path: root/stapprobes.5.in

.\" -*- nroff -*-
.TH STAPPROBES 5 @DATE@ "Red Hat"
.SH NAME
stapprobes \- systemtap probe points

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.SH DESCRIPTION
The following sections enumerate the variety of probe points supported
by the systemtap translator, and additional aliases defined by
standard tapset scripts.
.PP
The general probe point syntax is a "dotted-functor" sequence.  This
allows a breakdown of the event namespace into parts, somewhat like
the Domain Name System does on the Internet.  Each component
identifier may be parametrized by a string or number literal.  These
are all syntactically valid probe points:
.SAMPLE
kernel.function("foo").return
syscall(22)
user.inode("/bin/vi").statement(0x2222)
end
.ESAMPLE

.SS BEGIN/END

The probe points
.IR begin " and " end
are defined by the translator to refer to the time of session startup
and shutdown.  All "begin" probe handlers are run, in some sequence,
during the startup of the session.  All global variables will have
been initialized prior to this point.  All "end" probes are run, in
some sequence, during the
.I normal
shutdown of a session, such as in the aftermath of an
.I exit ()
function call, or an interruption from the user.  In the case of an
error-triggered shutdown, "end" probes are not run.  There are no
target variables available in either context.

.SS TIMERS

Intervals defined by the standard kernel "jiffies" timer may be used
to trigger probe handlers asynchronously.  Two probe point variants
are supported by the translator:
.SAMPLE
timer.jiffies(N)
timer.jiffies(N).randomize(M)
.ESAMPLE
The probe handler is run every N jiffies (a kernel-defined unit of
time, typically between 1 and 60 ms).  If the "randomize" component is
given, a linearly distributed random value in the range [-M..+M] is
added to N every time the handler is run.  N is restricted to a
reasonable range (1 to around a million), and M is restricted to be
smaller than N.  There are no target variables provided in either
context.  It is possible for such probes to be run concurrently on
a multi-processor computer.

.SS DWARF

This family of probe points uses symbolic debugging information for
the target kernel/module/program, as may be found in unstripped
executables, or the separate
.I debuginfo
packages.  They allow placement of probes logically into the execution
path of the target program, by specifying a set of points in the
source or object code.  When a matching statement executes on any
processor, the probe handler is run in that context.
.PP
Points in a kernel, which are identified by
module, source file, line number, function name, C label name, or some
combination of these.  This kind of "synchronous" event is deemed to
occur when any processor executes an instruction matched by the
specification.  Other families of probe points refer to "asynchronous"
events such as timers/counters rolling over, where there is no fixed
execution point that is related.  Each probe point specification may
match multiple physical locations, all of which are then probed.  A
probe declaration may also contain several comma-separated
specifications, all of which are probed.
.PP
Here is a list of probe point families currently supported.  The
.B .function
variant places a probe near the beginning of the named function, so that
parameters are available as context variables.  The
.B .return
variant places a probe at the moment of return from the named function, so
the return value is available as the "$retvalue" context variable.
The
.B .statement
variant places a probe at the exact spot, exposing those local variables
that are visible there.
.SAMPLE
kernel.function(PATTERN)
.br
kernel.function(PATTERN).return
.br
module(MPATTERN).function(PATTERN)
.br
module(MPATTERN).function(PATTERN).return
.br
kernel.statement(PATTERN)
.br
module(MPATTERN).statement(PATTERN)
.ESAMPLE
In the above list, MPATTERN stands for a string literal that aims to
identify the loaded kernel module of interest.  It may include "*" and
"?" wildcards.  PATTERN stands for a string literal that aims to
identify a point in the program.  It is made up of three parts.  The
first part is the name of a function, as would appear in the
.I nm
program's output.  This part may use the "*" and "?" wildcarding
operators to match multiple names.  The second part is optional, and
begins with the "@" character.  It is followed by a source file name
wildcard pattern, such as
.IR mm/slab* .
Finally, the third part is optional if the file name part was given,
and identifies the line number in the source file, preceded by a ":".
As an alternative, PATTERN may be a numeric constant, indicating an
(module-relative or kernel-absolute) address.
.PP
Some of the source-level variables, such as function parameters,
locals, globals visible in the compilation unit, may be visible to
probe handlers.  They may refer to these variables by prefixing their
name with "$" within the scripts.  In addition, a special syntax
allows limited traversal of structures, pointers, and arrays.
.TP
$var
refers to an in-scope variable "var".  If it's an integer-like type,
it will be cast to a 64-bit int for systemtap script use.  If it's
a string-like type (char *), it will be copied into a systemtap
string.
.TP
$var->field
traversal to a structure's field.  The indirection operator
may be repeated to follow more levels of pointers.
.TP
$var[N]
indexes into an array.  The index is given with a
literal number.

.SH EXAMPLES
.PP
Here are some example probe points, defining the associated events.
.TP
begin, end, end
refers to the startup and normal shutdown of the session.  In this
case, the handler would run once during startup and twice during
shutdown.
.TP
timer.jiffies(1000).randomize(200)
refers to a periodic interrupt, every 1000 +/- 200 jiffies.
.TP
kernel.function("*init*"), kernel.function("*exit*")
refers to all kernel functions with "init" or "exit" in the name.
.TP
kernel.function("*@kernel/sched.c:240")
refers to any functions within the "kernel/sched.c" file that span
line 240.
.TP
module("usb*").function("*sync*").return
refers to the moment of return from all functions with "sync" in the
name in any of the USB drivers.
.TP
kernel.statement(0xc0044852)
refers to the first byte of the statement whose compiled instructions
include the given address in the kernel.


.SH SEE ALSO
.IR stap (1)