Input from previous scanf appends to another scanf

Question

The program asks the user for input of names, IDs, etc, of 2 students, writes the data to a file and reads the data back from the file.

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

int main(){
    int fd, i = 0;
    char name[20], id[10], dob[10], gender[7], status[10];
    fd = open("file.dat", O_WRONLY|O_CREAT, S_IRWXU); //open file.dat for writing; if it does not exist, create it
    for (i = 0; i < 2; i++){
        //get input from user
        printf("\nFor student %d\n", (i + 1));
        printf("Enter name: ");
        scanf("%[^\n]%*c", name); //%[^\n]%*c - read everything up to (excluding) new line character
        printf("Enter ID: ");
        scanf("%[^\n]%*c", id);
        printf("Enter date of birth: ");
        scanf("%[^\n]%*c", dob);
        printf("Enter gender: ");
        scanf("%[^\n]%*c", gender);
        printf("Enter marital status: ");
        scanf("%[^\n]%*c", status);
        //write current student to file
        write(fd, name, 20);
        write(fd, id, 10);
        write(fd, dob, 10);
        write(fd, gender, 7);
        write(fd, status, 10);
    }
    close(fd);
    //display data from file
    char buf[20];
    fd = open("file.dat", O_RDONLY); //open file for reading
    for (i = 0; i < 2; i++){
        printf("\nFor student %d\n", (i + 1));
        read(fd, buf, 20); //read name
        printf("Name: %s\n", buf);
        read(fd, buf, 10); //read id
        printf("ID: %s\n", buf);
        read(fd, buf, 10); //read date of birth
        printf("Date of birth: %s\n", buf);
        read(fd, buf, 7); //read gender
        printf("Gender: %s\n", buf);
        read(fd, buf, 10); //read marital status
        printf("Marital status: %s\n", buf);
    }
}

The input part:

For student 1
Enter name: Mary Jane Smith
Enter ID: 1
Enter date of birth: 09/09/1990
Enter gender: Female
Enter marital status: Single

For student 2
Enter name: John Doe Paul
Enter ID: 2
Enter date of birth: 08/08/1993
Enter gender: Male
Enter marital status: Married

The display part:

For student 1
Name: Mary Jane Smith
ID: 1
Date of birth: 09/09/1990Smith
Gender: Female
Marital status: Single

For student 2
Name: John Doe Paul
ID: 2
Date of birth: 08/08/1993aul
Gender: Male
Marital status: Married

Some part of the name gets added to the date of birth. Why is this so and how do I solve this, please?

I have read that scanf is not reliable but I am required to use scanf in the program.

@BLUEPIXY I changed dob[10] to dob[11] but I still get the same error. — Saiyan

Balise Balise · Accepted Answer · 2015-09-15T21:51:14

The issue there is not in the scanf. The issue is in your buffer handling when you re-read the file. When you read the file, you read it repeatedly in buffer "buf", that is not reset in any way. Consequently, when you read 10 characters from your file, the previously inserted 20 characters from the name are still in the buffer and hence are displayed. You need to either terminate your strings properly (with '\0') or set your buffer to all-zeroes when you use it to ensure you won't get "old content" back.

Input from previous scanf appends to another scanf

1 Answers