程序在得到一个Segmentation fault这样的错误信息毫无保留地就跳出来了，遇到这样的问题让人很痛苦，查找问题不亚于你N多天辛苦劳累编写代码的难度。那么有没有更好的方法可以在产生SIGSEGV信号的时候得到调试可用的信息呢？看看下面的例程吧！
sigsegv.h
#ifndef __sigsegv_h__
#define __sigsegv_h__
#ifdef __cplusplus
extern "C" {
#endif
int setup_sigsegv();
#ifdef __cplusplus
}
#endif
#endif /* __sigsegv_h__ */
sigsegv.c
#define _GNU_SOURCE
#include <memory.h>
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <ucontext.h>
#include <dlfcn.h>
#include <execinfo.h>
#ifndef NO_CPP_DEMANGLE
#include <cxxabi.h>
#endif
#if defined(REG_RIP)
# define SIGSEGV_STACK_IA64
# define REGFORMAT "%016lx"
#elif defined(REG_EIP)
# define SIGSEGV_STACK_X86
# define REGFORMAT "%08x"
#else
# define SIGSEGV_STACK_GENERIC
# define REGFORMAT "%x"
#endif
static void signal_segv(int signum, siginfo_t* info, void*ptr) {
static const char *si_codes[3] = {"", "SEGV_MAPERR", "SEGV_ACCERR"};
size_t i;
ucontext_t *ucontext = (ucontext_t*)ptr;
#if defined(SIGSEGV_STACK_X86) || defined(SIGSEGV_STACK_IA64)
int f = 0;
Dl_info dlinfo;
void **bp = 0;
void *ip = 0;
#else
void *bt[20];
char **strings;
size_t sz;
#endif
fprintf(stderr, "Segmentation Fault!/n");
fprintf(stderr, "info.si_signo = %d/n", signum);
fprintf(stderr, "info.si_errno = %d/n", info->si_errno);
fprintf(stderr, "info.si_code = %d (%s)/n", info->si_code, si_codes[info->si_code]);
fprintf(stderr, "info.si_addr = %p/n", info->si_addr);
for(i = 0; i < NGREG; i++)
fprintf(stderr, "reg[%02d] = 0x" REGFORMAT "/n", i, ucontext->uc_mcontext.gregs[i]);
#if defined(SIGSEGV_STACK_X86) || defined(SIGSEGV_STACK_IA64)
# if defined(SIGSEGV_STACK_IA64)
ip = (void*)ucontext->uc_mcontext.gregs[REG_RIP];
bp = (void**)ucontext->uc_mcontext.gregs[REG_RBP];
# elif defined(SIGSEGV_STACK_X86)
ip = (void*)ucontext->uc_mcontext.gregs[REG_EIP];
bp = (void**)ucontext->uc_mcontext.gregs[REG_EBP];
# endif
fprintf(stderr, "Stack trace:/n");
while(bp && ip) {
if(!dladdr(ip, &dlinfo))
break;
const char *symname = dlinfo.dli_sname;
#ifndef NO_CPP_DEMANGLE
int status;
char *tmp = __cxa_demangle(symname, NULL, 0, &status);
if(status == 0 && tmp)
symname = tmp;
#endif
fprintf(stderr, "% 2d: %p <%s+%u> (%s)/n",
++f,
ip,
symname,
(unsigned)(ip - dlinfo.dli_saddr),
dlinfo.dli_fname);
#ifndef NO_CPP_DEMANGLE
if(tmp)
free(tmp);
#endif
if(dlinfo.dli_sname && !strcmp(dlinfo.dli_sname, "main"))
break;
ip = bp[1];
bp = (void**)bp[0];
}
#else
fprintf(stderr, "Stack trace (non-dedicated):/n");
sz = backtrace(bt, 20);
strings = backtrace_symbols(bt, sz);
for(i = 0; i < sz; ++i)
fprintf(stderr, "%s/n", strings[i]);
#endif
fprintf(stderr, "End of stack trace/n");
exit (-1);
}
int setup_sigsegv() {
struct sigaction action;
memset(&action, 0, sizeof(action));
action.sa_sigaction = signal_segv;
action.sa_flags = SA_SIGINFO;
if(sigaction(SIGSEGV, &action, NULL) < 0) {
perror("sigaction");
return 0;
}
return 1;
}
#ifndef SIGSEGV_NO_AUTO_INIT
static void __attribute((constructor)) init(void) {
setup_sigsegv();
}
#endif
main.c
#include "sigsegv.h"
#include <string.h>
int die() {
char *err = NULL;
strcpy(err, "gonner");
return 0;
}
int main() {
return die();
}
下面来编译上面的main.c程序看看将会产生什么样的信息呢，不过要注意的就是如果要在你的程序里引用sigsegv.h、sigsegv.c得到堆栈信息的话记得加上-rdynamic -ldl参数。
/data/codes/c/test/backtraces $ gcc -o test -rdynamic -ldl -ggdb -g sigsegv.c main.c
/data/codes/c/test/backtraces $ ./test
Segmentation Fault!
info.si_signo = 11
info.si_errno = 0
info.si_code = 1 (SEGV_MAPERR)
info.si_addr = (nil)
reg[00] = 0x00000033
reg[01] = 0x00000000
reg[02] = 0xc010007b
reg[03] = 0x0000007b
reg[04] = 0x00000000
reg[05] = 0xb7fc8ca0
reg[06] = 0xbff04c2c
reg[07] = 0xbff04c1c
reg[08] = 0xb7f8cff4
reg[09] = 0x00000001
reg[10] = 0xbff04c50
reg[11] = 0x00000000
reg[12] = 0x0000000e
reg[13] = 0x00000006
reg[14] = 0x080489ec
reg[15] = 0x00000073
reg[16] = 0x00010282
reg[17] = 0xbff04c1c
reg[18] = 0x0000007b
Stack trace:
1: 0x80489ec <die+16> (/data/codes/c/test/backtraces/test)
2: 0x8048a16 <main+19> (/data/codes/c/test/backtraces/test)
End of stack trace
/data/codes/c/test/backtraces $
下面用gdb来看看出错的地方左右的代码：
/data/codes/c/test/backtraces $ gdb ./test
gdb> disassemble die+16
Dump of assembler code for function die:
0x080489dc <die+0>: push %ebp
0x080489dd <die+1>: mov %esp,%ebp
0x080489df <die+3>: sub $0x10,%esp
0x080489e2 <die+6>: movl $0x0,0xfffffffc(%ebp)
0x080489e9 <die+13>: mov 0xfffffffc(%ebp),%eax
0x080489ec <die+16>: movl $0x6e6e6f67,(%eax)
0x080489f2 <die+22>: movw $0x7265,0x4(%eax)
0x080489f8 <die+28>: movb $0x0,0x6(%eax)
0x080489fc <die+32>: mov $0x0,%eax
0x08048a01 <die+37>: leave
0x08048a02 <die+38>: ret
End of assembler dump.
gdb>
也可以直接break *die+16进行调试，看看在出错之前的堆栈情况，那么下面我们再来看看代码问题到底出在什么地方了。
/data/codes/c/test/backtraces $ gdb ./test
gdb> break *die+16
Breakpoint 1 at 0x80489f2: file main.c, line 6.
gdb> list *die+16
0x80489f2 is in die (main.c:6).
1 #include "sigsegv.h"
2 #include <string.h>
3
4 int die() {
5 char *err = NULL;
6 strcpy(err, "gonner");
7 return 0;
8 }
9
10 int main() {
gdb>
现在看看定位错误将会多么方便，上面的调试指令中list之前break不是必须的，只是让你可以看到break其实就已经指出了哪一行代码导致 Segmentation fault了。如果你要发布你的程序你一般会为了减少体积不会附带调试信息的(也就是不加-ggdb -g参数)，不过没关系，你一样可以得到上面stack-trace信息，然后你调试之前只要加上调试信息即可。
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