我正在研究名为" Headfirst C"的C文本中的一些示例代码。我写了一个演示信号处理的练习应用程序,在完成章节后决定玩一下。我是一名曾经在LabVIEW工作的工程师(高度并发和直观的事件处理功能),因此我对使用警报和信号处理程序生成定期事件感兴趣。我的问题是:
在下面的示例代码中,丢弃位于stdin中的可变数量的用户输入的正确方法或最佳实践是什么?我把这个小应用程序写成了一个演示版,一个3秒钟的警报触发了一个烦人的"你好!"用于中断fgets调用的消息。然而,我注意到的是,如果用户在打字过程中被打断,当他最终按下输入时,输入的任何文本(已被中断或未被中断)将被回显。我想丢弃在用户按下回车之前中断的任何内容。
//Sample Program - Signal Handling & Alarms
//Header Includes
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
//Function Declarations
//Handler Functions
void diediedie(int sig);
void howdy(int sig);
//Handler Register Function
int catchSignal(int signum, void(*handler)(int));
//Variable declarations
//Declare interrupted flags
static int interrupted = 0;
//Program entrypoint
int main() {
//Register interrupt hander, catch errors
if(catchSignal(SIGINT, diediedie) == -1) {
fprintf(stderr, "Could not register interrupt handler");
exit(2);
}
//Register alarm handler, catch errors
if(catchSignal(SIGALRM, howdy) == -1) {
fprintf(stderr, "Could not register alarm handler");
exit(2);
}
//Create initial alarm trigger
alarm(3);
//Do something stupid while waiting for signals
char name[30];
printf("Enter your name: ");
//Keep waiting for user input even if interrupted by alarm signal
while(1) {
fgets(name, 30, stdin);
if(interrupted) {
// reset interrupted flag
interrupted = 0;
// ***** ADD CODE TO DISCARD INTERRUPTED INPUT HERE ******
}
else {
//echo user input and break out
printf("Hello %s\n", name);
break;
}
}
//end program
return 0;
}
//interrupt handler definition
void diediedie(int sig) {
//write some stuff, exit program
puts("Goodbye world!");
exit(1);
}
//alarm handler definition
void howdy(int sig) {
//set interrupted flag
interrupted = 1;
//write some annoying message
puts("howdy!");
//set another alarm trigger
alarm(3);
//**** COULD STDIN BE FLUSHED HERE? ****
}
//signal handler registration function definition
int catchSignal(int signum, void(*handler)(int)) {
//register handler
struct sigaction action;
action.sa_handler = handler;
sigemptyset(&action.sa_mask);
action.sa_flags = 0;
return sigaction(signum, &action, NULL);
}
警报处理程序中是否有适当的清除方法?请注意注释表明我对正确代码位置的想法。
我考虑过以下事项:
while(getchar() != EOF) {}
我也想知道,当fgets正在等待用户输入并且SIGALRM被提升时会发生什么?这个功能终止了吗?我观察到,如果我不包括while循环来检查中断的标志并做出适当的响应,程序将完成fgets,在屏幕上转储一些垃圾(我假设stdin的当前状态?)并结束程序
感谢您的任何建议!
答案 0 :(得分:1)
要做到这一点,您需要将终端设置为“原始”模式,其中每次击键都会立即返回到应用程序,而不是通过行编辑字符的解释累积(“熟”模式,这是默认的。)
当然,如果你不让内核处理行编辑(例如退格),那么你需要自己动手,这是相当多的工作。解释退格键以及您感兴趣的其他编辑命令并不难,但在终端上保持正确的外观是一件痛苦的事。
有关详情,请参阅man termios
。
答案 1 :(得分:1)
在Unix中,信号处理程序与您的代码带外发生。如果信号发生在阻塞系统调用的中间,系统调用将退出并将errno
设置为EINTR。但是我相信fgets()
正在为你处理这种干扰并继续进行而不会让你失去控制权。
如果您使用的是基于Unix的操作系统并从命令行输入输入,那么这里真正发生的是您正在熟食模式下从终端读取数据。在按下返回之前,您的程序不会从TTY获取任何数据。您需要将终端设置为“原始”模式。以下是与代码集成的示例:
//Sample Program - Signal Handling & Alarms
//Header Includes
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <termios.h>
//Function Declarations
//Handler Functions
void diediedie(int sig);
void howdy(int sig);
//Handler Register Function
int catchSignal(int signum, void(*handler)(int));
//Variable declarations
//Declare interrupted sa_flags
static int interrupted = 0;
static struct termios save_termios;
//Program entrypoint
int main() {
struct termios buf;
int fd = 1;
// This is derived from from Stevens, "Advanced Programming in the UNIX Environment"
if (tcgetattr(fd, &save_termios) < 0) /* get the original state */
return -1;
buf = save_termios;
buf.c_lflag &= ~(ECHO | ICANON | IEXTEN | ISIG);
/* echo off, canonical mode off, extended input
processing off, signal chars off */
buf.c_iflag |= BRKINT | ICRNL;
/* SIGINT on BREAK, CR-toNL on */
buf.c_cflag &= ~(CSIZE | PARENB);
/* clear size bits, parity checking off */
buf.c_cflag |= CS8;
/* set 8 bits/char */
buf.c_oflag &= ~(OPOST);
/* output processing off */
buf.c_cc[VMIN] = 1; /* 1 byte at a time */
buf.c_cc[VTIME] = 0; /* no timer on input */
if (tcsetattr(fd, TCSAFLUSH, &buf) < 0)
return -1;
//Register interrupt hander, catch errors
if(catchSignal(SIGINT, diediedie) == -1) {
fprintf(stderr, "Could not register interrupt handler");
exit(2);
}
//Register alarm handler, catch errors
if(catchSignal(SIGALRM, howdy) == -1) {
fprintf(stderr, "Could not register alarm handler");
exit(2);
}
//Create initial alarm trigger
alarm(3);
//Do something stupid while waiting for signals
char name[30];
printf("Enter your name: ");
//Keep waiting for user input even if interrupted by alarm signal
char nextchar = 0;
char *p;
p = name;
while(nextchar != '\n') {
nextchar = fgetc(stdin);
if (interrupted) {
// reset interrupted flag
interrupted = 0;
//Discard interrupted input by reseting 'p' to the start of the buffer
p = name;
*p = 0;
continue;
}
if (nextchar == '\n') {
*p = 0;
fputc('\r', stdout);
fputc('\n', stdout);
break;
}
// You'll have to handle some characters manually to emulate what the
// terminal does, or you could filter them out using a function like isprint()
//
if (nextchar == 127) {
// *** handle backspace
if (p > name) {
p--;
}
// TODO: To handle this right you'll have to backup the cursor on the screen
} else {
*p = nextchar;
p++;
}
fputc(nextchar, stdout);
// Handle buffer overflow
if (p-name == sizeof(name) - 1) {
*p = 0;
break;
}
}
// echo user input
printf("Input is: %s\r\n", name);
tcsetattr(1, TCSAFLUSH, &save_termios);
}
//interrupt handler definition
void diediedie(int sig) {
//write some stuff, exit program
puts("Goodbye world!");
tcsetattr(1, TCSAFLUSH, &save_termios);
exit(1);
}
//alarm handler definition
void howdy(int sig) {
//set interrupted flag
interrupted = 1;
//write some annoying message
puts("howdy!");
//set another alarm trigger
alarm(3);
}
// signal handler registration function definition
int catchSignal(int signum, void(*handler)(int)) {
//register handler
struct sigaction action;
action.sa_handler = handler;
sigemptyset(&action.sa_mask);
action.sa_flags = 0;
return sigaction(signum, &action, NULL);
}
请注意,您需要保存原始终端设置并在程序退出前恢复它们!如果遇到麻烦,可能会破坏终端设置。使用命令行中的reset
或stty sane
恢复理智的终端设置。有关termios数据结构的更多信息,请参见手册页。
您还可以使用ncurses之类的库来处理原始输入。