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/usr/include/x86_64-linux-gnu/sys/ucontext.h is in libc6-dev 2.27-3ubuntu1.

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/* Copyright (C) 2001-2018 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

#ifndef _SYS_UCONTEXT_H
#define _SYS_UCONTEXT_H	1

#include <features.h>

#include <bits/types.h>
#include <bits/types/sigset_t.h>
#include <bits/types/stack_t.h>


#ifdef __USE_MISC
# define __ctx(fld) fld
#else
# define __ctx(fld) __ ## fld
#endif

#ifdef __x86_64__

/* Type for general register.  */
__extension__ typedef long long int greg_t;

/* Number of general registers.  */
#define __NGREG	23
#ifdef __USE_MISC
# define NGREG	__NGREG
#endif

/* Container for all general registers.  */
typedef greg_t gregset_t[__NGREG];

#ifdef __USE_GNU
/* Number of each register in the `gregset_t' array.  */
enum
{
  REG_R8 = 0,
# define REG_R8		REG_R8
  REG_R9,
# define REG_R9		REG_R9
  REG_R10,
# define REG_R10	REG_R10
  REG_R11,
# define REG_R11	REG_R11
  REG_R12,
# define REG_R12	REG_R12
  REG_R13,
# define REG_R13	REG_R13
  REG_R14,
# define REG_R14	REG_R14
  REG_R15,
# define REG_R15	REG_R15
  REG_RDI,
# define REG_RDI	REG_RDI
  REG_RSI,
# define REG_RSI	REG_RSI
  REG_RBP,
# define REG_RBP	REG_RBP
  REG_RBX,
# define REG_RBX	REG_RBX
  REG_RDX,
# define REG_RDX	REG_RDX
  REG_RAX,
# define REG_RAX	REG_RAX
  REG_RCX,
# define REG_RCX	REG_RCX
  REG_RSP,
# define REG_RSP	REG_RSP
  REG_RIP,
# define REG_RIP	REG_RIP
  REG_EFL,
# define REG_EFL	REG_EFL
  REG_CSGSFS,		/* Actually short cs, gs, fs, __pad0.  */
# define REG_CSGSFS	REG_CSGSFS
  REG_ERR,
# define REG_ERR	REG_ERR
  REG_TRAPNO,
# define REG_TRAPNO	REG_TRAPNO
  REG_OLDMASK,
# define REG_OLDMASK	REG_OLDMASK
  REG_CR2
# define REG_CR2	REG_CR2
};
#endif

struct _libc_fpxreg
{
  unsigned short int __ctx(significand)[4];
  unsigned short int __ctx(exponent);
  unsigned short int __glibc_reserved1[3];
};

struct _libc_xmmreg
{
  __uint32_t	__ctx(element)[4];
};

struct _libc_fpstate
{
  /* 64-bit FXSAVE format.  */
  __uint16_t		__ctx(cwd);
  __uint16_t		__ctx(swd);
  __uint16_t		__ctx(ftw);
  __uint16_t		__ctx(fop);
  __uint64_t		__ctx(rip);
  __uint64_t		__ctx(rdp);
  __uint32_t		__ctx(mxcsr);
  __uint32_t		__ctx(mxcr_mask);
  struct _libc_fpxreg	_st[8];
  struct _libc_xmmreg	_xmm[16];
  __uint32_t		__glibc_reserved1[24];
};

/* Structure to describe FPU registers.  */
typedef struct _libc_fpstate *fpregset_t;

/* Context to describe whole processor state.  */
typedef struct
  {
    gregset_t __ctx(gregs);
    /* Note that fpregs is a pointer.  */
    fpregset_t __ctx(fpregs);
    __extension__ unsigned long long __reserved1 [8];
} mcontext_t;

/* Userlevel context.  */
typedef struct ucontext_t
  {
    unsigned long int __ctx(uc_flags);
    struct ucontext_t *uc_link;
    stack_t uc_stack;
    mcontext_t uc_mcontext;
    sigset_t uc_sigmask;
    struct _libc_fpstate __fpregs_mem;
  } ucontext_t;

#else /* !__x86_64__ */

/* Type for general register.  */
typedef int greg_t;

/* Number of general registers.  */
#define __NGREG	19
#ifdef __USE_MISC
# define NGREG	__NGREG
#endif

/* Container for all general registers.  */
typedef greg_t gregset_t[__NGREG];

#ifdef __USE_GNU
/* Number of each register is the `gregset_t' array.  */
enum
{
  REG_GS = 0,
# define REG_GS		REG_GS
  REG_FS,
# define REG_FS		REG_FS
  REG_ES,
# define REG_ES		REG_ES
  REG_DS,
# define REG_DS		REG_DS
  REG_EDI,
# define REG_EDI	REG_EDI
  REG_ESI,
# define REG_ESI	REG_ESI
  REG_EBP,
# define REG_EBP	REG_EBP
  REG_ESP,
# define REG_ESP	REG_ESP
  REG_EBX,
# define REG_EBX	REG_EBX
  REG_EDX,
# define REG_EDX	REG_EDX
  REG_ECX,
# define REG_ECX	REG_ECX
  REG_EAX,
# define REG_EAX	REG_EAX
  REG_TRAPNO,
# define REG_TRAPNO	REG_TRAPNO
  REG_ERR,
# define REG_ERR	REG_ERR
  REG_EIP,
# define REG_EIP	REG_EIP
  REG_CS,
# define REG_CS		REG_CS
  REG_EFL,
# define REG_EFL	REG_EFL
  REG_UESP,
# define REG_UESP	REG_UESP
  REG_SS
# define REG_SS	REG_SS
};
#endif

/* Definitions taken from the kernel headers.  */
struct _libc_fpreg
{
  unsigned short int __ctx(significand)[4];
  unsigned short int __ctx(exponent);
};

struct _libc_fpstate
{
  unsigned long int __ctx(cw);
  unsigned long int __ctx(sw);
  unsigned long int __ctx(tag);
  unsigned long int __ctx(ipoff);
  unsigned long int __ctx(cssel);
  unsigned long int __ctx(dataoff);
  unsigned long int __ctx(datasel);
  struct _libc_fpreg _st[8];
  unsigned long int __ctx(status);
};

/* Structure to describe FPU registers.  */
typedef struct _libc_fpstate *fpregset_t;

/* Context to describe whole processor state.  */
typedef struct
  {
    gregset_t __ctx(gregs);
    /* Due to Linux's history we have to use a pointer here.  The SysV/i386
       ABI requires a struct with the values.  */
    fpregset_t __ctx(fpregs);
    unsigned long int __ctx(oldmask);
    unsigned long int __ctx(cr2);
  } mcontext_t;

/* Userlevel context.  */
typedef struct ucontext_t
  {
    unsigned long int __ctx(uc_flags);
    struct ucontext_t *uc_link;
    stack_t uc_stack;
    mcontext_t uc_mcontext;
    sigset_t uc_sigmask;
    struct _libc_fpstate __fpregs_mem;
  } ucontext_t;

#endif /* !__x86_64__ */

#undef __ctx

#endif /* sys/ucontext.h */