25.1 The External Environment

25.1.1 Top level loop

The top level loop is the Common Lisp mechanism by which the user normally interacts with the Common Lisp system. This loop is sometimes referred to as the Lisp read-eval-print loop because it typically consists of an endless loop that reads an expression, evaluates it and prints the results.

The top level loop is not completely specified; thus the user interface is implementation-defined. The top level loop prints all values resulting from the evaluation of a form. Figure 25–1 lists variables that are maintained by the Lisp read-eval-print loop.

* + / -
** ++ //
*** +++ ///
Figure 25–1. Variables maintained by the Read-Eval-Print Loop

25.1.2 Debugging Utilities

Figure 25–2 shows defined names relating to debugging.

25.1.3 Environment Inquiry

Environment inquiry defined names provide information about the hardware and software configuration on which a Common Lisp program is being executed.

Figure 25–3 shows defined names relating to environment inquiry.

25.1.4 Time

Time is represented in four different ways in Common Lisp: decoded time, universal time, internal time, and seconds. Decoded time and universal time are used primarily to represent calendar time, and are precise only to one second. Internal time is used primarily to represent measurements of computer time (such as run time) and is precise to some implementation-dependent fraction of a second called an internal time unit, as specified by internal-time-units-per-second. An internal time can be used for either absolute and relative time measurements. Both a universal time and a decoded time can be used only for absolute time measurements. In the case of one function, sleep, time intervals are represented as a non-negative real number of seconds.

Figure 25–4 shows defined names relating to time.

Figure 25–4. Defined names involving Time. Decoded Time

A decoded time is an ordered series of nine values that, taken together, represent a point in calendar time (ignoring leap seconds):


An integer between 0 and 59, inclusive.


An integer between 0 and 59, inclusive.


An integer between 0 and 23, inclusive.


An integer between 1 and 31, inclusive (the upper limit actually depends on the month and year, of course).


An integer between 1 and 12, inclusive; 1 means January, 2 means February, and so on; 12 means December.


An integer indicating the year A.D. However, if this integer is between 0 and 99, the “obvious” year is used; more precisely, that year is assumed that is equal to the integer modulo 100 and within fifty years of the current year (inclusive backwards and exclusive forwards). Thus, in the year 1978, year 28 is 1928 but year 27 is 2027. (Functions that return time in this format always return a full year number.)

Day of week

An integer between 0 and 6, inclusive; 0 means Monday, 1 means Tuesday, and so on; 6 means Sunday.

Daylight saving time flag

A generalized boolean that, if true, indicates that daylight saving time is in effect.

Time zone

A time zone.

Figure 25–5 shows defined names relating to decoded time.

Figure 25–5. Defined names involving time in Decoded Time. Universal Time

Universal time is an absolute time represented as a single non-negative integer — the number of seconds since midnight, January 1, 1900 GMT (ignoring leap seconds). Thus the time 1 is 00:00:01 (that is, 12:00:01 a.m.) on January 1, 1900 GMT. Similarly, the time 2398291201 corresponds to time 00:00:01 on January 1, 1976 GMT. Recall that the year 1900 was not a leap year; for the purposes of Common Lisp, a year is a leap year if and only if its number is divisible by 4, except that years divisible by 100 are not leap years, except that years divisible by 400 are leap years. Therefore the year 2000 will be a leap year. Because universal time must be a non-negative integer, times before the base time of midnight, January 1, 1900 GMT cannot be processed by Common Lisp.

Figure 25–6. Defined names involving time in Universal Time. Internal Time

Internal time represents time as a single integer, in terms of an implementation-dependent unit called an internal time unit. Relative time is measured as a number of these units. Absolute time is relative to an arbitrary time base.

Figure 25–7 shows defined names related to internal time.

Figure 25–7. Defined names involving time in Internal Time. Seconds

One function, sleep, takes its argument as a non-negative real number of seconds. Informally, it may be useful to think of this as a relative universal time, but it differs in one important way: universal times are always non-negative integers, whereas the argument to sleep can be any kind of non-negative real, in order to allow for the possibility of fractional seconds.

Figure 25–8. Defined names involving time in Seconds.