#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()
{
    char campo1[5]="AAAA";
    char campo2[5]="XXXX";
    char salto='\n';
    trx.importe = 6938217;
    
    FILE *fp = fopen("Base.txt","ab");
    
    fprintf(fp,"%s",campo1);
    myfwrite((char*)&trx,sizeof(trx),fp);
    fprintf(fp,"%s",campo2);
    
    fputs(&salto,fp);
    
    fclose(fp);
    return 0;
}

AAAA[NUL][NUL][NUL][NUL][NUL]iÞiXXXX[LF]
XXXX

AAAA[NUL][NUL][NUL][NUL][NUL]iÞiXXXX[CR][LF]