The asctime function is defined in the time.h header file in C programming. It converts the time structure pointed to by timeptr into a string of the form "Sun Sep 16 01:03:52 1973\n". The returned string contains the abbreviated name of the day of the week, followed by the abbreviated name of the month, followed by the day of the month, followed by the time in the format "hh:mm:ss", followed by the year.

Here is the syntax for the asctime function:

char *asctime(const struct tm *timeptr);

The timeptr parameter is a pointer to a tm structure that contains the date and time information. The function returns a pointer to a null-terminated string that represents the date and time as described above. The string is statically allocated, so it should not be modified or freed by the caller.