In C,I cannot update when I close program database - c

Question

I made file that I want to update with the value when the program ends, then read the value from that file again next time the program runs.
For example: In this program I want to subtract an entered value from the saved one. So first I enter 3 then it gives 12. When I run the program again and enter 2, then it gives 13, but it should give 10 rather than 13.
#include<stdio.h>
#define PATH "/tmp/file"
int main()
{
FILE *file;
int age=15,s;
scanf("%d",&s);
file = fopen(PATH, "w");
age = age - s;
fprintf(file, "%d", age);
fclose(file);
file = fopen(PATH,"r");
if (file == NULL)
{
printf("files does not exist");
}
fscanf(file,"%d",&age);
printf("%d",age);
fclose(file);
}

Answer 1

The problem is that you always initialize age to 15, not to what's in your file. Then you write that age to the file, and immediately read it back.
If I understand you correctly, you want to read the file first (not overwrite it), then do your calculation, and write the result back.
#include<stdio.h>
#define PATH "/tmp/file"
int main()
{
FILE *file;
int age = 15, s;
file = fopen(PATH, "r");
if (file == NULL)
{
printf("files does not exist");
return 1;
}
fscanf(file, "%d", &age);
fclose(file);
scanf("%d", &s);
file = fopen(PATH, "w");
age = age - s;
fprintf(file, "%d", age);
fclose(file);
printf("%d", age);
}
Note that you should also do error handling for writing, not only for reading. Writing can fail if the file system is full, or if you don't have write permissions.
Your error handling isn't complete, though: in your code you print an error message, but still keep going. Accessing an invalid file pointer (i.e., NULL) will crash your program with a segmentation fault. I've added a return statement, but you could also put the rest of the logic in an else block.