### 9.1 Operating System ABI Variant Handling

gdb provides a mechanism for handling variations in OS ABIs. An OS ABI variant may have influence over any number of variables in the target architecture definition. There are two major components in the OS ABI mechanism: sniffers and handlers.

A sniffer examines a file matching a BFD architecture/flavour pair (the architecture may be wildcarded) in an attempt to determine the OS ABI of that file. Sniffers with a wildcarded architecture are considered to be generic, while sniffers for a specific architecture are considered to be specific. A match from a specific sniffer overrides a match from a generic sniffer. Multiple sniffers for an architecture/flavour may exist, in order to differentiate between two different operating systems which use the same basic file format. The OS ABI framework provides a generic sniffer for ELF-format files which examines the `EI_OSABI` field of the ELF header, as well as note sections known to be used by several operating systems.

A handler is used to fine-tune the `gdbarch` structure for the selected OS ABI. There may be only one handler for a given OS ABI for each BFD architecture.

The following OS ABI variants are defined in `defs.h`:

`GDB_OSABI_UNINITIALIZED`
  Used for struct gdbarch_info if ABI is still uninitialized.

`GDB_OSABI_UNKNOWN`
  The ABI of the inferior is unknown. The default `gdbarch` settings for the architecture will be used.

`GDB_OSABI_SVR4`
  UNIX System V Release 4.

`GDB_OSABI_HURD`
  GNU using the Hurd kernel.

`GDB_OSABI_SOLARIS`
  Sun Solaris.

`GDB_OSABI_OSF1`
  OSF/1, including Digital UNIX and Compaq Tru64 UNIX.

`GDB_OSABI_LINUX`
  GNU using the Linux kernel.

`GDB_OSABI_FREEBSD_AOUT`
  FreeBSD using the `a.out` executable format.

`GDB_OSABI_FREEBSD_ELF`
  FreeBSD using the ELF executable format.

`GDB_OSABI_NETBSD_AOUT`
  NetBSD using the `a.out` executable format.

`GDB_OSABI_NETBSD_ELF`
  NetBSD using the ELF executable format.

`GDB_OSABI_OPENBSD_ELF`
  OpenBSD using the ELF executable format.

`GDB_OSABI_WINCE`
  Windows CE.

`GDB_OSABI_GO32`
  DJGPP.

`GDB_OSABI_IRIX`
  Irix.

`GDB_OSABI_INTERIX`
  Interix (Posix layer for MS-Windows systems).

`GDB_OSABI_HPUX_ELF`
  HP/UX using the ELF executable format.

`GDB_OSABI_HPUX_SOM`
  HP/UX using the SOM executable format.

`GDB_OSABI_QNXNTO`
  QNX Neutrino.

`GDB_OSABI_CYGWIN`
  Cygwin.

`GDB_OSABI_AIX`
  AIX.

Here are the functions that make up the OS ABI framework:

— Function: const char *gdbarch_osabi_name (enum gdb_osabi osabi)

> Return the name of the OS ABI corresponding to osabi.

— Function: void gdbarch_register_osabi (enum bfd_architecture arch, unsigned long machine, enum gdb_osabi osabi, void (*init_osabi)(struct gdbarch_info info, struct gdbarch *gdbarch))

> Register the OS ABI handler specified by init_osabi for the architecture, machine type and OS ABI specified by arch, machine and osabi. In most cases, a value of zero for the machine type, which implies the architecture's default machine type, will suffice.

— Function: void gdbarch_register_osabi_sniffer (enum bfd_architecture arch, enum bfd_flavour flavour, enum gdb_osabi (*sniffer)(bfd *abfd))

> Register the OS ABI file sniffer specified by sniffer for the BFD architecture/flavour pair specified by arch and flavour. If arch is `bfd_arch_unknown`, the sniffer is considered to be generic, and is allowed to examine flavour-flavoured files for any architecture.

— Function: enum gdb_osabi gdbarch_lookup_osabi (bfd *abfd)

> Examine the file described by abfd to determine its OS ABI. The value `GDB_OSABI_UNKNOWN` is returned if the OS ABI cannot be determined.

— Function: void gdbarch_init_osabi (struct gdbarch info info, struct gdbarch *gdbarch, enum gdb_osabi osabi)

> Invoke the OS ABI handler corresponding to osabi to fine-tune the `gdbarch` structure specified by gdbarch. If a handler corresponding to osabi has not been registered for gdbarch's architecture, a warning will be issued and the debugging session will continue with the defaults already established for gdbarch.

— Function: void generic_elf_osabi_sniff_abi_tag_sections (bfd *abfd, asection *sect, void *obj)

> Helper routine for ELF file sniffers. Examine the file described by abfd and look at ABI tag note sections to determine the OS ABI from the note. This function should be called via `bfd_map_over_sections`.