Why are NOT the function parametters in the stack. They are in the register!

MCG LU Fengcheng
2005-11-02T06:11:14Z

Hello all

In my program, I have a confused problem. Any helps are welcome!

I have the following function with variable parameters
void PrintTrace(char* fileName, int lineNo, char* traceType, char* fmt,...)
{
va_list args;
char buf[MX_SIZE];

...........................................................
va_start(args,fmt);
vsnprintf(buf,MX_SIZE,fmt,args);
va_end(args);
..............................................................
}

Sometimes, program crash when PrintTrace is called. I use the gdb to analyze the problem.

root@aaa-n3s30#gdb a.out core.1462
GNU gdb 6.1.1
.......................................
Core was generated by `/home/lfc/a.out'.
Program terminated with signal 11, Segmentation fault.
.....................................................
#0 0x4013641b in vfprintf () from /lib/libc.so.6

Then I run the bt, get the output:
(gdb) bt
#0 0x4013641b in vfprintf () from /lib/libc.so.6
#1 0x40154b7c in vsnprintf () from /lib/libc.so.6
#2 0x40079316 in PrintTrace (fileName=0x400a07a0 "/home/lfc/test.c", lineNo=340, traceType=0x0, fmt=0x0) at test.c:248
#3 0x400934d2 in CmwInit (vceId=1003) at /home/lfc/test.c:340
#4 0x08049281 in main (argc=1, argv=0xbffff954) at test.c:463

In the PrintTrace, traceType and fmt are both NULL. It's impossible. I always give the valid pointer for the traceType and fmt.

I run f 2 and try to print the traceType and fmt address.
(gdb) f 2
#2 0x40079316 in PrintTrace (fileName=0x400a07a0 "/home/lfc/test.c", lineNo=340, traceType=0x0, fmt=0x0) at test.c:248
(gdb) p &traceType
Address requested for identifier "traceType" which is in register $eax
(gdb) p &fmt
Address requested for identifier "fmt" which is in register $eax

gdb complains fmt and traceType are in the register eax. Why not in the stack. In fact, lineNo and filename are in the stack. Gdb can print their address out.

I check the eax value. It is zero. I also check the stack. I found the fmt and traceType address were in the stack.

It's the gdb issue? I can't understand it.

Help me!!!!!!!!!!!!!!!!!!

B.R.
Lu