Re: __error in Scott Klement's e-Book -- RPG400-L


On 13/07/2006, at 2:47 PM, Mike Pantzopoulos - ((H/O)) wrote:

I was lulled into a false sense of security because the other APIS
(access, close, write etc.) didn't require that binding directory whenI
was progressively building up the code.

The Unix APIs are found via the QUSAPIBD binding directory which allILE compilers link to automatically.


So how do I know which APIs require which binding directory, or if they
require a binding directory at all?

Experience. All bindable APIs require binding by either specifying theservice program containing the API or the binding directory naming theservice program. The API documentation generally tells you whichservice program exports the function if it is bindable. The Unix APIsare a little different and don't provide that information. However, youcan find it if you examine the *BNDDIR objects and service programexports.

And why isn't the __errno function mentioned in the manual?

Because __errno is the internal function called to populate the C errnovariable. errno is documented in the C run time. When you prototype a Cfunction you are expected to find the C definition in the appropriate Hfile and map it to the RPGLE equivalent. In this case you would seefrom the C manual that errno is defined in the errno.h include. Thenyou'd look in the ERRNO member in QSYSINC/H to find the definition.There you'd see:


        #define errno (*__errno())

which tells the C pre-compiler to replace all occurrences of errno inthe source with (*__errno()) which is simply C-speak for the valuereturned by the __errno() function. I'll hold my tongue regarding theobviousness of this syntax. So now that you know it calls a functionyou need to find the function definition. Another look though errno.hwill find:


        extern int *__errno(void);

which simply means an external function that returns a pointer to aninteger and accepts no parameters is being defined. Converting that toRPG IV gives:


        D errno         PI                                      *       
extproc('__errno')

C allows you to define the pointer type of the return value on thedefinition (the int * bit) but RPG IV doesn't so you need to define abased integer to access the result.


        D errno         S                               10I 0   based(@errno)

It should be obvious that you can't have a function and a variable bothcalled errno--at least not until RPG IV gives us separate namespaces--so we need to change one of them. Because C code simply does:


        if (errno == EWOULDBLOCK )

sort of thing it would be nice to maintain similar coding style in RPGIV so we change the name of the function to something like getErrnogiving:


        D errno         S                                 10I 0 based(@errno)
        D getErrno      PI                                      *       
extproc('__errno')

In C you just test errno because the compiler sorts out calling thefunction automatically. In RPG IV you have to call getErrno() first toset the address of the errno variable before you can test it. There isa rumour to the effect that you need to do this each time you checkerrno (which is also what happens in C) but I've never had a problemwith just calling getErrno() in my RPG initialisation routine and thenjust reference errno as I need to throughout the program.

Note that the IBM examples in the C programmer's guide are flawed. Theydo stuff like:


        errno = 0;
        result = function();
        if (errno != 0 )
        {
                perror("Error occured.");
                exit(1);
        }

Even though they set errno to 0 before the function call it is betterpractice to test the result of the function and only reference errnoonce you know the function failed. That is never set it and only testit, thus:


        result = function();
        if (result == NULL )
        {
                switch ( errno )
                {
                        case EWOULDBLOCK:
                                /* Do something about it */
                                break;
                        default:
                                perror("function()");
                                exit(1);
                }
        }

Of course, in both case you need to know that the function setserrno--not all of them do. You also need to know the correct value forfailure from the function (usually NULL if a pointer or -1 if aninteger). The function documentation should tell you both things.



Regards,
Simon Coulter.
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