53, starting with the first Monday as the first day of week 01. +(Calculated from +.I tm_yday +and +.IR tm_wday .) +.TP +.B %x +The preferred date representation for the current locale without the time. +(The specific format used in the current locale can be obtained by calling +.BR nl_langinfo (3) +with +.B D_FMT +as an argument for the +.B %x +conversion specification, and with +.B ERA_D_FMT +for the +.B %Ex +conversion specification.) +(In the POSIX locale this is equivalent to +.BR %m/%d/%y .) +.TP +.B %X +The preferred time representation for the current locale without the date. +(The specific format used in the current locale can be obtained by calling +.BR nl_langinfo (3) +with +.B T_FMT +as an argument for the +.B %X +conversion specification, and with +.B ERA_T_FMT +for the +.B %EX +conversion specification.) +(In the POSIX locale this is equivalent to +.BR %H:%M:%S .) +.TP +.B %y +The year as a decimal number without a century (range 00 to 99). +(The +.B %Ey +conversion specification corresponds to the year since the beginning of the era +denoted by the +.B %EC +conversion specification.) +(Calculated from +.IR tm_year ) +.TP +.B %Y +The year as a decimal number including the century. +(The +.B %EY +conversion specification corresponds to +the full alternative year representation.) +(Calculated from +.IR tm_year ) +.TP +.B %z +The +.I +hhmm +or +.I \-hhmm +numeric timezone (that is, the hour and minute offset from UTC). (SU) +.TP +.B %Z +The timezone name or abbreviation. +.TP +.B %+ +.\" Nov 05 -- Not in Linux/glibc, but is in some BSDs (according to +.\" their man pages) +The date and time in +.BR date (1) +format. (TZ) +(Not supported in glibc2.) +.TP +.B %% +A literal \[aq]%\[aq] character. +.P +Some conversion specifications can be modified by preceding the +conversion specifier character by the +.B E +or +.B O +.I 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 +.BR %Ec , +.BR %EC , +.BR %Ex , +.BR %EX , +.BR %Ey , +.BR %EY , +.BR %Od , +.BR %Oe , +.BR %OH , +.BR %OI , +.BR %Om , +.BR %OM , +.BR %OS , +.BR %Ou , +.BR %OU , +.BR %OV , +.BR %Ow , +.BR %OW , +.BR %Oy , +where the effect of the +.B O +modifier is to use +alternative numeric symbols (say, roman numerals), and that of the +.B E +modifier is to use a locale-dependent alternative representation. +The rules governing date representation with the +.B E +modifier can be obtained by supplying +.B ERA +as an argument to a +.BR nl_langinfo (3). +One example of such alternative forms is the Japanese era calendar scheme in the +.B ja_JP +glibc locale. +.P +.BR strftime_l () +is equivalent to +.BR strftime (), +except it uses the specified +.I locale +instead of the current locale. +The behaviour is undefined if +.I locale +is invalid or +.BR LC_GLOBAL_LOCALE . +.SH RETURN VALUE +Provided that the result string, +including the terminating null byte, does not exceed +.I max +bytes, +.BR strftime () +returns the number of bytes (excluding the terminating null byte) +placed in the array +.IR s . +If the length of the result string (including the terminating null byte) +would exceed +.I max +bytes, then +.BR strftime () +returns 0, and the contents of the array are undefined. +.\" (This behavior applies since at least libc 4.4.4; +.\" very old versions of libc, such as libc 4.4.1, +.\" would return +.\" .I max +.\" if the array was too small.) +.P +Note that the return value 0 does not necessarily indicate an error. +For example, in many locales +.B %p +yields an empty string. +An empty +.I format +string will likewise yield an empty string. +.SH ENVIRONMENT +The environment variables +.B TZ +and +.B LC_TIME +are used. +.SH ATTRIBUTES +For an explanation of the terms used in this section, see +.BR attributes (7). +.TS +allbox; +lbx lb lb +l l l. +Interface Attribute Value +T{ +.na +.nh +.BR strftime (), +.BR strftime_l () +T} Thread safety MT-Safe env locale +.TE +.SH STANDARDS +.TP +.BR strftime () +C11, POSIX.1-2008. +.TP +.BR strftime_l () +POSIX.1-2008. +.SH HISTORY +.TP +.BR strftime () +SVr4, C89. +.\" FIXME strftime() is in POSIX.1-2001 and POSIX.1-2008, but the details +.\" in the standards changed across versions. Investigate and +.\" write up. +.TP +.BR strftime_l () +POSIX.1-2008. +.P +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 +.B %+ +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 +.BR date (1) +several extensions that could apply to +.BR strftime () +as well. +The +.B %F +conversion is in C99 and POSIX.1-2001. +.P +In SUSv2, the +.B %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). +.SH NOTES +.SS ISO\~8601 week dates +.BR %G , +.BR %g , +and +.B %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 +.RB ( %V ) +of the year 2009 +.RB ( %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. +.SS glibc notes +glibc provides some extensions for conversion specifications. +(These extensions are not specified in POSIX.1-2001, but a few other +systems provide similar features.) +.\" HP-UX and Tru64 also have features like this. +Between the \[aq]%\[aq] character and the conversion specifier character, +an optional +.I flag +and field +.I width +may be specified. +(These precede the +.B E +or +.B O +modifiers, if present.) +.P +The following flag characters are permitted: +.TP +.B _ +(underscore) +Pad a numeric result string with spaces. +.TP +.B \- +(dash) +Do not pad a numeric result string. +.TP +.B 0 +Pad a numeric result string with zeros even if the conversion +specifier character uses space-padding by default. +.TP +.B \[ha] +Convert alphabetic characters in result string to uppercase. +.TP +.B # +Swap the case of the result string. +(This flag works only with certain conversion specifier characters, +and of these, it is only really useful with +.BR %Z .) +.P +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. +.SH BUGS +If the output string would exceed +.I max +bytes, +.I errno +is +.I not +set. +This makes it impossible to distinguish this error case from cases where the +.I format +string legitimately produces a zero-length output string. +POSIX.1-2001 +does +.I not +specify any +.I errno +settings for +.BR strftime (). +.P +Some buggy versions of +.BR gcc (1) +complain about the use of +.BR %c : +.IR "warning: \`%c\[aq] yields only last 2 digits of year in some locales" . +Of course programmers are encouraged to use +.BR %c , +as it gives the preferred date and time representation. +One meets all kinds of strange obfuscations +to circumvent this +.BR gcc (1) +problem. +A relatively clean one is to add an +intermediate function +.P +.in +4n +.EX +size_t +my_strftime(char *s, size_t max, const char *fmt, + const struct tm *tm) +{ + return strftime(s, max, fmt, tm); +} +.EE +.in +.P +Nowadays, +.BR gcc (1) +provides the +.I \-Wno\-format\-y2k +option to prevent the warning, +so that the above workaround is no longer required. +.SH EXAMPLES +.B RFC\~2822-compliant date format +(with an English locale for %a and %b) +.P +.in +4n +.EX +"%a,\ %d\ %b\ %Y\ %T\ %z" +.EE +.in +.P +.B RFC\~822-compliant date format +(with an English locale for %a and %b) +.P +.in +4n +.EX +"%a,\ %d\ %b\ %y\ %T\ %z" +.EE +.in +.SS Example program +The program below can be used to experiment with +.BR strftime (). +.P +Some examples of the result string produced by the glibc implementation of +.BR strftime () +are as follows: +.P +.in +4n +.EX +.RB "$" " ./a.out \[aq]%m\[aq]" +Result string is "11" +.RB "$" " ./a.out \[aq]%5m\[aq]" +Result string is "00011" +.RB "$" " ./a.out \[aq]%_5m\[aq]" +Result string is " 11" +.EE +.in +.SS Program source +\& +.\" SRC BEGIN (strftime.c) +.EX +#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 \e"%s\e"\en", outstr); + exit(EXIT_SUCCESS); +} +.EE +.\" SRC END +.SH SEE ALSO +.BR date (1), +.BR time (2), +.BR ctime (3), +.BR nl_langinfo (3), +.BR setlocale (3), +.BR sprintf (3), +.BR strptime (3) |