IBM AS/400 manual Sample Service Program, Api

Sample Service Program

Sample Service Program

The following example shows how to create a service program CVTTOHEX whi converts character strings to their hexadecimal equivalent. Two paramete passed to the service program:

1. a character field (InString) to be converted

2.a character field (HexString) which will contain the 2-byte hexade alent

The field

HexString is used to contain

the

result

of

the

conversion

an

cate

the

length

of

the

string

to

be

converted. For example, if

a

ch

30 characters is passed, but

you

are only

interested

in

converting

th

would

pass

a

second

parameter

of 20 bytes (2 times

10). Based on t

the passed fields, the service program determines

the

length

to

handle

Figure 42

on

page 95

shows

the

source

for the service

program.

Figure

page 97

shows

the

/COPY member

containing

the

prototype

for

CvtToHex.

The basic logic of the procedure contained within the service program below:

1.Operational descriptors are used to determine the length of the eters.

2.The length to be converted is determined: it is the lesser of th

character string, or one-half the length of the hex string field.

3.Each character in the string is converted to a two-byte hexadeci using the subroutine GetHex.

Note	that GetHex		is coded		as	a subroutine rather than a subproce
to improve run-time performance.						An EXSR operation runs much faster tha
bound	call, and	in	this	example,		GetHex is called many times.
4. The	procedure	returns		to	its	caller.

The service program makes use of operational descriptors, which is an struct used when the precise nature of a passed parameter is not kn time, in this case the length. The operational descriptors are create

procedure

when

you

specify

the

operation

extender

(D)

on the

CALLB

o

when

OPDESC

is

specified

on

the

prototype.

To use the

operational

descriptors, the service program

must

call

the

API,

CEEDOD

(Retrieve Operational Descriptor). This API

requires certain

ters

which

must

be

defined for the CALLB

operation.

However,

it

is

t

eter which provides the information needed, namely, the length. For more information on operational descriptors, see “Using Operational Descriptors page 138.

94 ILE RPG for AS/400 Programmer's Guide