stapex (3stap)

These examples give a feel for basic systemtap syntax and control structures.

global odds, evens

probe begin {
  # "no" and "ne" are local integers
  for (i=0; i<10; i++) {
    if (i % 2) odds [no++] = i
    else evens [ne++] = i
  }
  delete odds[2]
  delete evens[3]
  exit ()
}

probe end {
  foreach (x+ in odds) {
    printf ("odds[%d] = %d\n", x, odds[x])
  }
  foreach (x in evens-) {
    printf ("evens[%d] = %d\n", x, evens[x])
  }
}

This prints: odds[1] = 1 odds[3] = 5 odds[4] = 7 odds[5] = 9 evens[5] = 8 evens[4] = 6 evens[2] = 2 evens[1] = 0 Note that all variables types are inferred, and that all locals and globals are automatically initialized.

This script prints the primes between 0 and 49. function isprime (x) {

  if (x < 2) return 0
  for (i=2; i<x; i++) {
    if (x % i == 0) return 0
    if (i * i > x) break
  }
  return 1

} probe begin {

  for (i=0; i<50; i++)
    if (isprime (i)) printf("%d\n", i)
  exit()

}

This script demonstrates recursive functions. function fibonacci(i) {

  if (i < 1) error ("bad number")
  if (i == 1) return 1
  if (i == 2) return 2
  return fibonacci (i-1) + fibonacci (i-2)

} probe begin {

  printf ("11th fibonacci number: %d\n", fibonacci (11))
  exit ()

} Any larger number may exceed the MAXACTION or MAXNESTING limits, and result in an error.

To trace entry and exit from a function, use a pair of probes: probe kernel.function("sys_mkdir") { println ("enter") } probe kernel.function("sys_mkdir").return { println ("exit") }

To list the probeable functions in the kernel, use the listings mode. % stap -l \[aq]kernel.function("*")\[aq]

To list the probeable functions and local variables in the kernel, use another listings mode. % stap -L \[aq]kernel.function("*")\[aq]

The directory to find more examples can be found in the stappaths (7) manual page, and online at http://sourceware.org/systemtap/examples/

stap(1)
stapprobes(3stap)
stappaths(7)