I often recommend using the C language runtime library functions in RPG IV as a supplement to the built-in functions and opcodes that are already part of the RPG IV language. This week, I want to introduce you to the C language's date and time functions. But first, let me review how you interface with the C language from RPG IV.

QC2LE Binding Directory

A special binding directory on every iSeries/i5 system contains the C language runtime library service programs. Most of you have probably seen or heard of it before, and many have perhaps used it. That binding directory is QC2LE, and it contains a list of service programs needed to allow the C language procedures to be called from RPG IV. The BNDDIR(QC2LE) keyword on the CRTBNDRPG and CRTPGM commands is all you need to use to include C runtime procedures in your RPG IV program. QC2LE is not an RPG IV-specific binding directory, however, and may be used by all ILE-targeted languages: CL, COBOL, C, or C++. But the C language implicitly uses QC2LE, so no BNDDIR parameter is required.

The C language has several interesting ways of evoking a procedure call. Fortunately, one of those methods allows the procedure to be called via a pointer to a procedure. IBM has exposed those functions in service programs. This is where the QC2LE binding directory comes in.

Since the C runtime library functions are exported from service programs, binding to them is as easy as writing an RPG IV procedure prototype. In fact, it is so easy to interface RPG IV with C that I would suggest that the RPG IV language supports every procedure the C language has, but since RPG IV has its own set of instructions, it is a better language than C. Take that, you C bigots!

In previous articles, I've illustrated how to convert character values to numeric using C runtime library functions atoll() and atoi(). This allows you to convert a number that is in text format (stored in a character field) into a true numeric value and then store that value in a numeric field. In other words, you can call a routine to do what the MOVE opcode already does. The benefit is that you can call atoll() from within an expression. Oh yes, and MOVE is not supported in the /free specification, so you'd have to switch to atoll() or atoi() to accomplish same thing.

Here's the prototype for the atoll() function:

Use the atoll() function as follows:

This technique can be used on every release since V3R7, so there's no confusion about which releases support this capability (as there is with %INT and %DEC).

The QC2LE binding directory should be inserted into all source members that call any C language routine. I make it a habit to include it on my source member's Header specifications, for example:

Remember that object names, such as the binding directory names above, must be in all uppercase letters; otherwise, the compiler will not find them.

Additional Date/Time Functions from C

RPG IV's support for date and time data types is pretty good. But while the traditional operation codes are stable and well known (ADDDUR, SUBDUR, EXTRCT, and TIME) they do not offer more than basic functionality.

There are C language runtime functions that do more than the traditional RPG IV operation codes. Some of the C functions that are useful include asctime, ctime, time, localtime, mktime, and strftime. While there are other date/time functions in the C compiler, the most commonly used date/time functions are summarized in the following table:

ASCTIME--Get Date/Time as Text

The asctime() procedure returns the character string form of the date and time value stored in a TM data structure. Here's the prototype:

The first parameter is a populated TM data structure value. The TM data structure must be initialized to the desired date and time before calling asctime(). Use time() and localtime() functions to initialize the TM structure, or initialize it yourself before calling this function.

The returned value is a pointer to a character string. The character string contains the date and time formatted for the local system. Use the %STR built-in function to copy the value to a character field. The format of the return value is similar to the following:

CTIME--Get Date/Time as Text

The ctime() procedure returns the character string form of the date and time value stored in the TIME_T parameter value. The TIME_T structure is nothing more than a data structure that contains a single Int4 value. Here's the prototype:

The first parameter is the TIME_T structure. This is a character data structure with a single 10i0 (4-byte integer) subfield that contains the date/time value in seconds. Use the time() procedure to retrieve the current TIME_T value.

The returned value is a pointer to a character string. The character string contains the date and time formatted for the local system. Use the %STR built-in function to copy the value to a character field. The format of the return value is similar to the following:

TIME--Get Current System Time in a TIME_T Structure

The time() function returns the current system clock time to a TIME_T structure. Here's the prototype:

The first parameter is optional. If specified, it will be set to the same time value as the return value. This allows the function to be called via CALLP or used in an expression. In both cases, the returned value is a TIME_T structure.

LOCALTIME()--Initialize TM Structure with Current Date/Time

The localtime() function converts a TIME_T value (if specified) to a TM structure that can be subsequently used by other C language date/time functions. Here's the prototype:

The first parameter is TIME_T structure. This is a character data structure with a single 10i0 (4-byte integer) subfield that contains the date/time value in seconds. This value is converted into a TM structure. Use this function with the time() procedure to retrieve the current time. For example, time(localtime()) returns a TIME_T structure for the current time.

The return value is a pointer to a TM structure. I often use a based data structure to reference the retuned data. For example:

Use this procedure to translate a TIME_T structure to a TM structure value that can be used by the asctime and mktime procedures.

MKTIME--Convert a TM Structure into a TIME_T Structure

The mktime() function is the complement of the LOCALTIME function. It converts a TM structure into a TIME_T value. Here's the prototype:

The first parameter is an initialized TM structure. The value in the TM structure is converted into a TIME_T integer value and returned.

Several data structures are used as templates for parameters, return values, and other data structures. These templates and data structures are illustrated below.

DIFFTIME--Calculate Difference Between to TIME_T Values

The difftime() procedure returns the number of seconds between the two TIME_T parameters. Here's the prototype:

The first parameter is a TIME_T structure that identifies the ending time. The second parameter is a TIME_T structure that identifies the starting time.

The returned value is a double (8F) value that identifies the number of seconds between the two values. To convert this to a more usable value, use the 4-byte binary (10i0) field referencing value:

The INT4 field is used to allow a 10i0 (4-byte binary) field to be declared "correctly" but with a more precise identification. It is used throughout the examples in this article.

TIME_T Data Structure Template

The TIME_T data structure contains the date and time in seconds.

TM Data Structure Template

Each subfield of this data structure is a 10i0 value. But since 10i0 is not widely recognized at this point, I've used a field reference to declare each subfield as an INT4 value. The reference field INT4 is declared as a 10i0 value.

The subfields of the TM data structure are described in more detail in the table that follows. Most of these subfields are zero-based, meaning that their values start at zero instead of 1. For example, to specify the month of the year, instead of 1 through 12, you specify 0 through 11 (0 for January, 11 for December).

Time Function Summary

Use the time() function to get the current system time into a TIME_T structure. This returns the time in seconds.

Use localtime() to convert a TIME_T structure to a TM structure. Omit the initial time parameter to retrieve the current clock time or use localtime(time()) to get the current system time in a TM structure.

So why do you care about these functions? Well, have you ever needed to figure out the number of years, months, days, hours, minutes, and seconds between two values (or a subset of same)? If so, the traditional RPG IV SUBDUR operation just doesn't cut it. With SUBDUR, you can only get the total number of minutes or seconds or days, etc. But you really want to find out if it has been 2 days, 3 hours, 15 minutes, and 20 seconds. With the difftime() function, you can!

Here's a simple example, in RPG IV, that illustrates how to populate the TM data structure with the number of years, months, days, etc. between two time values.

The first Calculation specification retrieves the current time and stores it in a TIME_T variable. After this routine runs, the MYTM data structure is populated with the duration between those two dates, broken down. We can take it further by using SUBDUR on a regular date or date/timestamp value to calculate the number of seconds between two dates; we can then move that result to the nDiff variable and call LOCALTIME.

Bob Cozzi is a programmer/consultant, writer/author, and software developer. His popular RPG xTools add-on subprocedure library for RPG IV is fast becoming a standard with RPG developers. His book The Modern RPG Language has been the most widely used RPG programming book for more than a decade. He, along with others, speaks at and produces the highly popular RPG World conference for RPG programmers.