Chapter 5. Understanding SystemTap Errors

This chapter explains the most common errors you may encounter while using SystemTap.

5.1. Parse and Semantic Errors

These types of errors occur while SystemTap attempts to parse and translate the script into C, prior to being converted into a kernel module. For example type errors result from operations that assign invalid values to variables or arrays.
parse error: expected foo, saw bar
The script contains a grammatical/typographical error. SystemTap detected type of construct that is incorrect, given the context of the probe.
The following invalid SystemTap script is missing its probe handlers:
probe vfs.write

It results in the following error message showing that the parser was expecting something other than the probe keyword in column 1 of line 2:
parse error: expected one of '. , ( ? ! { = +='
	saw: keyword at perror.stp:2:1
1 parse error(s).
parse error: embedded code in unprivileged script
The script contains unsafe embedded C code (blocks of code surrounded by %{ %}. SystemTap allows you to embed C code in a script, which is useful if there are no tapsets to suit your purposes. However, embedded C constructs are not safe; as such, SystemTap warns you with this error if such constructs appear in the script.
If you are sure of the safety of any similar constructs in the script and are member of stapdev group (or have root privileges), run the script in "guru" mode by using the option -g (that is stap -g script).
semantic error: type mismatch for identifier 'foo' ... string vs. long
The function foo in the script used the wrong type (that is %s or %d). This error will present itself in Example 5.1, “error-variable.stp”, because the function execname() returns a string the format specifier should be a %s, not %d.

Example 5.1. error-variable.stp

  printf ("%d(%d) open\n", execname(), pid())
semantic error: unresolved type for identifier 'foo'
The identifier (for example a variable) was used, but no type (integer or string) could be determined. This occurs, for instance, if you use a variable in a printf statement while the script never assigns a value to the variable.
semantic error: Expecting symbol or array index expression
SystemTap could not assign a value to a variable or to a location in an array. The destination for the assignment is not a valid destination. The following example code would generate this error:
probe begin { printf("x") = 1 }

while searching for arity N function, semantic error: unresolved function call
A function call or array index expression in the script used an invalid number of arguments/parameters. In SystemTap arity can either refer to the number of indices for an array, or the number of parameters to a function.
semantic error: array locals not supported, missing global declaration?
The script used an array operation without declaring the array as a global variable (global variables can be declared after their use in SystemTap scripts). Similar messages appear if an array is used, but with inconsistent arities.
semantic error: variable ’foo’ modified during ’foreach’ iteration
The array foo is being modifed (being assigned to or deleted from) within an active foreach loop. This error also displays if an operation within the script performs a function call within the foreach loop.
semantic error: probe point mismatch at position N, while resolving probe point foo
SystemTap did not understand what the event or SystemTap function foo refers to. This usually means that SystemTap could not find a match for foo in the tapset library. The N refers to the line and column of the error.
semantic error: no match for probe point, while resolving probe point foo
The events/handler function foo could not be resolved altogether, for a variety of reasons. This error occurs when the script contains the event kernel.function("blah"), and blah does not exist. In some cases, the error could also mean the script contains an invalid kernel file name or source line number.
semantic error: unresolved target-symbol expression
A handler in the script references a target variable, but the value of the variable could not be resolved. This error could also mean that a handler is referencing a target variable that is not valid in the context when it was referenced. This may be a result of compiler optimization of the generated code.
semantic error: libdwfl failure
There was a problem processing the debugging information. In most cases, this error results from the installation of a kernel-debuginfo RPM whose version does not match the probed kernel exactly. The installed kernel-debuginfo RPM itself may have some consistency/correctness problems.
semantic error: cannot find foo debuginfo
SystemTap could not find a suitable kernel-debuginfo at all.