How to use atoi conversion in C using gets_s - example code included

Question

I'm teaching myself about structures in C, and am having trouble compiling this code:

#include <stdio.h>
#include <stdlib.h>

struct Date {
    int Month;
    int Day;
    int Year;
};

void AddDecade(struct Date);

int main(int argc, char *argv[]) {
    struct Date BDay;
    char buffer[50];

    printf("What month were you born? ");
    BDay.Month = atoi(gets_s(buffer, 50));

    printf("What day were you born? ");
    BDay.Day = atoi(gets_s(buffer, 50));

    printf("What year were you born? ");
    BDay.year = atoi(gets_s(buffer, 50));

    printf("You were born on %d, %d, %d?\n", BDay.Month, BDay.Day, BDay.Year);

    AddDecade(BDay);

    printf("You will be 10 years older on %d, %d, %d\n", BDay.Month, BDay.Day, BDay.Year);
}

void AddDecade(struct Date Target) {
    Target.Year += 10;
}

The author of the code compiled it without error on a Windows machine, but on my Linux machine gcc gives the following errors:

07_04_structures2.c: In function ‘main’:

07_04_structures2.c:18:5: warning: passing argument 1 of ‘atoi’ makes pointer from integer without a cast [enabled by default]

/usr/include/stdlib.h:148:12: note: expected ‘const char *’ but argument is of type ‘int’

07_04_structures2.c:21:5: warning: passing argument 1 of ‘atoi’ makes pointer from integer without a cast [enabled by default]

/usr/include/stdlib.h:148:12: note: expected ‘const char *’ but argument is of type ‘int’

07_04_structures2.c:24:9: error: ‘struct Date’ has no member named ‘year’

07_04_structures2.c:24:5: warning: passing argument 1 of ‘atoi’ makes pointer from integer without a cast [enabled by default]

/usr/include/stdlib.h:148:12: note: expected ‘const char *’ but argument is of type ‘int’

Given this line:

BDay.Month = atoi(gets_s(buffer, 50));

My understanding is that gets_s copies a maximum of 50 bytes of input into buffer, and passes a pointer to the variable 'buffer', to atoi, but according to this error:

note: expected ‘const char *’ but argument is of type ‘int’

perhaps gets_s is at fault here? I've never used it before...

I'd be very thankful for a detailed explanation of what's wrong, and how to fix it.

Thanks a lot!

---

Update:

I implemented all of your recommendations and have come up with the following working code:

#include <stdio.h>
#include <stdlib.h>

struct Date {
    int Month; 
    int Day;
    int Year;
};

struct Date AddDecade(struct Date);

int main(int argc, char *argv[]) {
    struct Date BDay;
    char buffer[50];
    char *buffer_end;

    printf("What month were you born? ");
    fgets(buffer, sizeof(buffer), stdin);
    BDay.Month = strtol(buffer, &buffer_end, 10);

    printf("What day were you born? ");
    fgets(buffer, sizeof(buffer), stdin);
    BDay.Day = strtol(buffer, &buffer_end, 10);

    printf("What year were you born? ");
    fgets(buffer, sizeof(buffer), stdin);
    BDay.Year = strtol(buffer, &buffer_end, 10);

    printf("You were born on %d, %d, %d?\n", BDay.Month, BDay.Day, BDay.Year);

    BDay = AddDecade(BDay);

    printf("You will be 10 years older on %d, %d, %d\n", BDay.Month, BDay.Day, BDay.Year);
}

struct Date
AddDecade(struct Date Target) {
    Target.Year += 10;
    return Target;
}

This has been a great learning experience for me! Thank you! :)

Answer 1

get_s() isn't implemented in the Linux C library, so the compiler assumes the default of it being a function that returns and int takes whatever (promoted) parameters you pass it.

You'll need to recode to avoid using gets_s(). Probably the simplest fix is to use fgets() instead. Change the lines similar to:

BDay.Month = atoi(gets_s(buffer, 50));

with:

fgets(buffer, sizeof(buffer), stdin);
BDay.Month = atoi(buffer);

Answer 2

gets_s is a function defined in C11 and it is defined as an optional extension.

You have to compile your program with C11 support otherwise the gets_s function is not declared in stdio.h. When the function is not declared in stdio.h and there is no available prototype for the function like in your program, gcc will then implicity provide a prototype for gets_s that returns an int (implicit function declaration).

On gcc compiling with C11 is done with option -std=c11 on gcc 4.7 and -std=c1x in older gcc. Note that as of today support of C11 in gcc (and Linux C libraries like glibc or eglibc) is still very incomplete and gets_s may be also not declared even when C11 option is enabled (not to mention the extension is optional).

Answer 3

gets_s is not standard function in most versions of C language.

Since you didn't declare get_s, the compiler assume (as default) that it returns int, and hence the error.

Also, look at the error at line 24. (you should change year to Year)