System API's are not so easy to use as you would expect.
In many ILE-RPG programs you can find examples of calls to system API's and you may take advantage looking at how that was done.

Hereafter we provide a Where Used table of API's in MMAIL/QREPGLESRC source members:
(To display source members you need to install MMAIL)

 

Where Used of API's in MMAIL

API	MMAIL/QRPGLESRC source members
CDRCVRT, QTQCVRT Convert a Graphic Character String	YYYASCIEBC
QCDRCMDI Retrieve Command Information	CRTMMLIB2
QDCXLATE Convert Data	YYYQPRDEC	YYYSOCKET
QDMRTVFO Retrieve File Override Information	YYYPROC1
QDBRTVFD Retrieve Database File Description	XXXRTVDBFD
QLGRTVDC Retrieve Default CCSID	POP3PSSL	XXXCCSID
QGYCLST Close List	WRKWTR
QGYOLMSG Open list of non program messages	WRKWTR
QGYRPRTL Open List of Printers	WRKWTR
QMHRTVM Retrieve Message	CBX801	CBX8012V	XXXMIME
QMHSNDPM Send Program Message	AMHEADER	CVTSPLFPDF	EMLHTML2	EMLMSG	EMLPTUMSG
	EMLSAVF	EMLSMSIT1	EMLSMS1	EMLSPL	EMLSPLF
	EMLSPLF2	EMLSPLX	EMLSPL2	EMLSPL3	EMLSRCM
	EMLSTMF	EMLSTMF2	EMLTXTSTMF	MRGJOBSPLF	PRCUSRMAIL
	STMFCVT	XXXMIME	XXXSMS	XXXSTGIFS	YYYPROPERT
	ZIP
QMHSNDRM Send Reply Message	WRKWTR
QOKSCHD Search System Directory	XXXRTVDIRE
QSNDDTAQ Send Data Queue	RUNQSHCMD
QSPGETSP Get Spooled File Data	SPL2STMFRP
QSPROUTQ Retrieve Output Queue Information	WRKSPLF
QSPRWTRI Retrieve Writer Information	WRKWTR
QSYADVLE Add Validation List Entry	WRKVLDLE
QSYCHVLE Change Validation List Entry	WRKVLDLE
QSYCUSRS Check User Special Authorities	CBX8013
QSYFDVLE Find Validation List Entry	WRKVLDLE
QSYOLVLE Open List of Validation List Entries	WRKVLDLE
QSYRMVLE Remove Validation List Entry	WRKVLDLE
QSYRUSRI Retrieve User Information	XXXUSRPRF
QtmmSendMail Send MIME Mail	XXXMAILSND
QtmsCreateSendMail Create and Send MIME E-mail	EMLNOTE1
QUIADDLE Add List Entry	CBX801
QUIADDPA Add Print Application	CBX801E
QUICLOA Close Application	CBX801
QUIDLTL Delete List	CBX801
QUIGETLE Get List Entry	CBX801	CBX801E
QUIGETV Get Dialog Variable	CBX801	CBX801E
QUILNGTX Display Long Text	CBX801E
QUIOPNDA Open Display Application	CBX801
QUIOPNPA Open Print Application	CBX801
QUIPRTP Print Panel	CBX801	CBX801E
QUIPUTV Put Dialog Variable	CBX801E
QUIRMVLE Remove List Entry	CBX801E
QUIRMVPA Remove Print Application	CBX801E
QUIRTVLA Retrieve List Attributes	CBX801
QUISETLA Set List Attributes	CBX801	CBX801E
QUIUPDLE Update List Entry	CBX801E
QUSCRTUS Create User Space	CHGJBCCSID	MNGSPLF1	MRGJOBSPLF
QUSLJOB List Job	CHGJBCCSID
QUSLSPL List spooled files	MNGSPLF1	MRGJOBSPLF
QUSPTRUS Retrieve Pointer to User Space	SPL2STMFRP
QUSRJOBI Retrieve Job Information	F_TO_STMF	CBX801	CBX812X	POP3SSL	XXXMIME
QUSRMBRD Retrieve Member Description	XXXRTVMBRD
QUSRMVEP, QusRemoveExitProgram Remove Exit Program	CBX8013
QUSROBJD Retrieve Object Description	CBX8012V	XXXMIME
QUSRSPLA Retrieve Spooled File Attributes	EMLSPLF	EMLSPLF2	SPLA	XXXRTVSPLA
QUSRTVEI, QusRetrieveExitInformation Retrieve Exit Information	CBX801
QUSRTVUS Retrieve User Space	CHGJBCCSID	MRGJOBSPLF
QWCRNETA Retrieve Network Attributes	XXXMIME
QWCRSVAL Retrieve System Values	XXXMIME
QWCRTVTZ Retrieve Time Zone Description	XXXDATE822
QWVRCSTK Retrieve Call Stack	XXXWRKHTML
iconv() Code conversion	XXXCVTSTG	YYYCVT	YYYUTF8CV1	YYYUTF8CV2
QsyFindFirstValidationLstEntry Find First Validation List Entry	GETVLDLE
QsyFindNextValidationLstEntry Find Next Validation List Entry	GETVLDLE
QwpzHostPrintTransform Host Print Transform	SPL2STMFRP