18.1.7. Debugging Linux applications

Linux is a multi-tasking operating system in which each process has its own process address space, complete with private translation table mappings. This can make debug of some kinds of problems quite tricky.

Broadly speaking, there are two different debug approach used in Linux systems.

Linux applications are typically debugged using a GDB debug server running on the target, communicating with a host computer, usually through Ethernet. The kernel continues to operate normally while the debug session takes place. This method of debug does not provide access to the built-in hardware debug facilities. The target system is permanently in a running state. The server receives a connection request from the host debugger and then receives commands and provides data back to the host.

The host debugger sends a load request to the GDB server, which responds by starting a new process to run the application being debugged. Before execution begins, it uses the system call ptrace() to control the application process. All signals from this process are forwarded to the GDB server. Signals sent to the application instead go to the GDB server that can deal with the signal or forward it to the application being debugged.

To set a breakpoint, the GDB server inserts code that generates the SIGTRAP signal at the required location in the code. When this is executed, the GDB server is called and can then perform classic debugger tasks such as examining call stack information, variables or register contents.

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