stap command examples

stap – systemtap script translator/driver

LANGUAGE BASICS
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.

PROBING
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 'kernel.function("*")'

To list the probeable functions and local variables in the kernel, use another listings mode.

% stap -L 'kernel.function("*")'