As the previous speakers said, there are usually no complete solutions here, but help where it makes sense. The original C ++ code is very bad.There are many questionable places and without a description you would have to do reverse engineering here in order to port it useful. Most of it is rather easy to port. There are also places where the C ++ features are missing and you would have to make some effort. Here's a little help to get you started.
#include <stdio.h>
#include <stdlib.h>
typedef struct {
float p,t;
} attend;
void enter(attend *at);
void calculate(attend *at);
int readdate(int* date, int* month, int* year)
{
return 1;
}
int main()
{
attend o1;
...
return 0;
}