Compiling c code with bool without using c99 standard

问题

I've tried to compile a code using a bool variable in C and I've included the stdbool header but when I compiled it I didn't specify that I want to compile it with the c99 standard (so it was compiled with ANSI C standard) but it worked anyway. I was wondering why is that ? Here's the code :

#include <stdio.h>
#include <stdbool.h>

int main() {
    char name[20];
    printf("What's your name ? ");
    gets(name);
    printf("Nice to meet you %s.\n", name);
    bool exit = false;
    char c;
    printf("Do you wish to exit the program ? (Y/N) ");
    while (!exit) {
        c = getchar();
        if (c == '\n') {
            continue;
        }
        printf("Do you wish to exit the program ? (Y/N) ");
        if (c == 'Y' || c == 'y') {
            exit = true;
        }
    }
    printf("Have a nice day %s\n", name);
    return 0;
}

Also another question regarding to my code. In the part where you are being asked if you wish to exit the program, I've tested it with the following input : n n y

And for some reason it printed out to the console the question for the fourth time and I don't see why. I've set it so if the input is Y/y the next iteration in the while loop shouldn't take place but for some reason it printed it again, could someone explain me what I did wrong ?

EDIT : So I've edited the code a bit tried to test new things and I've noticed that with the following code if the user input is Y/y it won't come out of the loop :

#include <stdio.h>
#include <stdbool.h>

int main(int argc, char* argv[]) {
    for(int i = 0; i < argc; i++)
        printf("argv[%d] = %s\n", i, argv[i]);
    char name[20];
    printf("What's your name ? ");
    gets(name);
    char lastname[20];
    printf("%s what's your last name ? ", name);
    fgets(lastname, 20, stdin);
    int age;
    printf("%s %s what's your age? ", name, lastname);
    scanf("%d", &age);
    bool exit = false;
    char c;
    while (!exit) {
        printf("Do you wish to exit the program ? (Y/N) ");
        c = getchar();
        getchar();
        if (c == 'Y' || c == 'y')
            exit = true;
    }
    printf("Have a nice day %s %s.\n", name, lastname);
    return 0;
}

I don't know why I did it but I added a getchar() call before the while loop and tried to compile it this way and then the program worked fine, from this I assume that the fgets\scanf functions interfering with the getchar function but I'm not sure why could someone explain ?

回答1:

Most C compilers extend the base language with extensions. One such extension could be to let the stdbool.h work even in C90 mode. If you really want to, you can usually turn off most of the extensions with some compiler flag, e.g. for gcc use -std=c90. Not sure about extra headers, the file is still there after all, so it can probably be included regardless of mode.

For your second question, try stepping through the program, printing the value of c at each step. It should make it fairly obvious what's happening.

回答2:

About second question:

Your code deciding to leave program or not is strange, it can be done simpler way:

int c=0;
do{
    if(c!='\n')
        puts("Do you wish to exit the program? (Y/N) ");
    c=getchar();
}while(c!='y' && c!='Y');

When code is simpler, it's harder to make mistake in it.

回答3:

stdbool.h defines bool, true, and false for you - the header itself is not dependent on the language standard you pass in, so long as you have the header on your system. EDIT - this may only be the case for the stock osx header. gcc's header (4.8) doesn't look like it will work.

Your logic for exiting the loop doesn't look like how you describe what you want it to do. You're still testing the previous character in your y/Y section, even though you printed out the question again at that point. I suggest stepping through your code with a debugger - this will make it more clear to you what's going on. The debugger will be an incredibly important learning tool for you, now and forever. :)

You probably want something like this for your code (remove the first printf question in your code):

while (!exit) {
    printf("Do you wish to exit the program ? (Y/N) ");
    c = getchar();
    if (c == '\n') {
        continue;
    }
    if (c == 'Y' || c == 'y') {
        exit = true;
    }
}

来源：https://stackoverflow.com/questions/25534544/compiling-c-code-with-bool-without-using-c99-standard

标签

while-loop

boolean

c99

getchar