Writing strings to binary files in C

Question

I have some code that writes a char * to a binary file like so:

FILE *fp = fopen(path, "wb");
if (fp == NULL) {
    printf("Failed to open file at path: %s. %s\n", path, __PRETTY_FUNCTION__);
    return;
}

fwrite("Hello", 6, 1, fp);

The problem is that when I then read from that file like so:

FILE *fp = fopen(path, "rb");
if (fp == NULL) {
    printf("Failed to open file at path: %s. %s\n", path, __PRETTY_FUNCTION__);
    return;
}


char *str;
fread(str, 6, 1, fp);
printf("Got str: %s\n", str);

I get different results e.g. Oello or Mello. What is going on?

Answer 1

This code is incorrect:

char *str;
fread(str, 6, 1, fp);
printf("Got str: %s\n", str);

char *str; creates an uninitialized character pointer. Where's the memory that it points to?

char str[ 7 ];
fread(str, sizeof( str ) - 1, 1, fp);
str[ sizeof( str ) - 1 ] = '\0';
printf("Got str: %s\n", str);

would be better, although it still doesn't check the return value from fread() to ensure data is actually read.