## 12 Native Debugging

Several files control gdb's configuration for native support:

`gdb/config/arch/xyz.mh`
  Specifies Makefile fragments needed by a native configuration 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`.

  Maintainer's note: The ``.mh suffix is because this file originally contained ``Makefile fragments for hosting gdb on machine xyz. While the file is no longer used for this purpose, the ``.mh suffix remains. Perhaps someone will eventually rename these fragments so that they have a ``.mn suffix.

`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.

### 12.1 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.

### 12.2 ptrace

### 12.3 /proc

### 12.4 win32

### 12.5 shared libraries

### 12.6 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`.

`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.
`int gdbarch_fp0_regnum (`gdbarch`)`
  This functions normally returns 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.
`int gdbarch_get_longjmp_target (`gdbarch`)`
  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 function 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.

`I386_USE_GENERIC_WATCHPOINTS`
  An x86-based machine can define this to use the generic x86 watchpoint support; see I386_USE_GENERIC_WATCHPOINTS.
`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`.
`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`,` readsyms`)`
  Define this to expand into an expression that will cause the symbols in filename to be added to gdb's symbol table. If readsyms is zero symbols are not read but any necessary low level processing for filename is still done.
`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.