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### ⁠3.2.2. SystemTap Handler/Body

Consider the following sample script:
⁠

Example 3.4. helloworld.stp

    probe begin
    {
      printf ("hello world\n")
      exit ()
    }

In Example 3.4, “helloworld.stp”, the event `begin` (that is, the start of the session) triggers the handler enclosed in `{ }`, which simply prints `hello world` followed by a new-line, then exits.

Note

SystemTap scripts continue to run until the `exit()` function executes. If the users wants to stop the execution of the script, it can interrupted manually with Ctrl+C.
printf ( ) Statements
The `printf ()` statement is one of the simplest functions for printing data. `printf ()` can also be used to display data using a wide variety of SystemTap functions in the following format:

    		printf ("format string\n", arguments)

The format string specifies how arguments should be printed. The format string of Example 3.4, “helloworld.stp” instructs SystemTap to print `hello world`, and contains no format specifiers.
You can use the format specifiers `%s` (for strings) and `%d` (for numbers) in format strings, depending on your list of arguments. Format strings can have multiple format specifiers, each matching a corresponding argument; multiple arguments are delimited by a comma (`,`).

Note

Semantically, the SystemTap `printf` function is very similar to its C language counterpart. The aforementioned syntax and format for SystemTap's `printf` function is identical to that of the C-style `printf`.
To illustrate this, consider the following probe example:
⁠

Example 3.5. variables-in-printf-statements.stp

    probe syscall.open
    {
      printf ("%s(%d) open\n", execname(), pid())
    }

Example 3.5, “variables-in-printf-statements.stp” instructs SystemTap to probe all entries to the system call `open`; for each event, it prints the current `execname()` (a string with the executable name) and `pid()` (the current process ID number), followed by the word `open`. A snippet of this probe's output would look like:

    vmware-guestd(2206) open
    hald(2360) open
    hald(2360) open
    hald(2360) open
    df(3433) open
    df(3433) open
    df(3433) open
    hald(2360) open

SystemTap Functions
SystemTap supports a wide variety of functions that can be used as `printf ()` arguments. Example 3.5, “variables-in-printf-statements.stp” uses the SystemTap functions `execname()` (name of the process that called a kernel function/performed a system call) and `pid()` (current process ID).
The following is a list of commonly-used SystemTap functions:

tid()
  The ID of the current thread.
uid()
  The ID of the current user.
cpu()
  The current CPU number.
gettimeofday_s()
  The number of seconds since UNIX epoch (January 1, 1970).
ctime()
  Convert number of seconds since UNIX epoch to date.
pp()
  A string describing the probe point currently being handled.
thread_indent()
  This particular function is quite useful in providing you with a way to better organize your print results. The function takes one argument, an indentation delta, which indicates how many spaces to add or remove from a thread's "indentation counter". It then returns a string with some generic trace data along with an appropriate number of indentation spaces.
  The generic data included in the returned string includes a timestamp (number of microseconds since the first call to `thread_indent()` by the thread), a process name, and the thread ID. This allows you to identify what functions were called, who called them, and the duration of each function call.
  If call entries and exits immediately precede each other, it is easy to match them. However, in most cases, after a first function call entry is made several other call entries and exits may be made before the first call exits. The indentation counter helps you match an entry with its corresponding exit by indenting the next function call if it is not the exit of the previous one.
  Consider the following example on the use of `thread_indent()`:
  ⁠

  Example 3.6. thread_indent.stp

      probe kernel.function("*@net/socket.c").call
      {
        printf ("%s -> %s\n", thread_indent(1), probefunc())
      }
      probe kernel.function("*@net/socket.c").return
      {
        printf ("%s <- %s\n", thread_indent(-1), probefunc())
      }

  Example 3.6, “thread_indent.stp” prints out the `thread_indent()` and probe functions at each event in the following format:

      0 ftp(7223): -> sys_socketcall
      1159 ftp(7223):  -> sys_socket
      2173 ftp(7223):   -> __sock_create
      2286 ftp(7223):    -> sock_alloc_inode
      2737 ftp(7223):    <- sock_alloc_inode
      3349 ftp(7223):    -> sock_alloc
      3389 ftp(7223):    <- sock_alloc
      3417 ftp(7223):   <- __sock_create
      4117 ftp(7223):   -> sock_create
      4160 ftp(7223):   <- sock_create
      4301 ftp(7223):   -> sock_map_fd
      4644 ftp(7223):    -> sock_map_file
      4699 ftp(7223):    <- sock_map_file
      4715 ftp(7223):   <- sock_map_fd
      4732 ftp(7223):  <- sys_socket
      4775 ftp(7223): <- sys_socketcall

  This sample output contains the following information:
  - The time (in microseconds) since the initial `thread_indent()` call for the thread (included in the string from `thread_indent()`).
  - The process name (and its corresponding ID) that made the function call (included in the string from `thread_indent()`).
  - An arrow signifying whether the call was an entry (`<-`) or an exit (`->`); the indentations help you match specific function call entries with their corresponding exits.
  - The name of the function called by the process.
name
  Identifies the name of a specific system call. This variable can only be used in probes that use the event `syscall.system_call`.
target()
  Used in conjunction with `stap script -x process ID` or `stap script -c command`. If you want to specify a script to take an argument of a process ID or command, use `target()` as the variable in the script to refer to it. For example:
  ⁠

  Example 3.7. targetexample.stp

      probe syscall.* {
        if (pid() == target())
          printf("%s\n", name)
      }

  When Example 3.7, “targetexample.stp” is run with the argument `-x process ID`, it watches all system calls (as specified by the event `syscall.*`) and prints out the name of all system calls made by the specified process.
  This has the same effect as specifying `if (pid() == process ID)` each time you wish to target a specific process. However, using `target()` makes it easier for you to re-use the script, giving you the ability to pass a process ID as an argument each time you wish to run the script (that is, `stap targetexample.stp -x process ID`).

For more information about supported SystemTap functions, refer to `man stapfuncs`.

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