time(3) - glibc (time_t, its width, and the 2038 problem)

The simple calendar-time type and the function that reads the clock (manual chapter Date and Time, nodes Time Basics, Time Types, Getting the Time). Higher-resolution clocks: gettimeofday-settimeofday and clock_gettime(2).

Declaration and gates

time_t time(time_t *result);

Semantics

POSIX defines time_t as an integer counting seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds; ISO C only promises an arithmetic type whose values have no mandated meaning, and the manual points at difftime as the only portable arithmetic (see mktime-difftime). time stores the same value in *result when that is not NULL. The manual: glibc guarantees time_t is signed and all functions behave on negative values; it uses the same clock as clock_gettime(CLOCK_REALTIME_COARSE) where available, else CLOCK_REALTIME.

Gotchas and portability (width and Y2038)

• Width is a compile-time contract: POSIX.1-2024 requires time_t to be at least 64 bits; on glibc it “is 64 bits wide on all platforms … except that it is 32 bits wide on a few older platforms unless you define _TIME_BITS to 64” (manual, Time Types).
• _TIME_BITS=64 needs _FILE_OFFSET_BITS=64 too, uses 64-bit-time syscalls on kernels above 5.1, and otherwise falls back to legacy 32-bit syscalls (__USE_TIME64_REDIRECTS renames declarations like clock_gettime to __clock_gettime64) (manual, Feature Test Macros, _TIME_BITS).
• The manual is explicit: 32-bit time_t “stops working in the year 2038”; that is the Y2038 boundary.
• musl contrast: include/alltypes.h.in line 12 is TYPEDEF _Int64 time_t;, so musl 1.2.5 builds a 64-bit time_t on every architecture; delta tracked in the musl catalog.

References

• glibc manual nodes Time Basics, Time Types, Getting the Time; man3 time_t.3 (alias page for system_data_types.7, not cached locally).
• Header time.h; nm -D --with-symbol-versions on libc.so.6 (glibc 2.41); glibc NEWS 2.40 for the utmp/lastlog unsigned switch.