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);