Section: User Commands (1)
Updated: 2007-11-17
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stap - systemtap script translator/driver



stap [ OPTIONS ] - [ ARGUMENTS ]



The stap program is the front-end to the Systemtap tool. It accepts probing instructions (written in a simple scripting language), translates those instructions into C code, compiles this C code, and loads the resulting kernel module into a running Linux kernel to perform the requested system trace/probe functions. You can supply the script in a named file, from standard input, or from the command line. The program runs until it is interrupted by the user, or if the script voluntarily invokes the exit() function, or by sufficient number of soft errors.

The language, which is described in a later section, is strictly typed, declaration free, procedural, and inspired by awk. It allows source code points or events in the kernel to be associated with handlers, which are subroutines that are executed synchronously. It is somewhat similar conceptually to "breakpoint command lists" in the gdb debugger.

This manual corresponds to version 0.5.14.



The systemtap translator supports the following options. Any other option prints a list of supported options.
Increase verbosity. Produce a larger volume of informative (?) output each time option repeated.
Show help message.
Show version message.
Keep the temporary directory after all processing. This may be useful in order to examine the generated C code, or to reuse the compiled kernel object.
Guru mode. Enable parsing of unsafe expert-level constructs like embedded C.
Prologue-searching mode. Activate heuristics to work around incorrect debbugging information for $target variables.
Unoptimized mode. Disable unused code elision during elaboration.
Use bulk mode (percpu files) for kernel-to-user data transfer.
Collect timing information on the number of times probe executes and average amount of time spent in each probe.
Use NUM megabyte buffers for kernel-to-user data transfer. On a multiprocessor in bulk mode, this is a per-processor amount.
-p NUM
Stop after pass NUM. The passes are numbered 1-5: parse, elaborate, translate, compile, run. See the PROCESSING section for details.
Add the given directory to the tapset search directory. See the description of pass 2 for details.
Add the given C preprocessor directive to the module Makefile. These can be used to override limit parameters described below.
Look for the systemtap runtime sources in the given directory.
Build for given kernel release instead of currently running one.
Use the given name for the generated kernel object module, instead of a unique randomized name.
Send standard output to named file. In bulk mode, percpu files will start with FILE_ followed by the cpu number.
-c CMD
Start the probes, run CMD, and exit when CMD finishes.
-x PID
Sets target() to PID. This allows scripts to be written that filter on a specific process.



Any additional arguments on the command line are passed to the script parser for substitution. See below.



The systemtap script language resembles awk. There are two main outermost constructs: probes and functions. Within these, statements and expressions use C-like operator syntax and precedence.



Whitespace is ignored. Three forms of comments are supported:

# ... shell style, to the end of line
// ... C++ style, to the end of line
/* ... C style ... */
Literals are either strings enclosed in double-quotes (passing through the usual C escape codes with backslashes), or integers (in decimal, hexadecimal, or octal, using the same notation as in C). All strings are limited in length to some reasonable value (a few hundred bytes). Integers are 64-bit signed quantities, although the parser also accepts (and wraps around) values above positive 2**63.

In addition, script arguments given at the end of the command line may be expanded as literals. Use $1 ... $<NN> for casting as a numeric literal and @1 ... @<NN> for casting as string literal. The number of arguments may be accessed through $# (as a numeric literal) or through @# (as a string literal). These may be used in all contexts where literals are accepted, including preprocessing stage. Reference to an argument number beyond what was actually given is an error.



A simple conditional preprocessing stage is run as a part of parsing. The general form is similar to the cond ? exp1 : exp2 ternary operator:

The CONDITION is either an expression whose format is determined by its first keyword, or a string literals comparison or a numeric literals comparison.

If the first part is the identifier kernel_vr or kernel_v to refer to the kernel version number, with ("2.6.13-1.322FC3smp") or without ("2.6.13") the release code suffix, then the second part is one of the six standard numeric comparison operators <, <=, ==, !=, >, and >=, and the third part is a string literal that contains an RPM-style version-release value. The condition is deemed satisfied if the version of the target kernel (as optionally overridden by the -r option) compares to the given version string. The comparison is performed by the glibc function strverscmp.

If, on the other hand, the first part is the identifier arch to refer to the processor architecture, then the second part then the second part is one of the two string comparison operators == or !=, and the third part is a string literal for matching it. This comparison is simple string (in)equality.

Otherwise, the CONDITION is expected to be a comparison between two string literals or two numeric literals. In this case, the arguments are the only variables usable.

The TRUE-TOKENS and FALSE-TOKENS are zero or more general parser tokens (possibly including nested preprocessor conditionals), and are pasted into the input stream if the condition is true or false. For example, the following code induces a parse error unless the target kernel version is newer than 2.6.5:

%( kernel_v <= "2.6.5" %? **ERROR** %) # invalid token sequence

The following code might adapt to hypothetical kernel version drift:
probe kernel.function ( 
  %( kernel_v <= "2.6.12" %? "__mm_do_fault" %: 
     %( kernel_vr == "2.6.13-1.8273FC3smp" %? "do_page_fault" %:
        UNSUPPORTED %) %)
) { /* ... */ }

%( arch == "ia64" %?
   probe syscall.vliw = kernel.function("vliw_widget") {} 



Identifiers for variables and functions are an alphanumeric sequence, and may include "_" and "$" characters. They may not start with a plain digit, as in C. Each variable is by default local to the probe or function statement block within which it is mentioned, and therefore its scope and lifetime is limited to a particular probe or function invocation.

Scalar variables are implicitly typed as either string or integer. Associative arrays also have a string or integer value, and a a tuple of strings and/or integers serving as a key. Here are a few basic expressions.

var1 = 5
var2 = "bar"
array1 [pid()] = "name"     # single numeric key
array2 ["foo",4,i++] += 5   # vector of string/num/num keys
if (["hello",5,4] in array2) log ("yes")  # membership test

The translator performs type inference on all identifiers, including array indexes and function parameters. Inconsistent type-related use of identifiers signals an error.

Variables may be declared global, so that they are shared amongst all probes and live as long as the entire systemtap session. There is one namespace for all global variables, regardless of which script file they are found within. A global declaration may be written at the outermost level anywhere, not within a block of code. The following declaration marks a few variables as global. The translator will infer for each its value type, and if it is used as an array, its key types. Optionally, scalar globals may be initialized with a string or number literal.

global var1, var2, var3=4

Arrays are limited in size by the MAXMAPENTRIES variable -- see the SAFETY AND SECURITY section for details. Optionally, global arrays may be declared with a maximum size in brackets, overriding MAXMAPENTRIES for that array only. Note that this doesn't indicate the type of keys for the array, just the size.

global tiny_array[10], normal_array, big_array[50000]



Statements enable procedural control flow. They may occur within functions and probe handlers. The total number of statements executed in response to any single probe event is limited to some number defined by a macro in the translated C code, and is in the neighbourhood of 1000.
Execute the string- or integer-valued expression and throw away the value.
{ STMT1 STMT2 ... }
Execute each statement in sequence in this block. Note that separators or terminators are generally not necessary between statements.
Null statement, do nothing. It is useful as an optional separator between statements to improve syntax-error detection and to handle certain grammar ambiguities.
if (EXP) STMT1 [ else STMT2 ]
Compare integer-valued EXP to zero. Execute the first (non-zero) or second STMT (zero).
while (EXP) STMT
While integer-valued EXP evaluates to non-zero, execute STMT.
for (EXP1; EXP2; EXP3) STMT
Execute EXP1 as initialization. While EXP2 is non-zero, execute STMT, then the iteration expression EXP3.
foreach (VAR in ARRAY [ limit EXP ]) STMT
Loop over each element of the named global array, assigning current key to VAR. The array may not be modified within the statement. By adding a single + or - operator after the VAR or the ARRAY identifier, the iteration will proceed in a sorted order, by ascending or descending index or value. Using the optional limit keyword limits the number of loop iterations to EXP times. EXP is evaluted once at the beginning of the loop.
foreach ([VAR1, VAR2, ...] in ARRAY [ limit EXP ]) STMT
Same as above, used when the array is indexed with a tuple of keys. A sorting suffix may be used on at most one VAR or ARRAY identifier.
break, continue
Exit or iterate the innermost nesting loop (while or for or foreach) statement.
return EXP
Return EXP value from enclosing function. If the function's value is not taken anywhere, then a return statement is not needed, and the function will have a special "unknown" type with no return value.
Return now from enclosing probe handler.
delete ARRAY[INDEX1, INDEX2, ...]
Remove from ARRAY the element specified by the index tuple. The value will no longer be available, and subsequent iterations will not report the element. It is not an error to delete an element that does not exist.
delete ARRAY
Remove all elements from ARRAY.
delete SCALAR
Removes the value of SCALAR. Integers and strings are cleared to 0 and "" respectively, while statistics are reset to the initial empty state.



Systemtap supports a number of operators that have the same general syntax, semantics, and precedence as in C and awk. Arithmetic is performed as per typical C rules for signed integers. Division by zero or overflow is detected and results in an error.
binary numeric operators
* / % + - >> << & ^ | && ||
binary string operators
. (string concatenation)
numeric assignment operators
= *= /= %= += -= >>= <<= &= ^= |=
string assignment operators
= .=
unary numeric operators
+ - ! ~ ++ --
binary numeric or string comparison operators
< > <= >= == !=
ternary operator
cond ? exp1 : exp2
grouping operator
( exp )
function call
fn ([ arg1, arg2, ... ])
array membership check
exp in array
[exp1, exp2, ...] in array



The main construct in the scripting language identifies probes. Probes associate abstract events with a statement block ("probe handler") that is to be executed when any of those events occur. The general syntax is as follows:

Events are specified in a special syntax called "probe points". There are several varieties of probe points defined by the translator, and tapset scripts may define further ones using aliases. These are listed in the stapprobes(5) manual pages.

The probe handler is interpreted relative to the context of each event. For events associated with kernel code, this context may include variables defined in the source code at that spot. These "target variables" are presented to the script as variables whose names are prefixed with "$". They may be accessed only if the kernel's compiler preserved them despite optimization. This is the same constraint that a debugger user faces when working with optimized code. Some other events have very little context.

New probe points may be defined using "aliases". Probe point aliases look similar to probe definitions, but instead of activating a probe at the given point, it just defines a new probe point name as an alias to an existing one. There are two types of alias, i.e. the prologue style and the epilogue style which are identified by "=" and "+=" respectively.

For prologue style alias, the statement block that follows an alias definition is implicitly added as a prologue to any probe that refers to the alias. While for the epilogue style alias, the statement block that follows an alias definition is implicitly added as an epilogue to any probe that refers to the alias. For example:

probe = kernel.function("sys_read") {
  fildes = $fd

defines a new probe point, which expands to kernel.function(sys_read), with the given statement as a prologue. And
probe += kernel.function("sys_read") {
  fildes = $fd

defines a new probe point with the given statement as an epilogue.

Another probe definition may use the alias like this:

probe {
  printf("reading fd=%d, fildes)



Systemtap scripts may define subroutines to factor out common work. Functions take any number of scalar (integer or string) arguments, and must return a single scalar (integer or string). An example function declaration looks like this:
function thisfn (arg1, arg2) {
   return arg1 + arg2

Note the general absence of type declarations, which are instead inferred by the translator. However, if desired, a function definition may include explicit type declarations for its return value and/or its arguments. This is especially helpful for embedded-C functions. In the following example, the type inference engine need only infer type type of arg2 (a string).
function thatfn:string (arg1:long, arg2) {
   return sprint(arg1) . arg2

Functions may call others or themselves recursively, up to a fixed nesting limit. This limit is defined by a macro in the translated C code and is in the neighbourhood of 10.



The function names print, printf, sprint, and sprintf are specially treated by the translator. They format values for printing to the standard systemtap log stream in a more convenient way.

takes a single value of any type, and prints it
operates like print, but returns the formatted string instead of logging it.
takes a formatting string, and a number of values of corresponding types, and prints them all.
operates like printf, but like sprint, returns the formatted string instead of logging it.

The printf formatting directives similar to those of C, except that they are fully type-checked by the translator.

        x = sprintf("take %d steps forward, %d steps back\n", 3, 2)
        printf("take %d steps forward, %d steps back\n", 3+1, 2*2)
        bob = "bob"
        alice = "alice"
        printf("%s phoned %s %.4x times\n", bob, alice . bob, 3456)
        printf("%s except after %s\n", 
                sprintf("%s before %s", 
                        sprint(1), sprint(3)), 



It is often desirable to collect statistics in a way that avoids the penalties of repeatedly exclusive locking the global variables those numbers are being put into. Systemtap provides a solution using a special operator to accumulate values, and several pseudo-functions to extract the statistical aggregates.

The aggregation operator is <<<, and resembles an assignment, or a C++ output-streaming operation. The left operand specifies a scalar or array-index lvalue, which must be declared global. The right operand is a numeric expression. The meaning is intuitive: add the given number to the pile of numbers to compute statistics of. (The specific list of statistics to gather is given separately, by the extraction functions.)

    foo <<< 1
    stats[pid()] <<< memsize

The extraction functions are also special. For each appearance of a distinct extraction function operating on a given identifier, the translator arranges to compute a set of statistics that satisfy it. The statistics system is thereby "on-demand". Each execution of an extraction function causes the aggregation to be computed for that moment across all processors.

Here is the set of extractor functions. The first argument of each is the same style of lvalue used on the left hand side of the accumulate operation. The @count(v), @sum(v), @min(v), @max(v), @avg(v) extractor functions compute the number/total/minimum/maximum/average of all accumulated values. The resulting values are all simple integers.

Histograms are also available, but are more complicated because they have a vector rather than scalar value. @hist_linear(v,L,H,W) represents a linear histogram whose low/high/width parameters are given by the following three literal numbers. Similarly, @hist_log(v,N) represents a base-2 logarithmic histogram with the given number of buckets. N may be omitted, and defaults to 64. Printing a histogram with the print family of functions renders a histogram object as a tabular "ASCII art" bar chart.

probe foo {
  x <<< $value
probe end {  
  printf ("avg %d = sum %d / count %d\n",
          @avg(x), @sum(x), @count(x))
  print (@hist_log(v))



When in guru mode, the translator accepts embedded code in the script. Such code is enclosed between %{ and %} markers, and is transcribed verbatim, without analysis, in some sequence, into the generated C code. At the outermost level, this may be useful to add #include instructions, and any auxiliary definitions for use by other embedded code.

The other place where embedded code is permitted is as a function body. In this case, the script language body is replaced entirely by a piece of C code enclosed again between %{ and %} markers. This C code may do anything reasonable and safe. There are a number of undocumented but complex safety constraints on atomicity, concurrency, resource consumption, and run time limits, so this is an advanced technique.

The memory locations set aside for input and output values are made available to it using a macro THIS. Here are some examples:

function add_one (val) %{
  THIS->__retvalue = THIS->val + 1;
function add_one_str (val) %{
  strlcpy (THIS->__retvalue, THIS->val, MAXSTRINGLEN);
  strlcat (THIS->__retvalue, "one", MAXSTRINGLEN);

The function argument and return value types have to be inferred by the translator from the call sites in order for this to work. The user should examine C code generated for ordinary script-language functions in order to write compatible embedded-C ones.



A set of builtin functions and probe point aliases are provided by the scripts installed under the /usr/share/systemtap/tapset directory. These are described in the stapfuncs(5) and stapprobes(5) manual pages.



The translator begins pass 1 by parsing the given input script, and all scripts (files named *.stp) found in a tapset directory. The directories listed with -I are processed in sequence, each processed in "guru mode". For each directory, a number of subdirectories are also searched. These subdirectories are derived from the selected kernel version (the -R option), in order to allow more kernel-version-specific scripts to override less specific ones. For example, for a kernel version 2.6.12-23.FC3 the following patterns would be searched, in sequence: 2.6.12-23.FC3/*.stp, 2.6.12/*.stp, 2.6/*.stp, and finally *.stp Stopping the translator after pass 1 causes it to print the parse trees.

In pass 2, the translator analyzes the input script to resolve symbols and types. References to variables, functions, and probe aliases that are unresolved internally are satisfied by searching through the parsed tapset scripts. If any tapset script is selected because it defines an unresolved symbol, then the entirety of that script is added to the translator's resolution queue. This process iterates until all symbols are resolved and a subset of tapset scripts is selected.

Next, all probe point descriptions are validated against the wide variety supported by the translator. Probe points that refer to code locations ("synchronous probe points") require the appropriate kernel debugging information to be installed. In the associated probe handlers, target-side variables (whose names begin with "$") are found and have their run-time locations decoded.

Next, all probes and functions are analyzed for optimization opportunities, in order to remove variables, expressions, and functions that have no useful value and no side-effect. Embedded-C functions are assumed to have side-effects unless they include the magic string /* pure */. Since this optimization can hide latent code errors such as type mismatches or invalid $target variables, it sometimes may be useful to disable the optimizations with the -u option.

Finally, all variable, function, parameter, array, and index types are inferred from context (literals and operators). Stopping the translator after pass 2 causes it to list all the probes, functions, and variables, along with all inferred types. Any inconsistent or unresolved types cause an error.

In pass 3, the translator writes C code that represents the actions of all selected script files, and creates a Makefile to build that into a kernel object. These files are placed into a temporary directory. Stopping the translator at this point causes it to print the contents of the C file.

In pass 4, the translator invokes the Linux kernel build system to create the actual kernel object file. This involves running make in the temporary directory, and requires a kernel module build system (headers, config and Makefiles) to be installed in the usual spot /lib/modules/VERSION/build. Stopping the translator after pass 4 is the last chance before running the kernel object. This may be useful if you want to archive the file.

In pass 5, the translator invokes the systemtap auxiliary program staprun program for the given kernel object. This program arranges to load the module then communicates with it, copying trace data from the kernel into temporary files, until the user sends an interrupt signal. Any run-time error encountered by the probe handlers, such as running out of memory, division by zero, exceeding nesting or runtime limits, results in a soft error indication. Soft errors in excess of MAXERRORS block of all subsequent probes, and terminate the session. Finally, staprun unloads the module, and cleans up.



See the stapex(5) manual page for a collection of samples.



The systemtap translator caches the pass 3 output (the generated C code) and the pass 4 output (the compiled kernel module) if pass 4 completes successfully. This cached output is reused if the same script is translated again assuming the same conditions exist (same kernel version, same systemtap version, etc.). Cached files are stored in the $SYSTEMTAP_DIR/cache directory, which may be periodically cleaned/erased by the user.



Systemtap is an administrative tool. It exposes kernel internal data structures and potentially private user information. It acquires root privileges to actually run the kernel objects it builds using the sudo command applied to the staprun program. The latter is a part of the Systemtap package, dedicated to module loading and unloading (but only in the white zone), and kernel-to-user data transfer. Since staprun does not perform any additional security checks on the kernel objects it is given, it would be unwise for a system administrator to give even targeted sudo privileges to untrusted users.

The translator asserts certain safety constraints. It aims to ensure that no handler routine can run for very long, allocate memory, perform unsafe operations, or in unintentionally interfere with the kernel. Use of script global variables is suitably locked to protect against manipulation by concurrent probe handlers. Use of guru mode constructs such as embedded C can violate these constraints, leading to kernel crash or data corruption.

The resource use limits are set by macros in the generated C code. These may be overridden with the -D flag. A selection of these is as follows:

Maximum number of recursive function call levels, default 10.
Maximum length of strings, default 128.
Maximum number of iterations to wait for locks on global variables before declaring possible deadlock and skipping the probe, default 1000.
Maximum number of statements to execute during any single probe hit (with interrupts disabled), default 1000.
Maximum number of statements to execute during any single probe hit which is executed with interrupts enabled (such as begin/end probes), default (MAXACTION * 10).
Maximum number of rows in any single global array, default 2048.
Maximum number of soft errors before an exit is triggered, default 0, which means that the first error will exit the script.
Maximum number of skipped reentrant probes before an exit is triggered, default 100.
Minimum number of free kernel stack bytes required in order to run a probe handler, default 1024. This number should be large enough for the probe handler's own needs, plus a safety margin.

In case something goes wrong with stap or staprun after a probe has already started running, one may safely kill both user processes, and remove the active probe kernel module with rmmod. Any pending trace messages may be lost.

In addition to the methods outlined above, the generated kernel module also uses overload processing to make sure that probes can't run for too long. If more than STP_OVERLOAD_THRESHOLD cycles (default 500000000) have been spent in all the probes on a single cpu during the last STP_OVERLOAD_INTERVAL cycles (default 1000000000), the probes have overloaded the system and an exit is triggered.

By default, overload processing is turned on for all modules. If you would like to disable overload processing, define STP_NO_OVERLOAD.



Systemtap data directory for cached systemtap files, unless overridden by the SYSTEMTAP_DIR environment variable.
Temporary directory for systemtap files, including translated C code and kernel object.
The automatic tapset search directory, unless overridden by the SYSTEMTAP_TAPSET environment variable.
The runtime sources, unless overridden by the SYSTEMTAP_RUNTIME environment variable.
The location of kernel module building infrastructure.
The location of kernel debugging information when packaged into the kernel-debuginfo RPM.
The auxiliary program supervising module loading, interaction, and unloading.



stapprobes(5), stapfuncs(5), stapex(5), lket(5), awk(1), sudo(8), gdb(1)



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