How to write a file owned by root in C?

Question

How can I to write into a file owned by root, for example in /etc ? In particular I need to have the executable owned by a normal user therefore I can't run it as root nor call it with sudo.

I know this is not a specific C problem, however I'd like to know a possible solution in C. The same solution in another language may help as well.

Example using a C programming language

/*
 How can I write into a file owned by root user?

 -rwxrwxr-x  1 user user 8480 ott 17 11:48 test
 -rw-rw-r--  1 user user  432 ott 17 11:48 test.c
 -rw-rw-r--  1 root root   20 ott 17 11:48 textfile.txt

 $ ./test 
Test how to write a file owned by root
File can't be opened
Segmentation fault (core dumped)
*/

#include <stdio.h>

int main(int argc, char **argv)
{
    printf("Test how to write a file owned by root\n");

    FILE *fp;
 
    fp = fopen("textfile.txt", "w");
 
    if (fp != NULL) {
        fprintf(fp, "Wite success!\n");
        printf("\n Details successfully written to the file\n\n");
    }
    else
        printf("File can't be opened\n");
 
    fclose(fp);

    return 0;
}

Thank you