Linux程序設計第四版 筆記

1：5.2節中與終端進行對話

      如果不希望程序中與用戶交互的部分被重定向，單允許其他的輸入和輸出唄重定向，你就需要將與用戶交互的部分與stdout和stderr分開，因此，可以直接對/dev/tty進行讀寫，該設備始終指向當前終端或當前的登錄會話。代碼如下：

#include <stdio.h>
#include <unistd.h>

char *menu[] = {
    "a - add new record",
    "d - delete record",
    "q - quit",
    NULL,
};

int getchoice(char *greet, char *choices[], FILE *in, FILE *out);

int main()
{
    int choice = 0;
    FILE *input;
    FILE *output;

    if (!isatty(fileno(stdout))) {
        fprintf(stderr,"You are not a terminal, OK.\n");
    }

    input = fopen("/dev/tty", "r");
    output = fopen("/dev/tty", "w");
    if(!input || !output) { 
        fprintf(stderr,"Unable to open /dev/tty\n");
        exit(1);
    }

    do {
        choice = getchoice("Please select an action", menu, input, output);
        printf("You have chosen: %c\n", choice);
    } while (choice != 'q');
    exit(0);
}

int getchoice(char *greet, char *choices[], FILE *in, FILE *out)
{
    int chosen = 0;
    int selected;
    char **option;

    do {
        fprintf(out,"Choice: %s\n",greet);
        option = choices;
        while(*option) {
            fprintf(out,"%s\n",*option);
            option++;
        }
        do {
            selected = fgetc(in);
        } while (selected == '\n');
        option = choices;
        while(*option) {
            if(selected == *option[0]) {
                chosen = 1;
                break;
            }
            option++;
        }
        if(!chosen) {
            fprintf(out,"Incorrect choice, select again\n");
        }
    } while(!chosen);
    return selected;
}

執行效果：

$ ./a.out >file
You are not a terminal, OK.
Choice: Please select an action
a - add new record
d - delete record
q - quit
d
Choice: Please select an action
a - add new record
d - delete record
q - quit
q
$ cat file
You have chosen: d
You have chosen: q
$ 

源碼解析：

其實就是把不需要重定向的信息，即仍然要顯示在屏幕上的信息顯示在設備/dev/tty中，而本來通過stdin/stdout/stderr的信息就重定向到file文件中了，不顯示到屏幕上。