Question

#include "stdlib.h"
#include "stdio.h"
#include "string.h"
#include "termios.h"

int main (int ac, char* av[]) {
  struct termios ttyinfo;
  int result;

  result = tcgetattr(0, &ttyinfo);
  if (result == -1) {
    perror("cannot get params about stdin");
    exit (1);
  }

  if (av[1] == "stop" && av[2] == "A") {
    printf ("Stop: ^%c\n", ttyinfo.c_cc[VSTOP] - 19 + 'A');
  }
  if (av[1] == "start" && av[2] == "^Q") {
    printf ("Stop: ^%c\n", ttyinfo.c_cc[VSTOP] - 3 + 'A');
  }
  return 0;
}

我正在学习Linux，这段代码用C语言编写。使用命令行显示字符更改。例如：./示例停止A.但是，它不会在屏幕上显示任何内容。

Answer 1

使用C时应该打开警告，你很可能会发现失败的原因。如果你用它来用Clang编译它

gcc -Wall -std=c11 -pedantic goo.c

你会遇到这些错误：

goo.c:19:13: warning: result of comparison against a string literal is unspecified (use strncmp instead) [-Wstring-compare] 
if (av[1] == "stop" && av[2] == "A")
        ^  ~~~~~~
goo.c:19:32: warning: result of comparison against a string literal is unspecified (use strncmp instead) [-Wstring-compare]
if (av[1] == "stop" && av[2] == "A")
                           ^  ~~~
goo.c:24:13: warning: result of comparison against a string literal is unspecified (use strncmp instead) [-Wstring-compare]
 if (av[1] == "start" && av[2] == "^Q")
        ^  ~~~~~~~
goo.c:24:33: warning: result of comparison against a string literal is unspecified (use strncmp instead) [-Wstring-compare]
if (av[1] == "start" && av[2] == "^Q")

您需要使用字符串比较函数比较字符串。您无法使用==按照您的方式比较字符串。尝试这样的事情：

#include "stdlib.h"
#include "stdio.h"
#include "string.h"
#include "termios.h"

int main (int ac, char* av[])
{
  struct termios ttyinfo;
  int result;
  result = tcgetattr(0, &ttyinfo);
  if (result == -1) {
    perror("cannot get params about stdin");
    exit (1);
  }

  if(ac > 2) {
    if (strcmp(av[1], "stop") == 0 && strcmp(av[2], "A") == 0) {
      printf ("Stop: ^%c\n", ttyinfo.c_cc[VSTOP] - 19 + 'A');
    }   
    if (strcmp(av[1], "start") == 0 && strcmp(av[2], "^Q") == 0) {
      printf ("Stop: ^%c\n", ttyinfo.c_cc[VSTOP] - 3 + 'A');
    }   
  }
  else {
    printf("Need two arguments\n");
  }
  return 0;
}

阅读strncmp和strcmp。特别要确保知道为什么以及何时strncmp优于strcmp。

为什么这个程序不打印任何东西？

1 个答案: