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10.179 ctime

POSIX specification:

Gnulib module: ctime

Portability problems fixed by Gnulib:

Portability problems not fixed by Gnulib:

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.

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