Fatal error: glibc. Trying to put char in file

Question

I'm trying to put and get+print four char characters in a file by promting the user for four chars. However, there is an error (Fatal error: glibc detected an invalid stdio handle).

After debugging I found the fault at the line fputc(c[3], putc). After prompting the user for the fourth time, it cannot put the fourth char c[3] in the file.

Please help me to understand.

Here's my code:

    FILE* putc = fopen("test3.txt", "w");

    if(putc == NULL)
    {
        return 1;
    }

    char c[4];

    for(int i = 0; i < 4; i++)
    {
        printf("char: ");
        scanf("%s", &c[i]);
        fputc(c[i], putc);
    }


    fclose(putc);

    FILE* getc = fopen("test3.txt", "r");

    if(getc == NULL)
    {
        return 1;
    }

    char abc;

    while ((abc = fgetc(getc)) != EOF)
    {
        printf("%c", abc);
    }
    printf("\n");

    fclose(getc);

Answer 1

scanf("%s", &c[i]) writes a null character after the input it reads and stores. c is defined with only four elements, so, when i is three, the null character is written outside of the array c, corrupting other memory in the program.

One fix is to make c larger and scan with scanf("%1s", &c[i]).

Another is to scan with scanf(" %c", &c[i]), to skip spaces and read a single character, which is put into c[i] with no terminating null character.