The CL Corner: Alternatives to the CEE Date and Time APIs

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Have you tried using the C run-time date and time APIs?

 

In the two most recent articles of this column (here and here), we have looked at the CEE date and time APIs and seen how easy it is, when working with Lilian seconds, to perform operations such as adding six hours to a time, comparing two time values, etc. without regard to the number of seconds in a minute, the number of minutes in an hour, and so on. This capability is not unique to the CEE APIs. It also exists with other APIs that are provided standard with your i operating system. Today, we will look at one such alternative set of APIs, the C language run-time functions--functions that you can also use from ILE CL. As a reminder, the task we are working on is to send an alert from one program (CHKLSTSND in the original article) when another program (SNDUPD) has not successfully run in over six hours.

 

Similar to how the CEE APIs support time as simply a sequential number (using either days or seconds) starting from October 14, 1582, C APIs support time as a sequential number. In the case of C run-time, the sequential number represents the number of seconds since January 1, 1970, UTC. This value is typically defined as a 4-byte integer data type (TYPE(*INT) in CL), which is both good and bad. A "good" aspect is that CL natively supports 4-byte integers. This means we don't have to worry about converting CEE floating point values to decimal values as we did with the Copy Numeric Value (CPYNV) API in previous articles. A "bad" aspect is that a 4-byte integer cannot hold anywhere near the number of seconds that an 8-byte floating point value can. So, using default C run-time support, we cannot work with time values that are greater than Tuesday, January 19, 2038, at 03:14:07 (shades of Y2K!). We'll come back to this consideration later in the article.

 

UTC time is obtained by calling the Determine Current Time API "time" (note that the API name is lowercase and that case is important when using the C run-time APIs). The API has one output parameter where the current UTC time, in seconds since January 1, 1970, is stored. The API can also return the current UTC time as a return value (RTNVAL on the CALLPRC command) from the API call. The time API is documented in the ILE C/C++ Run-Time Library Functions manual. The parameter list for time is given below:

 

time_t time(time_t *timeptr);

If you have never worked with the C language, this parameter list may look rather strange. As the time API is a C run-time API, it is documented using a language-defined style, in this case what is known as a C prototype. There are more details on how to read this parameter list (or prototype) in my book IBM System i APIs at Work, Second Edition, but essentially the definition to the left of the API name (time_t in this case) is a return value from the API, and definitions to the right of the API name are parameters you pass to the API. The time API is expecting one parameter (named timeptr), which is a pointer (the * indicates that the data type is that of a pointer) to a data type of time_t. time_t is defined within the C language as a 4-byte signed integer. The following Send Update 3 program, SNDUPD3, shows how the time API is called and the resulting time then stored in the data area QGPL/LSTSNDTIM3. But prior to running SNDUPD3, the data area LSTSNDTIM3 needs to be created using this command:

 

CRTDTAARA DTAARA(QGPL/LSTSNDTIM3) TYPE(*CHAR) LEN(4)

Pgm                                                      

Dcl        Var(&Snd_Int)   Type(*Int)                    

Dcl        Var(&Snd_Char)  Type(*Char) Stg(*Defined) +   

             Len(4) DefVar(&Snd_Int)                     

                                                         

/* Send the updates and then:                         */ 

                                                         

CallPrc    Prc('time') Parm(&Snd_Int)                    

ChgDtaAra  DtaAra(QGPL/LSTSNDTIM3) Value(&Snd_Char)      

EndPgm                                                   

 

Some explanation of what is being done in the SNDUPD3 program is in order. We first define the 4-byte signed integer &Snd_Int, which will hold the current UTC time that the time API returns to us as a parameter. This variable corresponds to the time_t definition found in the time API's parameter list/prototype. As data areas cannot be defined as being an integer (only *DECIMAL, *CHAR, *LGL, and *DDM are supported) and we defined the data area LSTSNDTIM3 as *CHAR, SNDUPD3 also redefines the variable &Snd_Int as a 4-byte character field named &Snd_Char. This is necessary in order to successfully use the CHGDTAARA command later in the program to reflect the time of the last successful transmission. We could have also created LSTSNDTIM3 as a *DECIMAL data area, used CHGVAR to convert &Snd_Int to a *DECIMAL variable &Snd_Dec, and written &Snd_Dec to LSTSNDTIM3, but using this redefinition capability of CL works quite well and avoids having to run a CHGVAR command.

 

Note that the ability to redefine variables as we have done with &Snd_Int and &Snd_Char is a feature of CL only available starting with V5R4. If you are on a previous release, you should simply define one *CHAR variable with a length of 4 bytes. This variable would then be used with both the API call and the CHGDTAARA command.

 

With the data definitions out of the way, SNDUPD3 calls the time API, passing the parameter &Snd_Int. This parameter is being passed with the default *BYREF parameter-passing convention, which means the CL compiler will pass a pointer to &Snd_Int implicitly. This *BYREF behavior meets the needs of the time API--namely, that a pointer be passed to a 4-byte signed integer variable. As the time API returns the current UTC time in the &Snd_Int parameter, there is no need for us to specify a RTNVAL on the CALLPRC, so we don't.

 

Having called the time API, &Snd_Int is now updated with the number of seconds since January 1, 1970, UTC. The CHGDTAARA command writes this value (using the &Snd_Char definition) to the data area.

 

To compile SNDUPD3 on a V6R1 system, you can simply use this command:

 

CRTBNDCL PGM(SNDUPD3)

 

To compile SNDUPD3 on a previous release (or on V6R1 if you like performing extra steps), you need to follow a two-step process:

 

CRTCLMOD MODULE(SNDUPD3)

CRTPGM PGM(SNDUPD3) BNDDIR(QC2LE)

 

The following Check Last Send 3 program, CHKLSTSND3, demonstrates how to use the time value stored in LSTSNDTIM3 and determine if more than six hours have elapsed since the last successful update by SNDUPD3. CHKLSTSND3 does assume that the SNDUPD3 program has been run at least one time in order to initialize the LSTSNDTIM3 data area.

 

             Pgm                                                     

             Dcl        Var(&Snd_Int)   Type(*Int)                   

             Dcl        Var(&Snd_Char)  Type(*Char) Stg(*Defined) +  

                          Len(4) DefVar(&Snd_Int)                    

             Dcl        Var(&Cur_Int)    Type(*Int)                  

             Dcl        Var(&Alert_Time) Type(*Int)  Value(21600)    

             Dcl        Var(&Delay_Time) Type(*Dec)                  

             Dcl        Var(&Status)     Type(*Char) Len(1)          

                                                                     

 Loop:       RtvDtaAra  DtaAra(QGPL/LSTSNDTIM3) RtnVar(&Snd_Char)    

             CallPrc    Prc('time') Parm(&Cur_Int)                    

                                                                     

             If         Cond((&Cur_Int - &Snd_Int) > &Alert_Time) +  

                          Then(Do)                                   

                             SndPgmMsg Msg('Time to send alert') +   

                                ToPgmQ(*Ext)                         

                            ChgVar Var(&Delay_Time) Value(300)       

                            EndDo                                    

            Else       Cmd(ChgVar Var(&Delay_Time) +                 

                         Value(&Alert_Time - (&Cur_Int - &Snd_Int) + 

                         + 1))                                       

                                                                      

            RtvJobA    EndSts(&Status)                               

            If         Cond(&Status *NE '1') Then(Do)                

                       DlyJob Dly(&Delay_Time)                       

                       GoTo CmdLbl(Loop)                             

                       EndDo                                         

                                                                     

            EndPgm                                                    

 

CHKLSTSND3 basically reads the LSTSNDTIM3 data area by reversing the approach used in SNDUPD3. The program reads the data area into the variable &Snd_Char and then works with the value as a signed integer using the &Snd_Int definition. Processing-wise, you should see that the logic is similar to, but requires less work than, what we had to perform in CHKLSTSND when using the CEE APIs and working with floating-point time values.

 

To compile CHKLSTSND3, you would again use either the one-step or two-step process given previously for the SNDUPD3 program.

 

If you are on a release prior to V5R4 and are not able to use *DEFINED storage, then you should again define one *CHAR variable with a length of 4 bytes. Also define a separate *INT variable and then use the %bin built-in support of CL to convert the *CHAR variable value to the *INT variable:

 

Dcl        Var(&Snd_Char) Type(*Char) Len(4)       

Dcl        Var(&Snd_Int)  Type(*Int)               

ChgVar     Var(&Snd_Int) Value(%Bin(&Snd_Char))    

 

Similar to how we enhanced the CEE versions of SNDUPD and CHKLSTSND with the more operator-friendly programs SNDUPD2 and CHKLSTSND2, in the next article, we'll show how to make similar changes to SNDUPD3 and CHKLSTSND3.

 

Before we close, let's revisit the discussion of date ranges that we brought up earlier. As mentioned in the introduction, a 4-byte signed integer time_t data type cannot represent time values that exceed January 19, 2038, at 03:14:07 (similar if you will to how a two-digit number cannot exceed 99). The year 2038 may seem like a long time away, but as we learned with Y2K, there's no time like today to start worrying about how to format date and time values. In V6R1, IBM provides an enhanced version of the time API called time64. The time64 API works just like the time API but uses an 8-byte signed integer to store the number of seconds since January 1, 1970. This enhancement allows the API to support time values, still measured in seconds, since January 1, 1970, up to the year 9999. There are two considerations to keep in mind, though. One is that if you use the C run-time time APIs prior to V6R1, you should keep track of where the APIs are used and be prepared to change in the future to the "64" versions of the APIs. This change should be relatively painless. The second consideration is that ILE CL does not have direct support for 8-byte signed integers, so we're also back to using the Copy Numeric Value API approach that was used with the CEE APIs, where the number of seconds is stored as a floating point value. If you go with the time64 approach, the Copy Numeric Value format definition for an 8-byte signed integer would be x'00000800000000'.   

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