I was trying to track down a problem in Java where shared memory was not being accessed as it should. This was too early in the JVM startup for trace – so how to debug it?
One option I tried was using a cuckoo function. A cuckoo is well know for laying an egg in another bird’s nest; the nest owner then treats the cuckoo’s egg as its own. I inserted some code between Java and a C function, and Java called my code.
The Java code uses the C run time function shmget() — Get a shared memory segment. I wanted to write a stub function to display parameters, and then call the system version of shmget.
To do this I had to get the Java program to use my cuckoo version of shmget.
My program
void wto( char * msg) {
// time_t ltime;
// time(<ime);
//printf(" the time is %s", ctime(<ime));
struct __cons_msg cmsg;
int rc;
int cmsg_cmd = 0;
/* fill in the __cons_msg structure */
cmsg.__format.__f1.__msg = msg;
cmsg.__format.__f1.__msg_length = strlen(msg);
rc = __console(&cmsg,NULL,&cmsg_cmd);
}
// this has the same signature as the run time code
cpshmget(
key_t key, size_t size, int shmflg)
{
char buffer[200];
int i, j;
j = sprintf(buffer, ">shmget key %8.8x sz %i flg 0x%8.8x\0", key,size,shmflg);
wto(buffer);
int myresult = shmget(key,size,shmflg);
//sprintf(buffer,"<shmget1 r %i e %i e2 0x%8.8x\0",myresult, errno, __errno2());
//wto(buffer);
//myresult = shmget(key,size,shmflg);
return myresult;
}
I compiled it using a shell script (I found it easier than a make file)
name2=cp
p1=" -DNDEBUG -O3 -qarch=10 -qlanglvl=extc99 -q64"
p2="-Wc,DLL -D_XOPEN_SOURCE_EXTENDED -D_POSIX_THREADS"
p2="-D_XOPEN_SOURCE_EXTENDED -D_POSIX_THREADS"
p3="-D_OPEN_SYS_FILE_EXT -qstrict "
p4="-Wa,asa,goff -qgonumber -qenum=int"
p5="-I."
p6=""
p7="-Wc,ASM,SHOWINC,ASMLIB(//'SYS1.MACLIB') "
p8="-Wc,LIST(c.lst),SOURCE,NOWARN64,XREF -Wa,LIST,RENT"
/bin/xlc $p1 $p2 $p3 $p4 $p5 $p6 $p7 $p8 -c $name2.c -o $name2.o
It is used as an object .o code – not as an executable module.
Bind it to the Java code
I wanted to cuckoo the module libj9prt29.so .
I saved a copy of the original module libj9prt29.so as libj9prt29.so.orig. I could then include this into the binder, and not worry about any of my old code staying around.
When I needed “a clean run”, without any of my code, I copied libj9prt29.so.orig over libj9prt29.so .
The bind job
//IBMBIND JOB 1,MSGCLASS=H
// SET BPARM='CASE=MIXED,SIZE=(900K,124K),XREF,RMODE=ANY,CALL=YES'
// SET CPARM='MAP,DYNAM=DLL,LIST=ALL,AMODE=64,TERM=YES'
//BIND EXEC PGM=IEWL,REGION=0M,PARM='&CPARM,&BPARM'
//SYSLIB DD DSN=CEE.SCEEBND2,DISP=SHR
// DD DSN=CBC.SCCNOBJ,DISP=SHR
// DD DSN=SYS1.CSSLIB,DISP=SHR
//Z8921 DD DSN=CEE.SCEELIB(CELQS003),DISP=SHR
//SYSLMOD DD PATH='/usr/lpp/java/J17.0_64/lib/default/',
// PATHDISP=(KEEP,KEEP)
//SYSPRINT DD SYSOUT=*
//SYSLIN DD *
ORDER CELQSTRT
ENTRY CELQSTRT
INCLUDE /u/tmp/java/cp.o
change shmget(cpshmget)
INCLUDE /usr/lpp/java/J17.0_64/lib/default/libj9prt29.so.orig
include Z8921
name libj9prt29.so
//
Notes:
- You specify SYSLMOD as a directory and specify the “name” later. This was not obvious from the documentation
- INCLUDE /u/tmp/java/cp.o copies in my code
- change shmget(cpshmget) says any code which is included after this, cuckoo the entry point shmget to point to cpshmget
- INCLUDE /usr/lpp/java/J17.0_64/lib/default/libj9prt29.so.orig include the original module
- include Z8921 = CEE.SCEELIB(CELQS003) contains statements like IMPORT CODE64,CELQV003,’shmget’,280 which defines where shmget is really defined
- name libj9prt29.so save it as this name in //SYSLMOD
This worked well – but…
I originally had
wto(buffer);
int myresult = shmget(key,size,shmflg);
sprintf(buffer,"<shmget1 r %i e %i e2 0x%8.8x\0",myresult, errno, __errno2());
wto(buffer);
//myresult = shmget(key,size,shmflg);
But the wto function, which invokes __console(), sets errno and __errno2. The values passed back to Java were the values from the __console function, not the shmget function.
Issuing the myresult = shmget(key,size,shmflg); a second time set the errno and __errno2 – but if the request was to create, or delete, issuing it a second time gave a different error code. For example if “delete” was specified – the first time worked, the second time gave “not found”, and this confused the Java program calling this function.
ColinPaice 