Ups is a source level C and C++ debugger that runs under X11. Fortran is also supported on some systems.

It runs in a window with two major regions: one showing the current state of the target program data and the other showing the currently executing source code. A key feature of ups is that the variables display is persistent: when you add a variable to the display it stays there as you step through the code. The current stack trace (which function called which) is always visible.

Ups includes a C interpreter which allows you to add fragments of code simply by editing them into the source window (the source file itself is not modified). This lets you add debugging printf calls without recompiling, relinking (or even restarting) the target program. You can also add conditional breakpoints in a natural way - you just add a statement like "if (i == 73) #stop" at the appropriate place in the source window.