Why do I get a warning “implicit declaration of function 'fopen_s'” and how can I get rid of it?

Question

I want to make a C program that creates files for my lab class.

But with this code I am getting a warning with the fopen_s function.

#include <stdio.h>

int main(void)
{
    int i , j , seira , sum , temp;

    printf("Give the number of exercise series:\n");
    scanf("%d" , &seira);
    printf("Give me the number of total exercises\n");
    scanf("%d" , &sum);
    
    FILE *fp;
    char name[FILENAME_MAX];
    j = seira;
    temp = sum;

    if (j < 10)
    {
        for (i = 1; i <= temp; i++)
        {
            _snprintf(name , sizeof(name) , "Homework0%d_Group03_%d.c", j , i);
            fopen_s(&fp , name , "w");
            fclose(fp);
        }
    }
    else if (j >= 10)
    {
        for (i = 1; i <= temp; i++)
        {
            _snprintf(name , sizeof(name) , "Homework%d_Group03_%d.c", j , i);
            fopen_s(&fp , name , "w");
            fclose(fp);
        }
    }

    return 0;
}

This is the warning I get with gcc:

How can I get rid of this warning?

Answer 1

You are using

fopen_s(&fp, name, "w");

According to https://en.cppreference.com/w/c/io/fopen , the function is declared in stdio.h , which you are including, but it is only available since the C11 standard.

And,

fopen_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including stdio.h .

So you need to enable C11 with -std=c11 if necessary (see How to enable c11 on later versions of gcc? ), and you need to define the __STDC_WANT_LIB_EXT1__ macro, if your compiler supports the function – which, as mentioned in the comments, seems unlikely. Otherwise, you cannot use the function.