为什么fwrite＆fread打印垃圾？

Question

When The following code is run, why is there extra garbage printed alongside the HALLO string?

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    FILE *binfile;

    binfile = fopen("binary.bin", "wb+");

    char arr[5] = "HALLO";
    int integer = 1;
    double doub = 1.2345;

    fwrite(arr, sizeof(arr), 1, binfile);
    fwrite(&integer, sizeof(int), 1, binfile);
    fwrite(&doub, sizeof(double), 1, binfile);

    fclose(binfile);

    binfile = fopen("binary.bin", "r");

    char arr2[5];
    int integer2;
    double doub2;

    fread(arr2, sizeof(arr2), 1, binfile);
    fread(&integer2, sizeof(int), 1, binfile);
    fread(&doub2, sizeof(double), 1, binfile);

    printf("%s\n", arr2);
    printf("%d\n", integer2);
    printf("%lf\n", doub2);

    fclose(binfile);
    return EXIT_SUCCESS;
}

Answer 1

Becuase you have not allowed for the null terminator on the string.

Ok, some more detail. You have not made it clear what you're trying to do, so let's assume you want to write 5 char bytes. The problem is that when you read back your 5 chars you read them back into a char array (fine) then try to print that array with no null terminator - not fine.

So either read them back into a new array and set

newarray[5]=0;

or print it out safe in the knowledge that you have 5 chars

printf("%5.5s",array);

(something like that anyway)

Answer 2

printf("%s\\n", arr2) interprets arr2 as a pointer to a string, not as a char array. Since your char array does not end with a zero terminator, printf happily prints whatever is stored in the memory locations following the char array.

This code snippet should work:

char arr2[5+1];
fread(arr2, sizeof(arr2) - 1, 1, binfile);
arr2[sizeof(arr2) - 1] = '\0';
printf("%s\n", arr2);