How to resolve REG_EIP undeclared (First use in this function ) error on Linux 32 bit machine?

Question

I have been coming across errors in compilation of my signal handler program written in C language with gcc in displaying the dumped register values after occurance of Segmentation fault. When i tried to access it using the code:

void print_registers(FILE *fd, ucontext_t *ctx, bool fpu = false)
{
        const char *flags_str[] = {
                "CF", 0, "PF", 0, "AF", 0, "ZF", "SF", "TP", "IF", "DF",
                "OF", 0, 0, "NT", 0, "RF", "VM", "AC", "VIF", "VIP", "ID"
        };

        greg_t *regs = ctx->uc_mcontext.gregs;

        void *eip[1] = { (void*)regs[REG_EIP] };

        char **symbol = backtrace_symbols(eip, 1);

        fprintf(fd, "Registers:\neip is at ");

         backtrace_symbols_fd(eip, 1, fd->_fileno);

        size_type flags = regs[REG_EFL];
        fprintf(fd, "eflags: %x [ ", flags);
        for (size_type i = 0; i < sizeof(flags_str) / sizeof(flags_str[0]); ++i) {
                if (!flags_str[i]) continue;
                if (flags & (1 << i)) fprintf(fd, "%s ", flags_str[i]);
        }
        size_type iopl = (flags & 0x3000) >> 12;

            fprintf(fd, "] iopl: %i\n"
                                        "eax: %08x\tebx: %08x\tecx: %08x\tedx: %08x\n"
                                        "esi: %08x\tedi: %08x\tebp: %08x\tesp: %08x\n"
                                        "cs: %04x\tgs: %04x\tfs: %04x\n"
                                        "ds: %04x\tes: %04x\tss: %04x\n",
                                        iopl,
                                        regs[REG_EAX], regs[REG_EBX], regs[REG_ECX], regs[REG_EDX],
                                        regs[REG_ESI], regs[REG_EDI], regs[REG_EBP], regs[REG_ESP],
                                        regs[REG_CS], regs[REG_GS], regs[REG_FS],
                                        regs[REG_DS], regs[REG_ES], regs[REG_SS]);
                    }
                }

I tried the code by adding

                 #include<sys/ucontext.h> 

as well as

                 #define _GNU_SOURCE
                 #ifndef REG_EIP
                 #define REG_EIP 0x23b46F
                 #endif

But, the error appearing is:

 ‘REG_EIP’ undeclared (first use in this function) 
 (Each undeclared identifier is reported only once  for each function it appears in.)

and the error is appearing for all registers

I tried many documents...but couldn't get the solution. Can anybody share the details for resolving this error.

Advance thanks to all repliers

Answer 1

I believe you should either have #define _GNU_SOURCE as the first line of your source file, or better put -D_GNU_SOURCE in your CFLAGS (on the command line). Then make sure you include <signal.h> and <ucontext.h>.

Answer 2

Try defining __USE_GNU before including <ucontext.h:

#define __USE_GNU
#include <ucontext.h>

You don't need to include <sys/ucontext.h> explicitly, <ucontext.h> will do that.