add answerback.c, tells us if we are on putty

answerback.c

@@ -0,0 +1,130 @@
+#define _POSIX_C_SOURCE 200112L
+
+#include <signal.h> /* for signal handling */
+#include <stdio.h> /* fopen(), et al. */
+#include <unistd.h> /* for ssize_t, read, write */
+#include <stdlib.h> /* for EXIT_SUCCESS, EXIT_FAILURE */
+#include <termios.h> /* ctermid, et al. */
+
+int main()
+{
+	const char *cterm = ctermid(NULL);
+
+	if (cterm == NULL)
+	{
+		fputs("Cannot get the path to the console", stderr);
+	}
+
+	FILE *fp;
+	if((fp = fopen(cterm, "r+b")) == NULL)
+	{
+		perror("open");
+		return EXIT_SUCCESS;
+	}
+
+	int fd = fileno(fp);
+	if (fd == -1)
+	{
+		perror("fileno");
+		return EXIT_FAILURE;
+	}
+
+	setbuf(fp, NULL);
+
+	sigset_t new_sig;
+	if (sigemptyset(&new_sig) == -1)
+	{
+		perror("sigemptyset");
+		return EXIT_FAILURE;
+	}
+	if (sigaddset(&new_sig, SIGINT) == -1)
+	{
+		perror("sigaddset");
+		return EXIT_FAILURE;
+	}
+	if (sigaddset(&new_sig, SIGTSTP) == -1)
+	{
+		perror("sigaddset");
+		return EXIT_FAILURE;
+	}
+	sigset_t old_sig;
+	if (sigprocmask(SIG_BLOCK, &new_sig, &old_sig) == -1)
+	{
+		perror("sigprocmask");
+		return EXIT_FAILURE;
+	}
+
+	struct termios old_term;
+	if (tcgetattr(fd, &old_term) == -1)
+	{
+		perror("tcgetattr");
+		return EXIT_FAILURE;
+	}
+
+	struct termios new_term = old_term;
+	new_term.c_lflag &= ~(ECHO | ECHOE | ECHOK | ECHONL | ICANON);
+	new_term.c_cc[VMIN] = 0;
+	new_term.c_cc[VTIME] = 10;
+
+	if (tcsetattr(fd, TCSAFLUSH, &new_term) == -1)
+	{
+		perror("tcsetattr");
+		if (tcsetattr(fd, TCSAFLUSH, &old_term) == -1)
+		{
+			perror("tcsetattr");
+		}
+		return EXIT_FAILURE;
+	}
+
+	char code[1] = { 5 };
+	for (;;)
+	{
+		ssize_t ret = write(fd, code, sizeof(code));
+		if (ret == -1)
+		{
+			perror("write");
+			tcsetattr(fd, TCSAFLUSH, &old_term);
+			return EXIT_FAILURE;
+		}
+		else if (ret > 0)
+		{
+			break;
+		}
+	}
+
+	char buffer[16] =  { 0 };
+	ssize_t ret = read(fd, buffer, sizeof(buffer) - 1);
+	if (ret == -1)
+	{
+		perror("read");
+		tcsetattr(fd, TCSAFLUSH, &old_term);
+		return EXIT_FAILURE;
+	}
+	buffer[ret] = '\0';
+
+	if (tcsetattr(fd, TCSAFLUSH, &old_term) == -1)
+	{
+		perror("tcsetattr");
+		return EXIT_FAILURE;
+	}
+
+	if (sigprocmask(SIG_SETMASK, &old_sig, NULL) == -1)
+	{
+		perror("sigprocmask");
+		return EXIT_FAILURE;
+	}
+
+	if (fclose(fp) == EOF)
+	{
+		perror("fclose");
+		return EXIT_FAILURE;
+	}
+
+	if (ret > 0)
+	{
+		puts(buffer);
+		return EXIT_SUCCESS;
+	}
+
+	return EXIT_FAILURE;
+}