10.179 ctime

POSIX specification:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/ctime.html

Gnulib module: ctime

Portability problems fixed by Gnulib:

• On native Windows platforms (mingw, MSVC), this function works incorrectly when the environment variable TZ has been set by Cygwin.

Portability problems not fixed by Gnulib:

• This function is deprecated in C23. Likewise, POSIX says this function is obsolescent and it is planned to be removed in a future version. Portable applications can use localtime_r and strftime (or even sprintf) instead. However, localtime_r can fail and strftime is locale dependent.
• This function may overflow its internal buffer if its argument specifies a time before the year 1000 or after the year 9999.
• This function may dereference an internal null pointer if its argument specifies a time before the year INT_MIN+1900 or after the year INT_MAX+1900.
• The ctime function need not be reentrant, and consequently is not required to be thread safe. Implementations of ctime typically write the timestamp into static buffer. If two threads call ctime at roughly the same time, you might end up with the wrong date in one of the threads, or some undefined string.
• Native Windows platforms (mingw, MSVC) support only a subset of time zones supported by GNU or specified by POSIX. See tzset.

Although ctime and related functions asctime, asctime_r and ctime_r formerly were plausible to use, they are now unsafe in general, and should be avoided.

Decades ago when time_t was narrow and there was no strftime or internationalization, code could call these functions and then select the parts needed. For example, in Unix 7th Edition /usr/src/cmd/ls.c (1979):

cp = ctime(&p->lmtime);
if(p->lmtime < year)
        printf(" %-7.7s %-4.4s ", cp+4, cp+20); else
        printf(" %-12.12s ", cp+4);

This had well-defined behavior when time_t was only 32 bits and so was OK for circa 1979 platforms.

However, today’s platforms have a time_t so wide that the year might not be in the range [1000, 9999]. In this case the behavior of ctime is undefined and some platforms behave badly, overrunning a buffer or dereferencing an internal null pointer; and even on platforms where no undefined behavior occurs, the 7th Edition code generates wrong output for out-of-range years, because it incorrectly assumes that every year is represented by exactly four digits.