JTC1/SC22/WG14
N689
SC22/WG14 N689 J11/97-052
GENERAL WORDING ISSUES (clause 7)
Clive D.W. Feather
[email protected]
1997-05-15
This document is an attempt to identify all the minor issues I can find
in clause 7 of the Standard. I am using draft 9 pre 3 as my starting point.
It omits those items which I have previously proposed, or have been
addressed in a DR. It also omits a couple of matters which I am handling
separately.
There is a companion paper for subclauses 1 to 6, and there may be another
for the Annexes.
I hope it is unnecessary to produce formal proposals for all of these,
but I will attempt to identify the issue and/or generate new wording (I
hope that, in at least some cases, new wording will make the problem
obvious).
=======================================================================
Item 1:
The following code:
int *a;
/* ... */
a [1] = (realloc)(a, a [0]);
is strictly conforming because of the sequence point before realloc is
called and the one before it returns. However, if the parentheses are
removed, the call might be replaced by a macro and the sequence points
are lost. This is surprising to most users of the standard library.
Add to the end of 7.1.2 paragraph 5:
In addition, the macro shall expand to code that contains a sequence
point after the evaluation of all arguments and before any other
action, and a sequence point at the end of evaluation.
Alternatively, add a footnote:
Such macros might not contain the sequence points that the
corresponding function calls do.
====
Item 2:
There was a long discussion some time ago about the following code:
printf ("%n foo %n", &i, &i);
and whether it is strictly conforming. I would suggest that we need the
following somewhere in 7.1 (either as 7.1.8, or as part of 7.1.7):
Except where explicitly stated, there are no sequence points during
the evaluation of a library function. Where a function's action is
described in sequential terms, or one function is defined in terms
of calls to another, this is for the purpose of describing the final
effect, and does not require the events to actually occur in that
order, or for an actual call to the other function to occur.
Nevertheless, there is a sequence point immediately before the
function is called (as specified by subclause 6.3.2.2), and
immediately before it returns.
Example
The call:
int i;
(printf) ("%n %n", &i, &i)
invokes undefined behaviour, because it assigns to i twice between
the same pair of sequence points. Even though printf is defined in
terms of calls to putc(), it is not required for such a call
actually to occur, nor for there to be a sequence point before and
after outputting the space.
====
Item 3:
Add to the end of 7.11 paragraph 3:
It is permitted to create a pointer to a va_list and pass that
pointer to another function, in which case the original function
may make further use of the original list after the other function
returns.
====
Item 4:
I can see nothing to prevent EOF being defined, on a 16-bit int system,
as -65535, which is, after all, a negative integral constant expression.
However, this does not have a negative value.
More generally, while some symbols have restrictions on them such as
"suitable to be used as the 3rd argument of foo()", and others are never
passed to or returned by functions, there are still some loopholes.
To fix this, change 7.12.1 paragraph 3 to read, in part:
EOF
which expands to an integral constant expression, with type /int/ and
a negative value, that is returned ...
There are no doubt several other such cases. Making a quick check I can
find:
* EDOM, ERANGE, SIGABRT, SIGFPE, SIGILL, SIGINT, SIGSEGV, and SIGTERM are
all positive integral expressions that need to be stored in an /int/, but
are not restricted in this way.
* CLOCKS_PER_SEC does not have a specified type.
====
Item 5:
Part 1:
Change the first sentence of subclause 7.1.2 paragraph 1 to read:
Each library function is declared, with a type that includes a
prototype, in a /header/ [111].
The as-if rule means that this need not be done literally, provided that
the effects of argument assignment rather than default promotion (other
than trailing varargs, of course) will happen to all library function
calls.
Part 2:
One effect of part 1 is to guarantee that a diagnostic will be generated
if the corresponding argument has an unconvertable type:
#include <stdlib.h>
/* ... */
int main (void)
{ return (abs)("0"); }
However, this guarantee does not apply if a macro happens to be
provided:
#include <stdlib.h>
/* ... */
int main (void)
{ return abs ("0"); }
It would be convenient to provide this latter guarantee. This could be
done by adding at the end of 7.1.2:
It shall also ensure that, if the macro is called with an argument
whose type is such that it cannot be converted to the type of the
corresponding parameter, the diagnostic required by subclause
6.3.2.2 shall be generated, as if the macro had been suppressed.
The intent of this change is *not* to introduce some new kind of macro,
but merely to ensure that all the arguments are used in ways such that
the expanded macro will cause the necessary diagnostic. However, it is
not immediately obvious that this can be done.
====
Item 6:
A careful reading of subclause 7.3.1 shows that, for characters outside
the 95-element minimal execution character set, there are two sets of
classification macros that are significant. For each set, a character
can belong to at most one member of the set. The following table shows
these sets, examples of characters within those sets taken from the
minimal 95, and cases that cannot happen:
isprint() iscntrl() [neither]
isalpha() 'A' forbidden =1=
isspace() ' ' '\n' =2=
[neither] ':' '\b' '\0'
The interesting cases are those marked =1= and =2=; any characters with
these properties must be locale-specific.
The question turns on the intended meaning of "printable". The current
definition requires the character to occupy a position on a printing
device. In this case, such characters do make sense - =1= could be a "dead"
character that overprints another one, or =2= could be a hair-thin space.
In this case, attach a footnote to subclauses 7.3.1.2 (isalpha()),
7.3.1.6 (islower()), and 7.3.1.10 (isupper()):
[*] The additional characters might not be printing characters; for
example, they may be "dead" characters that overprint the preceeding
or following character and are thus not "printing".
However, it is questionable whether the term "one printing position" still
has a meaning in this day of proportional-spaced output devices, and
whether there is a need for a better definition of "printable". In this
case:
Change the definition to:
The term "printing character" refers to a member of an implementation-
defined set of characters, each of which has a characteristic appearance
on a display device and usually occupies one printing position;
in subclauses 7.3.1.2 (isalpha()), 7.3.1.6 (islower()), and 7.3.1.10
(isupper()), change:
... locale-specific set of characters ...
to:
... locale-specific set of printing characters ...
and in 7.3.1.9 (isspace()) change it to:
... locale-specific set of printing or control characters ...
====
Item 7:
There are a couple of places where <ctype.h> is less than clear.
Attach a footnote to subclause 7.3.1.2 (isalpha()):
[*] The functions islower() and isupper() test true or false
separately for each of these additional characters; all four
combinations are possible.
In subclause 7.3.2.1 (tolower()), change the text to:
Description
The tolower function converts an uppercase letter to a corresponding
lowercase letter.
Returns
If the argument is a character for which isupper is true and there
are one or more corresponding characters for which islower is true,
the tolower function returns one of the corresponding characters
(always the same one for any given locale); otherwise, the argument
is returned unchanged.
and make the corresponding changes to subclause 7.3.2.2 (toupper()).
====
Item 8:
In subclause 7.9.1.1 (setjmp()) there is a heading "Environmental
constraint". This implies that the sentence is a Constraint, and that
violation requires a diagnostic. It is reported that very few
implementations generate such a diagnostic, and that most
implementations correctly handle other contexts.
Therefore change the heading to "Environmental restriction" or just make
this part of the semantics. Possibly add at the end:
If the invocation appears in any other context, the behaviour is
undefined.
====
Item 9:
There is no obvious reason why the following functions - va_list forms
of the *scanf family - are not provided by the Standard; they should be
added:
7.12.6:
vfscanf (FILE * restrict, const char * restrict, va_list)
vscanf (const char * restrict, va_list)
vsscanf (const char * restrict, const char * restrict, va_list)
7.18.2
vfwscanf (FILE * restrict, const wchar_t * restrict, va_list)
vwscanf (const wchar_t * restrict, va_list)
vswscanf (const wchar_t * restrict, const wchar_t * restrict,
va_list)
====
Item 10:
Locales are currently treated as extremely opaque. It is not possible to
determine whether two locales are equivalent in a category. It is not
even sensible to compare locale strings for equality; the string
returned need not be the same as the string passed in, even if it was
also the string returned from a previous call. That is:
char *loc;
char copy_loc [LARGE_ENOUGH];
loc = setlocale (LC_COLLATE, "C");
if (strcmp (loc, "C") != 0)
do_something (); // This can happen
assert (strlen (loc) < LARGE_ENOUGH);
strcpy (copy_loc, loc);
loc = setlocale (LC_COLLATE, "C");
if (strcmp (loc, copy_loc) != 0)
do_something (); // This can happen
I realize that most systems store most locales in files, and therefore
comparing for functional equality is not as simple as it might seem.
However, I would recommend the following as a minimum:
(1) Add to 7.5.1.1 (setlocale()) paragraph 8:
Furthermore, if this string value is passed to the setlocale
function with the same category, the result shall be the same string
value.
(2) Add either a function to compare two locale strings for functional
equivalence in a category, or a function to compare a locale string with
the current locale in a category. Functional equivalence is defined as:
No behaviour defined in clause 7, other than the result of the
setlocale function, changes as a result of changing the locale.
Note that "strictly conforming" is not a good term to use in any
comparison.
====
Item 11:
The localeconv() function discusses monetary and non-monetary
formatting, especially the former, but provides no easy way to implement
it. The natural place to do this is the printf() family of functions.
Therefore add to 7.12.6.1 (fprintf()):
Flag , (comma):
for d, i, o, u, x, X, f, F, e, E, g, G, a, and A conversions, the
output shall be grouped in accordance with the /thousands_sep/ and
/grouping/ fields of the locale. For other conversions, the
behaviour is undefined.
Format or flag $ (dollar):
[It is unclear whether this is better as a flag or a format.]
Generate a formatted monetary quantity. If it is a format, the
argument is a double (or long double if L is included). The plus and
space flags act as if the output already included a sign (even if it
does not). The # flag specifies international formatting. The minus
and zero flags can be used. If no precision is specified, the value
of /frac_digits/ or /int_frac_digits/ from the current locale is
used; if that is CHAR_MAX, the precision is unspecified. [If it is a
flag, this would overrule the normal meaning of the precision.]
Issues:
[comma]
Should there be a mechanism to allow the grouping to depend on the
format (e.g. decimal output grouped in threes, hex output grouped in
fours) ?
I am informed that there are circumstances where the /thousands_sep/
character is different for each grouping. For example, a notation
commonly used in Japan (particularly in newspapers) places characters
meaning "myriad", "hundred million", "billion" and so on between the
groups. This would require changing the separator to be a list of
strings, and providing a convention to indicate this (for example, using
CHAR_MAX as the first byte of the string).
[dollar]
I've used the normal rule that the specified precision overrides the
default. An alternative would be that the precision applies only if the
locale-specified value is CHAR_MAX. Which is preferable, or should there
be a way to choose ?
If $ is a flag and is used with %d, should it scale the value to the
appropriate number of fractional digits ? For example, "%$6.2d" might
indicate that the integer is to be printed in /ddd.dd/ form, with 12345
being printed as "123.45". Should %$d and %$i behave differently in this
case ? If $ is a format, should there be an equivalent for integral
types ?
Since this proposal was drafted, it has been pointed out to me that any use
of $ will conflict with the X/Open mechanisms, which use descriptors of the
form "%1$d", "$*2$3$d", and "%*6$.*5$4$d".
====
Item 12:
The results of localeconv() should be completely specified for the C
locale. All strings are empty and all chars are CHAR_MAX except:
decimal_point "."
mon_decimal_point "."
positive_sign "+"
negative_sign "-"
====
Item 13:
Change the last sentence of subclause 7.10.1.1 (signal()) paragraph 2 to
read:
An invocation of such a function because of a signal, or of any
further functions called by that invocation (other than functions
in the standard library), is called a /signal handler/.
Change subclause 7.10.1.1 paragraph 4 to read:
If the signal occurs other than as the result of calling the abort
or raise function, the behaviour is undefined if the signal handler
includes a call to any function in the standard library other than:
- the abort, exit, or longjmp functions, or
- the signal function itself, with the first argument equal to the
signal number corresponding to the signal that caused the
invocation of the handler,
or refers to any object with static storage duration other than
- by assigning a value to a static storage duration object of type
volatile sig_atomic_t,
- as part of the first argument to a call to the longjmp
function[*], or
- as part of the execution of the abort, exit, or longjmp functions
(but not as part of any signal handler or function registered with
the atexit function called from them).
Furthermore, if such a call to the signal function results in a
SIG_ERR return, the value of errno is indeterminate [161].
[*] That is, given:
static jmp_buf env;
then:
longjmp (env, 0);
is valid, but:
auto jmp_buf e2;
memcpy (e2, env, sizeof env);
longjmp (e2, 0);
even though the value of /env/ will eventually be used in such a
call.
I suspect that subclause 7.10.1.1 paragraph 7 also needs to say "... for
the most recent successful call ...".
====
Item 14:
The Standard is somewhat unclear about the details of stdio buffering.
For example, considering output (the analogous situation happens with
input) a call to fputc() can have one of the following effects:
(1) the character is sent to the underlying system;
(2) the character is written to a buffer;
(3) the character is written to a buffer and then a number of characters
are sent to the underlying system from the buffer;
(4) a number of characters are sent to the underlying system from a
buffer, and then the character is written to the buffer.
In case (1), failure can be reported in a straightforward manner, and it
can be assumed that case (2) never fails. The question is: what will
happen if cases (3) or (4) have a failure during the output, but not
directly as a result of that character (that is, the error occurs
earlier on in the buffer) ?
The present wording of the Standard implies that an error in outputting
a character can only be reported on that call to fputc(), and not on any
subsequent call. This needs to be changed, or buffering becomes a
nonsense - the implementation would be required to *predict* whether a
write will succeed. A suitable location is 7.12.3, and the wording needs
to say something along the following lines:
If output is buffered, then it may be transmitted to the host
environment at any subsequent call to fputc(), and shall be
transmitted no later than the next fflush() call or when the stream
is closed. Thus a call to fputc() may fail and set the error
indicator on the stream because of the earlier output. Similarly, if
input is buffered, a call to fgetc() may cause the error indicator
to be set even though the same call on an unbuffered stream would
not (because the error is associated with a later character in the
input).
Even if the data is successfully transmitted to the host environment,
it is possible for an error to occur within the latter. If this happens
after the stream has been closed, it can not be reported to the
application; if it occurs earlier, it is implementation-defined when
it is so reported.
A secondary issue is: can the buffer be sent to the underlying system
other than within a call to fputc(); is asynchronous I/O permitted ? If
so, then:
When a stream is buffered, characters may be transmitted to or from
the host environment other than as part of a library function, and
thus the error indicator for the stream may be set outside such a
function (the indicator can only be cleared as part of a function
that explicitly states it does so).
====
Item 15:
Is there a need to provide a way to make the three standard streams be
binary, in the same way that they can already be made wide ? Without it,
there's no strictly-conforming way to write "cat". Even with it there is
the trailing zero byte problem.
====
Item 16:
There is no way to determine whether two fpos_t values represent the
same position in a file. Therefore, it is not possible to do the
following:
open a file
read through it, looking for some mark
note the position using fgetpos()
rewind
read through it again to the same position, using calls to fgetpos()
to determine where you are, rather than recalculating it
I suggest the following function be added to subclause 7.12.9:
struct fcmppos fcmppos (fpos_t* a, fpos_t* b, FILE *stream)
Compares two fpos_t values that refer to the given stream; if either
argument is a null pointer, the result of a call to fgetpos() on the
stream is used instead. The resulting structure contains at least
the following fields:
int before; // Less than, equal to, or greater than zero according
// to whether /a/ is before, at the same location as,
// or after /b/ in the file.
int mbstate; // Zero if the two positions have the same multibyte
// parsing status.
If the stream has been written to at any point before the later of
the two positions, the behaviour is undefined.
====
Item 17:
Add to subclause 7.13.4.2 (atexit()) paragraph 2:
Whether the function is called on abnormal program termination is
implementation-defined.
or it could be unspecified.
====
Item 18:
Change the last sentence of subclause 7.13.3 paragraph 1 to:
The value of a pointer that refers to freed space, or to space that
has subsequently been moved, is indeterminate.
In subclause 7.13.3.4 (realloc ()) paragraph 2, change:
or if the space has been deallocated by a call to the /free/ or
/realloc/ function,
to:
or if the space has been deallocated or moved by a call to the
/free/ or /realloc/ function,
Further to this, the Standard provides no way to determine whether
realloc() has moved the memory; this is something you want to do if you
have pointers to within the block of memory. If it hasn't moved, the
returned pointer will compare equal to the pointer argument. But if it
has, you cannot make the comparison because a pointer to freed memory
(and thus to moved memory) is indeterminate, and the comparison is
undefined behaviour (unless you go through hoops).
What is the reasoning behind this last part - making a legitimate value
suddenly become illegitimate ? Should it be changed, or is some other
mechanism to determine this desirable.
====
Item 19:
The specification of the comparison functions for bsearch() and qsort()
(7.13.5.1 and 7.13.5.2) is insufficient to safely code them. In
particular, it does not address the following issues.
(1) Are the pointers to objects within the base array (or the key
object), or can they be to copies ?
(2) Can the comparison alter the values of the pointed-to objects ?
(3) If so, does the alteration persist ?
(4) What are the requirements on the consistency of the comparison
results ?
I propose that comparisons are not allowed to alter the values, and
therefore that the implementation can pass pointers to copies of the
objects. [This, of course, invalidates an item in one of my articles in
CUJ :-]
Therefore add something like the following:
The comparison function shall behave as follows. The implementation
shall ensure that the second argument when called from /bsearch/, or
both arguments when called from /qsort/, shall be pointers to an
element of the array, or to a copy of such an element. The first
argument when called from /bsearch/ shall equal /key/. The function
shall make its comparison based on the pointed-to objects, and not
the specific addresses passed to it.
The comparison function shall not alter the contents of the array.
The implementation may reorder elements of the array between calls
to the comparison function, but shall not alter the contents of any
individual element.
When the same object (consisting of /size/ bytes, irrespective of
its current position in the array) is passed more than once to the
comparison function, the results shall be consistent with one
another. That is, for /qsort/ they shall define a total ordering on
the array, and for /bsearch/ the same object shall always compare
the same way with the key.
A sequence point occurs immediately before and immediately after
each call to the comparison function, and also between any call to
the comparison function and any movement of the objects passed as
arguments to that call.
If it is felt desirable that the pointers *shall* always point into the
array, then replace the first paragraph by:
The comparison function shall behave as follows. The implementation
shall ensure that the second argument when called from /bsearch/, or
both arguments when called from /qsort/, shall be pointers to
elements of an array whose first member is at /base/ and whose size
is /size/ bytes [1]. The first argument when called from /bsearch/
shall equal /key/.
[1] That is, if the value passed is /p/, then the following
expressions are always non-zero:
((char *) p - (char *) base) % size == 0
(char *) p >= (char *) base
(char *) p < (char *) base + nmemb * size
====
Item 20:
Modify the definition of a conversion specifier in 7.15.3.5 (strftime())
to allow the following:
* the - flag
* the 0 flag
* a field width
* a precision, with the meaning that fprintf assigns for %d and %s.
====
Item 21:
The conversion carried out by localtime does not provide any way of
determining the time zone used, and the normalization done by mktime
does not specify how DST changes are handled. Similarly, many systems
are now aware of leap seconds, but the Standard is not clear on how
these are to be handled. Adding this information is not trivial, because
there is no obvious way to extend /struct tm/ in a compatible manner.
This proposal therefore contains a kludge.
[The following is not final wording; it is first necessary to agree the
semantics.]
Add the following fields to struct tm:
int tm_version; /* version number of the structure layout */
int tm_utcoffset; /* offset from UTC in minutes - [-1439, +1439] */
int tm_leapsecs; /* leap seconds applied */
int tm_xisdst; /* daylight saving time flag - [-1, +1439] */
and add the following macros to <time.h>, all constant integral
expressions capable of being stored in an object of type int:
_EXTENDED_TM
_NO_LEAP_SECONDS
_LOCALTIME
The /gmtime/ function shall set tm_utcoffset to 0, while the /localtime/
function shall set it according to the local time zone, including any
DST corrections; a negative value for tm_utcoffset indicates ahead of
UTC, so that PDT is represented by +420. If the implementation is unable
to determine the local zone, /localtime/ shall set this field _LOCALTIME
and /gmtime/ shall fail.
Both functions shall set tm_isdst to represent whether DST is (believed
to be) in effect at the represented time, and tm_xisdst to -1, 0, or the
(positive) size of the DST offset, in minutes, according as whether
tm_isdst is less than, equal to, or greater than zero.
Both functions shall set tm_leapsecs to indicate the number of leap
seconds that have been applied to the resulting value (if tm_sec == 60,
the relevant leap second is *not* included in the count). If the
implementation is not aware of leap seconds, it shall set tm_leapsecs to
_NO_LEAP_SECONDS.
Both functions shall set tm_version to 1.
The /mktime/ function shall behave as follows. If the tm_isdst field is
equal to _EXTENDED_TM, then the tm_version field shall be 1. The broken
down time is normalized according to the following rules, and also
converted to a /time_t/ representation.
If the call is successful, a second call to /mktime/ with the resulting
/struct tm/ value shall always leave it unchanged and return the same
value as the first call.
If the call is successful and the normalized time is exactly representable
as a /time_t/ value, then the normalized broken-down time, and the
broken-down time generated by converting the result of /mktime/ as if by
a call to /localtime/, shall be identical except that, if the tm_isdst
member of the former originally had the value _EXTENDED_TM, it shall remain
unchanged.
A time is normalized according to the following rules. The principle
behind normalization is that the date is converted to a number of
seconds past some epoch, and then converted back to the correct
normalized form.
If the tm_isdst member does not equal _EXTENDED_TM, then the rules shall
be applied as if:
- tm_leapsecs is _NO_LEAP_SECONDS;
- tm_utcoffset is _LOCALTIME;
- tm_xisdst is -1, 0, or +60 according to whether tm_isdst is less than,
equal to, or greater than zero.
All dates are in the Gregorian calendar. Thus a value of -800 for
/tm_year/ represents 1100 CE, while a value of -2000 represents -100 CE
(99 BCE); neither are leap years, while -2300 (-400 CE, 399 BCE) is.
The value of /tm_leapsecs/ is the number of leap seconds applied (the
value of UTC-UT0) at the represented time. It should therefore be added
to the value determined by (days*86400 + hours*3600 + mins*60 +
seconds). If the value is _NO_LEAP_SECONDS, then the implementation
should determine the correct number if it can, and use 0 otherwise.
The value of /tm_utcoffset/ is a number of minutes to be added to the
time to convert it to UTC. The value _LOCALTIME is a request for the
implementation to determine this; if it is unknown, it should assume
that local time is UTC plus any DST offset determined from /tm_xisdst/.
If /tm_mon/ is outside the range [0, 11], it shall be converted to that
range by adding or subtracting a multiple of 12 and adjusting the year
accordingly. This shall then be used to determine the number of days in
the year prior to the month. Thus /tm_year/ == 97 and /tm_mon/ == -8
represents May of 1996, a leap year.
Apart from this, the final date can be determined simply by adding
together the various fields, each with a suitable weight, to get the
number of seconds past the epoch.
The normalization should be exact provided that there is no unreasonable
overflow. I would consider reasonable limitations to be that each of the
following expressions are in the range [-1<<30,+1<<30]:
tm_year * 366
tm_mon * 31
tm_mday
tm_hour * 3600
tm_min * 60
tm_sec
tm_leapsecs
tm_utcoffset * 60
tm_xisdst * 60 [if nonnegative, else tm_xisdst must be -1]
This would ensure that separate "seconds in the day" and "days since
epoch" calculations won't overflow in 32 bits.
====
Item 22:
Add the following alternate forms to the conversion specifiers in
7.15.3.5 (strftime()), taking effect when the # flag is included in the
specifier.
%#w - weekday (1=Monday, 7=Sunday)
%#W - ISO 8601 week number ) If %W would be zero, the date is
%#y - ISO 8601 week number year % 100 ) treated as belonging to week 53
%#Y - ISO 8601 week number year ) of the previous year
%#Z - timezone in "+0800" notation, with + being west of Greenwich.
====
Item 23:
In subclause 7.15.1, change the range of tm_sec to [0,60] and remove
footnote 201. See various WG14 mailing list items (e.g. 3482) or:
# The International Earth Rotation Service periodically uses leap seconds
# to keep UTC to within 0.9 s of TAI (atomic time); see
# Terry J Quinn, The BIPM and the accurate measure of time,
# Proc IEEE 79, 7 (July 1991), 894-905.
====
Item 24:
Subclause 7.15.3.5 (strftime()) is unclear on how the values of the
members of /timeptr/ affect the result, especially if they are outside
the normal range.
Add one of the following sets of wording:
EITHER:
If the value of any member of the structure pointed to by /timeptr/
is out of the normal range, or the values are not consistent with
one another [*], the behaviour is undefined.
[*] For example, the contents represent "30th Feb", "29th Feb 1997",
or "Monday 10th May 1997".
OR:
If the value of any member of the structure pointed to by /timeptr/
is out of the normal range, or the values are not consistent with
one another [*], the value returned and the contents of the array
are unspecified.
[*] For example, the contents represent "30th Feb", "29th Feb 1997",
or "Monday 10th May 1997".
OR:
The characters placed in the array by each conversion specifier depend
on a member of the structure pointed to by /timeptr/, as specified in
brackets in the description. If this value is outside the normal range,
the characters stored are unspecified.
and add the following to each specifier:
%a [tm_wday]
%A [tm_wday]
%b [tm_mon]
%B [tm_mon]
%c [all]
%d [tm_mday]
%H [tm_hour]
%I [tm_hour]
%j [tm_yday]
%m [tm_mon]
%M [tm_min]
%p [tm_hour]
%S [tm_sec]
%U [tm_year, tm_mday]
%w [tm_wday]
%W [tm_year, tm_mday]
%x [all]
%X [all]
%y [tm_year]
%Y [tm_year]
%Z [tm_isdst]
If item 19 is accepted, then %Z becomes [tm_utcoffset, tm_isdst, tm_xisdst].
====
Item 25:
All the conversion specifiers in subclause 7.15.3.5 (strftime()) should
have values specified for the C locale.
=======================================================================
END OF DOCUMENT