The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

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Sunday, Debtember 02, 2007

Multiple system calls

I finally found the answer to little conundrum. But first, a bit of a recap—the code for __getpid (the function that acually does the system call) that I presented:

0804e380 <__getpid>:
 804e380:       b8 14 00 00 00          mov    $0x14,%eax
 804e385:       cd 80                   int    $0x80
 804e387:       c3                      ret

That was generated from the output from objdump, and in order to get that, I had to compile the program using gcc -g -static -o t1 t1.o t1a.o, but note the -static bit there. Normally, such routines are part of a shared library that aren't included in the final executable, but by adding -static when compiling, the routines in the standard libraries are included in the final result. And had I run the statically compiled version, I would have seen that the code that calls the system supplied __getpid() would have taken 11 minutes to run.

What bit of code was I actually testing?

(gdb) disassemble __getpid
Dump of assembler code for function getpid:
0x00820730 <getpid+0>:  mov    %gs:0x4c,%edx
0x00820737 <getpid+7>:  test   %edx,%edx
0x00820739 <getpid+9>:  mov    %edx,%eax
0x0082073b <getpid+11>: jle    0x82073e <getpid+14>
0x0082073d <getpid+13>: ret    
0x0082073e <getpid+14>: jne    0x820752 <getpid+34>
0x00820740 <getpid+16>: mov    %gs:0x48,%eax
0x00820746 <getpid+22>: test   %eax,%eax
0x00820748 <getpid+24>: nop    
0x00820749 <getpid+25>: lea    0x0(%esi),%esi
0x00820750 <getpid+32>: jne    0x82073d <getpid+13>
0x00820752 <getpid+34>: mov    $0x14,%eax
0x00820757 <getpid+39>: call   *%gs:0x10
0x0082075e <getpid+46>: test   %edx,%edx
0x00820760 <getpid+48>: jne    0x82073d <getpid+13>
0x00820762 <getpid+50>: mov    %eax,%gs:0x48
0x00820768 <getpid+56>: ret    
0x00820769 <getpid+57>: nop    
0x0082076a <getpid+58>: nop    
0x0082076b <getpid+59>: nop    
0x0082076c <getpid+60>: nop    
0x0082076d <getpid+61>: nop    
0x0082076e <getpid+62>: nop    
0x0082076f <getpid+63>: nop    
End of assembler dump.

Ah! That makes more sense then!

It's basically checking to see if the getpid() system call has been made, and if not, call it once, then cache the value for later calls to this routine. You also won't notice an int $80 here, but that's because the shared library version of __getpid() uses a different method of making a system call than the traditional int $80, although the older method is still supported, which is why I suspect the static version of the system libraries use int $80 to make system calls—to support older systems that might not support the newer system call mechanism.


As long as they refrain from installing WiFi, it should be okay

For Mark, who may or may not find this horrifying (he used to work on car diagnostic software, so he's had more than enough exposure to automobile based networks): BMW testdriving IP as the underlying protocol for car computers (link via Instapundit).

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