#include <stdio.h>
 
struct transaccion
{
       long long importe;
} trx;
 
void myfwrite(char * buffer, size_t bytes, FILE *fp) {
    while(bytes)
        fputc(buffer[--bytes],fp);
}
 
int main(){
    trx.importe = 6938217;
    FILE *fp = fopen("prueba.bin","ab");
    myfwrite(&trx,sizeof(trx),fp);
    fclose(fp);
    return 0;
}
 
 

#include <iostream>
#include <fstream>
 
struct transaccion {
    long long importe;
} trx;
 
class myofstream : public std::basic_ofstream<char> {
public:
    std::ostream& mywrite(const char* s, std::streamsize n) {
        while(n)
            put(s[--n]);
        return *this;
    }
};
 
int main() {
    trx.importe=6938217;
    myofstream f;
    f.open("prueba.txt",std::ios::app|std::ios::binary);
    f.mywrite((const char*)&trx,sizeof(trx));
    f.close();
    return 0;
}