/*  We start by defining a RECORD structure
    and then create NRECORDS versions each recording their number.
    These are appended to the file records.dat.  */

#include <unistd.h>
#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>

typedef struct {
    int integer;
    char string[24];
} RECORD;

#define NRECORDS (100)

int main()
{
    RECORD record, *mapped;
    int i, f;
    FILE *fp;

    fp = fopen("records.dat","w+");
    for(i=0; i<NRECORDS; i++) {
        record.integer = i;
        sprintf(record.string,"RECORD-%d",i);
        fwrite(&record,sizeof(record),1,fp);
    }
    fclose(fp);

/*  We now change the integer value of record 43 to 143
    and write this to the 43rd record's string.  */

    fp = fopen("records.dat","r+");
    fseek(fp,43*sizeof(record),SEEK_SET);
    fread(&record,sizeof(record),1,fp);

    record.integer = 143;
    sprintf(record.string,"RECORD-%d",record.integer);

    fseek(fp,43*sizeof(record),SEEK_SET);
    fwrite(&record,sizeof(record),1,fp);
    fclose(fp);

/*  We now map the records into memory
    and access the 43rd record in order to change the integer to 243
    (and update the record string), again using memory mapping.  */

    f = open("records.dat",O_RDWR);
    mapped = (RECORD *)mmap(0, NRECORDS*sizeof(record), 
                          PROT_READ|PROT_WRITE, MAP_SHARED, f, 0);

    mapped[43].integer = 243;
    sprintf(mapped[43].string,"RECORD-%d",mapped[43].integer);

    msync((void *)mapped, NRECORDS*sizeof(record), MS_ASYNC);
    munmap((void *)mapped, NRECORDS*sizeof(record));
    close(f);

    exit(0);
}