C programming - reading numbers from text file

Question

I'm trying to make a sort of a database program and ran into a few issues with reading integers from a text file in C.

I have the following code:

#include <stdio.h>

int main(){
    int index;
    FILE * fp;

    if((fp = fopen("read_file.txt","r+")) == NULL){
        perror("Cannot open file");
        printf("\nCreating new file...");
        if((fp = fopen("read_file.txt","w+")) == NULL){
            perror("\nCannot create file.. Terminating..");
            return -1;
        }
    }
    fputs("INDEX = 3",fp);
    fscanf(fp, "INDEX = %d",&index);
    printf("index = %d\n",index);

    fclose(fp);
    return 0;
}

When i try to run the program it outputs "index = 16", i tried using fgets and sscanf but same thing happens. With strings however it decides to print out a bunch of characters that don't make sense.

Answer 1

What you see in undefined behavior because you write a string to the file and try to scan INDEX = %d which is not there in the file because of the file pointer is pointing after INDEX = 3

You need to rewind(fp) before scanning.

fputs("INDEX = 3",fp);
rewind(fp);
if( fscanf(fp, "INDEX = %d",&index) != 1)
printf("Scanning failes\n");
else
printf("INDEX = %d\n",index);