Ctime

ctime is a function in the C standard library, declared in time.h, that converts a calendar time expressed as a time_t value into a human-readable string. It takes a pointer to a time_t object and returns a pointer to a statically allocated C string containing the local time representation; the exact format is implementation-defined but is commonly similar to a date and time like "Www Mmm dd hh:mm:ss yyyy" followed by a newline. The returned string is null-terminated.

Because ctime uses a static internal buffer, successive calls overwrite the same memory, so the result is not thread-safe. If multiple threads need to format times concurrently, alternative functions or synchronization are required. Portable programs that require thread safety can use ctime_r on POSIX systems, which requires the caller to provide a buffer, or ctime_s in Microsoft environments, which writes into a caller-provided buffer and reports errors.

Usage typically involves obtaining a time_t value (for example, by calling time) and then passing its address to ctime to obtain a string for display. The output reflects the local time zone setting of the running environment, rather than Coordinated Universal Time (UTC). For UTC formatting, other facilities such as gmtime along with asctime or strftime can be used.

ctimes are part of the broader family of time formatting functions, which also include asctime, localtime, gmtime, and strftime. These functions together enable conversion of time values to human-friendly representations and formatted strings suitable for display, storage, or logging.