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:

probe vfs.read
{
  reads[execname()] ++
}

In Example 3.16, “vfsreads.stp”, the first time that the probe returns the process name gnome-terminal (that is, 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.