可執行程序執行流程

./hello_world這個命令在linux內核系統中具體是怎麼執行的。文件系統使用busybox，對應的linux內核調用過程。

busybox中shell解析命令行

int ash_main(int argc, char **argv)
{

 state4: /* XXX ??? - why isn't this before the "if" statement */
		cmdloop(1);

}

static int
cmdloop(int top)
{
	union node *n;
	struct stackmark smark;
	int inter;
	int numeof = 0;

    printf("1. cmdloop\n");
	TRACE(("cmdloop(%d) called\n", top));
	for (;;) {
		} else if (nflag == 0) {
			/* job_warning can only be 2,1,0. Here 2->1, 1/0->0 */
            printf("3. cmdloop\n");
			job_warning >>= 1;
			numeof = 0;
			evaltree(n, 0);
			printf("4. cmdloop\n");
		}

}


static void
evaltree(union node *n, int flags)
{
	int checkexit = 0;
	void (*evalfn)(union node *, int);
	unsigned isor;
	int status;
	if (n == NULL) {
		TRACE(("evaltree(NULL) called\n"));
		goto out;
	}


	case NCMD:
		evalfn = evalcommand;
		printf("2. evaltree ncmd switch\n");
 checkexit:
		if (eflag && !(flags & EV_TESTED))
			checkexit = ~0;
		goto calleval;

 calleval:
			evalfn(n, flags);
			break;
		}
}


static void
evalcommand(union node *cmd, int flags)
{
	static const struct builtincmd bltin = {
		"\0\0", bltincmd
	};
	struct stackmark smark;

	/* Execute the command. */
	switch (cmdentry.cmdtype) {
	default:
     	printf("2. cmdentry.cmdtype default.\n");
		/* Fork off a child process if necessary. */
		if (!(flags & EV_EXIT) || trap[0]) {
			INT_OFF;
			jp = makejob(cmd, 1);
			if (forkshell(jp, cmd, FORK_FG) != 0) {
				exitstatus = waitforjob(jp);
				INT_ON;
				break;
			}
        	printf("3. cmdentry.cmdtype forkshell.\n");

			FORCE_INT_ON;
		}
		listsetvar(varlist.list, VEXPORT|VSTACK);
		shellexec(argv, path, cmdentry.u.index);
		/* NOTREACHED */
}

命令執行過程中這裏面會調用兩個系統調用forkshell和shellexec，通過shellexec舉例看系統調用

static void
shellexec(char **argv, const char *path, int idx)
{
	char *cmdname;
	int e;
	char **envp;
	int exerrno;

	clearredir(1);
	envp = environment();
	if (strchr(argv[0], '/')
#if ENABLE_FEATURE_SH_STANDALONE
	 || find_applet_by_name(argv[0])
#endif
	) {
		tryexec(argv[0], argv, envp);
		e = errno;
	} else {
		e = ENOENT;
		while ((cmdname = padvance(&path, argv[0])) != NULL) {
			if (--idx < 0 && pathopt == NULL) {
				tryexec(cmdname, argv, envp);
				if (errno != ENOENT && errno != ENOTDIR)
					e = errno;
			}
			stunalloc(cmdname);
		}
	}

}

static void
tryexec(char *cmd, char **argv, char **envp)
{
	int repeated = 0;

 repeat:
#ifdef SYSV
	do {
		execve(cmd, argv, envp);
	} while (errno == EINTR);
#else
	execve(cmd, argv, envp);
#endif
}

execve函數是gcc編譯工具提供的庫文件實現glibc-2.3.6/sysdeps/unix/sysv/linux/execve.c中定義

int
__execve (file, argv, envp)
     const char *file;
     char *const argv[];
     char *const envp[];
{
#if __BOUNDED_POINTERS__
  {
    char *const *v;
    int i;
    char *__unbounded *__unbounded ubp_argv;
    char *__unbounded *__unbounded ubp_envp;
    char *__unbounded *__unbounded ubp_v;

    for (v = argv; *v; v++)
      ;
    i = v - argv + 1;
    ubp_argv = (char *__unbounded *__unbounded) alloca (sizeof (*ubp_argv) * i);
    for (v = argv, ubp_v = ubp_argv; --i; v++, ubp_v++)
      *ubp_v = CHECK_STRING (*v);
    *ubp_v = 0;

    for (v = envp; *v; v++)
      ;
    i = v - envp + 1;
    ubp_envp = (char *__unbounded *__unbounded) alloca (sizeof (*ubp_envp) * i);
    for (v = envp, ubp_v = ubp_envp; --i; v++, ubp_v++)
      *ubp_v = CHECK_STRING (*v);
    *ubp_v = 0;

    return INLINE_SYSCALL (execve, 3, CHECK_STRING (file), ubp_argv, ubp_envp);
  }
#else
  return INLINE_SYSCALL (execve, 3, file, argv, envp);
#endif
}
weak_alias (__execve, execve)

最終調用到INLINE_SYSCALL宏在頭文件glibc-2.3.6/sysdeps/unix/sysv/linux/arm/sysdep.h中實現

/* Define a macro which expands into the inline wrapper code for a system
   call.  */
#undef INLINE_SYSCALL
#define INLINE_SYSCALL(name, nr, args...)				\
  ({ unsigned int _sys_result = INTERNAL_SYSCALL (name, , nr, args);	\
     if (__builtin_expect (INTERNAL_SYSCALL_ERROR_P (_sys_result, ), 0))	\
       {								\
	 __set_errno (INTERNAL_SYSCALL_ERRNO (_sys_result, ));		\
	 _sys_result = (unsigned int) -1;				\
       }								\
     (int) _sys_result; })

#undef INTERNAL_SYSCALL_DECL
#define INTERNAL_SYSCALL_DECL(err) do { } while (0)

#undef INTERNAL_SYSCALL
#define INTERNAL_SYSCALL(name, err, nr, args...)		\
  ({ unsigned int _sys_result;					\
     {								\
       register int _a1 asm ("a1");				\
       LOAD_ARGS_##nr (args)					\
       asm volatile ("swi	%1	@ syscall " #name	\
		     : "=r" (_a1)				\
		     : "i" (SYS_ify(name)) ASM_ARGS_##nr	\
		     : "memory");				\
       _sys_result = _a1;					\
     }								\
     (int) _sys_result; })

最終會調用到arm的switch指令進入軟中斷轉入操作系統中斷處理中進行處理。