GDB Internals - Native Debugging

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# Native Debugging

Several files control GDB's configuration for native support:

``gdb/config/arch/xyz.mh'`
  Specifies Makefile fragments needed when hosting or native on machine xyz. In particular, this lists the required native-dependent object files, by defining ``NATDEPFILES=...'`. Also specifies the header file which describes native support on xyz, by defining ``NAT_FILE= nm-xyz.h'`. You can also define ``NAT_CFLAGS'`, ``NAT_ADD_FILES'`, ``NAT_CLIBS'`, ``NAT_CDEPS'`, etc.; see ``Makefile.in'`.
``gdb/config/arch/nm-xyz.h'`
  (``nm.h'` is a link to this file, created by configure). Contains C macro definitions describing the native system environment, such as child process control and core file support.
``gdb/xyz-nat.c'`
  Contains any miscellaneous C code required for this native support of this machine. On some machines it doesn't exist at all.

There are some "generic" versions of routines that can be used by various systems. These can be customized in various ways by macros defined in your ``nm-xyz.h'` file. If these routines work for the xyz host, you can just include the generic file's name (with ``.o'`, not ``.c'`) in `NATDEPFILES`.

Otherwise, if your machine needs custom support routines, you will need to write routines that perform the same functions as the generic file. Put them into `xyz-nat.c`, and put `xyz-nat.o` into `NATDEPFILES`.

``inftarg.c'`
  This contains the target_ops vector that supports Unix child processes on systems which use ptrace and wait to control the child.
``procfs.c'`
  This contains the target_ops vector that supports Unix child processes on systems which use /proc to control the child.
``fork-child.c'`
  This does the low-level grunge that uses Unix system calls to do a "fork and exec" to start up a child process.
``infptrace.c'`
  This is the low level interface to inferior processes for systems using the Unix `ptrace` call in a vanilla way.

## Native core file Support

``core-aout.c::fetch_core_registers()'`
  Support for reading registers out of a core file. This routine calls `register_addr()`, see below. Now that BFD is used to read core files, virtually all machines should use `core-aout.c`, and should just provide `fetch_core_registers` in `xyz-nat.c` (or `REGISTER_U_ADDR` in `nm-xyz.h`).
``core-aout.c::register_addr()'`
  If your `nm-xyz.h` file defines the macro `REGISTER_U_ADDR(addr, blockend, regno)`, it should be defined to set `addr` to the offset within the ``user'` struct of GDB register number `regno`. `blockend` is the offset within the "upage" of `u.u_ar0`. If `REGISTER_U_ADDR` is defined, ``core-aout.c'` will define the `register_addr()` function and use the macro in it. If you do not define `REGISTER_U_ADDR`, but you are using the standard `fetch_core_registers()`, you will need to define your own version of `register_addr()`, put it into your `xyz-nat.c` file, and be sure `xyz-nat.o` is in the `NATDEPFILES` list. If you have your own `fetch_core_registers()`, you may not need a separate `register_addr()`. Many custom `fetch_core_registers()` implementations simply locate the registers themselves.

When making GDB run native on a new operating system, to make it possible to debug core files, you will need to either write specific code for parsing your OS's core files, or customize ``bfd/trad-core.c'`. First, use whatever `#include` files your machine uses to define the struct of registers that is accessible (possibly in the u-area) in a core file (rather than ``machine/reg.h'`), and an include file that defines whatever header exists on a core file (e.g. the u-area or a ``struct core'`). Then modify `trad_unix_core_file_p()` to use these values to set up the section information for the data segment, stack segment, any other segments in the core file (perhaps shared library contents or control information), "registers" segment, and if there are two discontiguous sets of registers (e.g. integer and float), the "reg2" segment. This section information basically delimits areas in the core file in a standard way, which the section-reading routines in BFD know how to seek around in.

Then back in GDB, you need a matching routine called `fetch_core_registers()`. If you can use the generic one, it's in ``core-aout.c'`; if not, it's in your ``xyz-nat.c'` file. It will be passed a char pointer to the entire "registers" segment, its length, and a zero; or a char pointer to the entire "regs2" segment, its length, and a 2. The routine should suck out the supplied register values and install them into GDB's "registers" array.

If your system uses ``/proc'` to control processes, and uses ELF format core files, then you may be able to use the same routines for reading the registers out of processes and out of core files.

## ptrace

## /proc

## win32

## shared libraries

## Native Conditionals

When GDB is configured and compiled, various macros are defined or left undefined, to control compilation when the host and target systems are the same. These macros should be defined (or left undefined) in ``nm-system.h'`.

`ATTACH_DETACH`
  If defined, then GDB will include support for the `attach` and `detach` commands.
`CHILD_PREPARE_TO_STORE`
  If the machine stores all registers at once in the child process, then define this to ensure that all values are correct. This usually entails a read from the child. [Note that this is incorrectly defined in ``xm-system.h'` files currently.]
`FETCH_INFERIOR_REGISTERS`
  Define this if the native-dependent code will provide its own routines `fetch_inferior_registers` and `store_inferior_registers` in ``HOST-nat.c'`. If this symbol is not defined, and ``infptrace.c'` is included in this configuration, the default routines in ``infptrace.c'` are used for these functions.
`FILES_INFO_HOOK`
  (Only defined for Convex.)
`FP0_REGNUM`
  This macro is normally defined to be the number of the first floating point register, if the machine has such registers. As such, it would appear only in target-specific code. However, /proc support uses this to decide whether floats are in use on this target.
`GET_LONGJMP_TARGET`
  For most machines, this is a target-dependent parameter. On the DECstation and the Iris, this is a native-dependent parameter, since <setjmp.h> is needed to define it. This macro determines the target PC address that longjmp() will jump to, assuming that we have just stopped at a longjmp breakpoint. It takes a CORE_ADDR * as argument, and stores the target PC value through this pointer. It examines the current state of the machine as needed.
`KERNEL_U_ADDR`
  Define this to the address of the `u` structure (the "user struct", also known as the "u-page") in kernel virtual memory. GDB needs to know this so that it can subtract this address from absolute addresses in the upage, that are obtained via ptrace or from core files. On systems that don't need this value, set it to zero.
`KERNEL_U_ADDR_BSD`
  Define this to cause GDB to determine the address of `u` at runtime, by using Berkeley-style `nlist` on the kernel's image in the root directory.
`KERNEL_U_ADDR_HPUX`
  Define this to cause GDB to determine the address of `u` at runtime, by using HP-style `nlist` on the kernel's image in the root directory.
`ONE_PROCESS_WRITETEXT`
  Define this to be able to, when a breakpoint insertion fails, warn the user that another process may be running with the same executable.
`PROC_NAME_FMT`
  Defines the format for the name of a ``/proc'` device. Should be defined in ``nm.h'` only in order to override the default definition in ``procfs.c'`.
`PTRACE_FP_BUG`
  mach386-xdep.c
`PTRACE_ARG3_TYPE`
  The type of the third argument to the `ptrace` system call, if it exists and is different from `int`.
`REGISTER_U_ADDR`
  Defines the offset of the registers in the "u area".
`SHELL_COMMAND_CONCAT`
  If defined, is a string to prefix on the shell command used to start the inferior.
`SHELL_FILE`
  If defined, this is the name of the shell to use to run the inferior. Defaults to `"/bin/sh"`.
`SOLIB_ADD (filename, from_tty, targ)`
  Define this to expand into an expression that will cause the symbols in filename to be added to GDB's symbol table.
`SOLIB_CREATE_INFERIOR_HOOK`
  Define this to expand into any shared-library-relocation code that you want to be run just after the child process has been forked.
`START_INFERIOR_TRAPS_EXPECTED`
  When starting an inferior, GDB normally expects to trap twice; once when the shell execs, and once when the program itself execs. If the actual number of traps is something other than 2, then define this macro to expand into the number expected.
`SVR4_SHARED_LIBS`
  Define this to indicate that SVR4-style shared libraries are in use.
`USE_PROC_FS`
  This determines whether small routines in ``*-tdep.c'`, which translate register values between GDB's internal representation and the /proc representation, are compiled.
`U_REGS_OFFSET`
  This is the offset of the registers in the upage. It need only be defined if the generic ptrace register access routines in ``infptrace.c'` are being used (that is, ``infptrace.c'` is configured in, and `FETCH_INFERIOR_REGISTERS` is not defined). If the default value from ``infptrace.c'` is good enough, leave it undefined. The default value means that u.u_ar0 points to the location of the registers. I'm guessing that `#define U_REGS_OFFSET 0` means that u.u_ar0 is the location of the registers.
`CLEAR_SOLIB`
  objfiles.c
`DEBUG_PTRACE`
  Define this to debug ptrace calls.

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