Worse is worse

Last week I presented my work on the ZeroBUGS debugger at Amazon.com. I started my talk by saying that the debugging support in Linux is in line with the worse is better principle: the building blocks are rudimentary for the sake of keeping the implementation simple.

For example, there is no native BreakpointEvent notification. Rather, the debugger implementer needs to keep track of all breakpoints; if a SIGTRAP occurs at the address where an active breakpoint exists, then it is most likely because the said breakpoint was hit.

Another solution is to use PTRACE_GETSIGINFO (available since kernel 2.3.99) and inspect the siginfo_t structure:

    struct siginfo_t {
                  int      si_signo;    /* Signal number */
                  int      si_errno;    /* An errno value */
                  int      si_code;     /* Signal code */
                  pid_t    si_pid;      /* Sending process ID */
                  uid_t    si_uid;      /* Real user ID of sending process */
                  int      si_status;   /* Exit value or signal */
                  clock_t  si_utime;    /* User time consumed */
                  clock_t  si_stime;    /* System time consumed */
                  sigval_t si_value;    /* Signal value */
                  int      si_int;      /* POSIX.1b signal */
                  void    *si_ptr;      /* POSIX.1b signal */
                  void    *si_addr;     /* Memory location which caused fault */
                  int      si_band;     /* Band event */
                  int      si_fd;       /* File descriptor */
              }

For a SIGTRAP signal, the si_code field may be TRAP_BRKPT, in which case we know that the program hit a breakpoint.

At any rate, it is up to the debugger application to create higher-level abstractions, based on signals and ptrace notifications.

Linux is not the easiest nor most pleasant system to program on, but the implementation is so simple a child could understand it. Ahem.