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3.5.3. Incrementing Associated Values

Use ++ to increment the associated value of a unique key in an array, as in:
array_name[index_expression] ++
Again, you can also use a handler function for your index_expression. For example, if you wanted to tally how many times a specific process performed a read to the virtual file system (using the event vfs.read), you can use the following probe:
Example 3.16. vfsreads.stp
probe vfs.read
{
  reads[execname()] ++
}

In Example 3.16, “vfsreads.stp”, the first time that the probe returns the process name gnome-terminal (i.e. the first time gnome-terminal performs a VFS read), that process name is set as the unique key gnome-terminal with an associated value of 1. The next time that the probe returns the process name gnome-terminal, SystemTap increments the associated value of gnome-terminal by 1. SystemTap performs this operation for all process names as the probe returns them.