Re: probing a plt

[Stan Cox <scox at redhat dot com>]

On 09/02/2011 02:35 PM, Stan Cox wrote:
> I would expect the __libc_start_main probe to have appeared first and
> the printf probe to have been hit twice

I'm seeing this puzzling behavior for this simple program:
  char *abc = "abcdefghijklmnopqrstuvwxyz";
  char *xyz = "zyxwvutsrqpomnlkjihgfedcba";
  int x;
  struct timeval tp;
  struct timezone tzp;
  gettimeofday (&tp, &tzp);
  printf ("length %d %s %d%d\n", strlen(abc), abc, tp.tv_sec, tp.tv_usec);
  printf ("length %d %s\n", strlen(xyz), xyz);
  puts ("a string");
  return 0;
If I set a break at every callsite for a function in the plt and at the 
plt entry then gdb does what I expect; I get matching "plt ref" 
(callsite) and "plt entry" (foo@plt entry)
GDB:
plt ref __libc_start_main @400494 rip=0x400494
plt entry __libc_start_main @0000000000400460 rip=0x400460
plt entry  @0000000000400420 rip=0x400420
plt ref gettimeofday @400580 rip=0x400580
plt entry gettimeofday @0000000000400450 rip=0x400450
plt entry  @0000000000400420 rip=0x400420
plt ref printf @4005cd rip=0x4005cd
plt entry printf @0000000000400430 rip=0x400430
plt entry  @0000000000400420 rip=0x400420
length 26 abcdefghijklmnopqrstuvwxyz 1315342376852950
plt ref printf @40060f rip=0x40060f
plt entry printf @0000000000400430 rip=0x400430
length 26 zyxwvutsrqpomnlkjihgfedcba
plt ref puts @40061b rip=0x40061b
plt entry puts @0000000000400440 rip=0x400440
plt entry  @0000000000400420 rip=0x400420
a string

However stap gives output in an unusual order and the printf plt entry 
is only hit once.  (The gdb breakpoints and awk probes are automatically 
generated with an awk script and are thus equivalent)
length 26 abcdefghijklmnopqrstuvwxyz 1315342362875422
length 26 zyxwvutsrqpomnlkjihgfedcba
a string
STAP:
plt ref __libc_start_main @400494 rip=0x400470
plt entry  @0000000000400420 rip=0x400408
plt entry printf @0000000000400430 rip=0x400408
plt entry puts @0000000000400440 rip=0x400408
plt entry gettimeofday @0000000000400450 rip=0x400408
plt entry __libc_start_main @0000000000400460 rip=0x400408
plt ref gettimeofday @400580 rip=0x400554
plt ref printf @4005cd rip=0x400554
plt ref printf @40060f rip=0x400554
plt ref puts @40061b rip=0x400554