gnu-netcat执行部分原理分析(1)

/* 如果根本没有给出 args，则从 stdin 中获取它们并生成 argv */
/* Cmd line: */
if (argc == 1)
    netcat_commandline_read(&argc, &argv);

  /* 检查命令行开关 */
while (TRUE) {
    int option_index = 0;
    static const struct option long_options[] = {
    { "close",    no_argument,        NULL, 'c' },
    { "debug",    no_argument,        NULL, 'd' },
    { "exec",    required_argument,    NULL, 'e' },
    { "gateway",    required_argument,    NULL, 'g' },
    { "pointer",    required_argument,    NULL, 'G' },
    { "help",    no_argument,        NULL, 'h' },
    { "interval",    required_argument,    NULL, 'i' },
    { "listen",    no_argument,        NULL, 'l' },
    { "tunnel",    required_argument,    NULL, 'L' },
    { "dont-resolve", no_argument,        NULL, 'n' },
    { "output",    required_argument,    NULL, 'o' },
    { "local-port",    required_argument,    NULL, 'p' },
    { "tunnel-port", required_argument,    NULL, 'P' },
    { "randomize",    no_argument,        NULL, 'r' },
    { "source",    required_argument,    NULL, 's' },
    { "tunnel-source", required_argument,    NULL, 'S' },
#ifndef USE_OLD_COMPAT
    { "tcp",    no_argument,        NULL, 't' },
    { "telnet",    no_argument,        NULL, 'T' },
#else
    { "tcp",    no_argument,        NULL, 1 },
    { "telnet",    no_argument,        NULL, 't' },
#endif
    { "udp",    no_argument,        NULL, 'u' },
    { "verbose",    no_argument,        NULL, 'v' },
    { "version",    no_argument,        NULL, 'V' },
    { "hexdump",    no_argument,        NULL, 'x' },
    { "wait",    required_argument,    NULL, 'w' },
    { "zero",    no_argument,        NULL, 'z' },
    { 0, 0, 0, 0 }
    };

c = getopt_long(argc, argv, "cde:g:G:hi:lL:no:p:P:rs:S:tTuvVxw:z",
            long_options, &option_index);
    if (c == -1)
      break;
    switch (c) {
        ...
        case 'e':            /* exec执行参数 */
        if (opt_exec)
        // 错误流 | 退出 | 报错日志(当e出现多次)
        ncprint(NCPRINT_ERROR | NCPRINT_EXIT,
            _("Cannot specify `-e' option double"));
        // 执行并返还.
        opt_exec = strdup(optarg);
        break;
        ...
    }

/* 将子进程进行重定向到socket */
dup2(ncsock->fd, STDIN_FILENO);    /* fiddlage 的精确顺序 */
close(ncsock->fd);            /* 显然是至关重要的;这是*/
dup2(STDIN_FILENO, STDOUT_FILENO);    /* swiped directly out of "inetd". */
dup2(STDIN_FILENO, STDERR_FILENO);    /* also duplicate the stderr channel */

/* 执行一个外部文件，使其 stdin/stdout/stderr 成为实际的socket */

static void ncexec(nc_sock_t *ncsock)
{
  int saved_stderr;
  char *p;
  assert(ncsock && (ncsock->fd >= 0));

  /* 保存 stderr fd，因为我们以后可能需要它 */
  saved_stderr = dup(STDERR_FILENO);

  /* 将子进程进行重定向到socket */
  dup2(ncsock->fd, STDIN_FILENO);    /* fiddlage 的精确顺序 */
  close(ncsock->fd);            /* 显然是至关重要的;这是*/
  dup2(STDIN_FILENO, STDOUT_FILENO);    /* swiped directly out of "inetd". */
  dup2(STDIN_FILENO, STDERR_FILENO);    /* also duplicate the stderr channel */

  /*更改已执行程序的标签*/
  if ((p = strrchr(opt_exec, '/')))
    p++;            /*较短的argv[0]*/
  else
    p = opt_exec;

  /* 用新的过程替换此过程 */
#ifndef USE_OLD_COMPAT
  execl("/bin/sh", p, "-c", opt_exec, NULL);
#else
  execl(opt_exec, p, NULL);
#endif
  dup2(saved_stderr, STDERR_FILENO);
  ncprint(NCPRINT_ERROR | NCPRINT_EXIT, _("Couldn't execute %s: %s"),
      opt_exec, strerror(errno));
}                /* 结束ncexec() */

int execl(const char *path, const char *arg, ...);

// clang hello.c -o hello
#include <stdio.h>
int main(int argc,char *argv[],char *envp[]){
        printf("Filename: %s\n",argv[0]);
        printf("%s %s\n",argv[1],argv[2]);
        return 0;
}

// clang return.c -o return
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
        char *temp,*temp1,*temp2;
        temp="test";  //Filename
        temp1="Funny";
        temp2="world";

        execl("hello",temp,temp1,temp2,NULL);
        printf("Error");
        return 0;
}a
/* return:
 * Filename: test
 * Funny world
**/