Too Much Milk Problem

Your family drinks milk every morning. Mom and Dad are both used to checking the fridge when they arrive home. If milk runs out, they will leave home to buy some. The fridge is so small that only one bottle of milk can be put in it at a time. Mom and Dad always arrive home at different time.

Time	Dad	Mom
3:00	Arrive Home
3:05	Look into fridge, no milk
3:10	Leave for supermarket
3:15		Arrive Home
3:20	Arrive at Supermarket	Look into fridge, no milk
3:25	Buy Milk	Leave for supermarket
3:30	Arrive home, put milk into fridge
3:35		Arrive at Supermarket
3:40		Buy milk
3:45		Arrive home, put milk into fridge
3:50		Oh ......

Here are two programs simulating the above situation. When process mom or dad buys a bottle of milk, it writes string "milk" to a file fridge (the file is empty at the beginning). Compile and run these two programs and see what will happen:

gcc mom.c -o mom
gcc dad.c -o dad
./mom & ./dad &

/*mom.c*/
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>
int main(int argc, char *argv[]) {
    int fd;
    printf("Mom comes home.\n");
    printf("Mom checks the fridge.\n");
    fd = open("fridge", O_CREAT | O_RDWR | O_APPEND, 0777);
    if (lseek(fd, 0, SEEK_END) == 0) {
        printf("Mom goes to buy milk...\n");
        sleep(2);
        write(fd, "milk ", 5);
        printf("Mom puts milk in fridge and leaves.\n");
        if (lseek(fd, 0, SEEK_END) > 5)
            printf("What a waste of food! The fridge can not hold so much milk!\n");
    } else {
        printf("Mom closes the fridge and leaves.\n");
    }
    close(fd);
    return 0;
}

/*dad.c*/
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>
int main(int argc, char *argv[]) {
    int fd;
    printf("Dad comes home.\n");
    sleep(rand() % 2 + 1);
    printf("Dad checks the fridge.\n");
    fd = open("fridge", O_CREAT | O_RDWR | O_APPEND, 0777);
    if (lseek(fd, 0, SEEK_END) == 0) {
        printf("Dad goes to buy milk...\n");
        sleep(rand() % 2 + 1);
        write(fd, "milk ", 5);
        printf("Dad puts milk in fridge and leaves.\n");
        if (lseek(fd, 0, SEEK_END) > 5)
            printf("What a waste of food! The fridge can not hold so much milk!\n");
    } else {
        printf("Dad closes the fridge and leaves.\n");
    }
    close(fd);
    return 0;
}

Analysis

If we runmom.c and dad.c many times (remember to rm fridge between two runs), we could see different results as shown above. The result of these two programs depends on the order in which the fridge is accessed. This is an example of race condition.