Attempting to create a FAT file system in C++?

Question

I'm attempting to create a FAT file system I understand the basic principle of how its supposed to set up and I'm using a struct like this for each FAT entry

struct FATEntry
{
    char      name[20];  /* Name of file */
    uint32_t  pos;       /* Position of file on disk (sector, block, something else) */
    uint32_t  size;      /* Size in bytes of file */
    uint32_t  mtime;     /* Time of last modification */
};

I'm essentially creating a 2 MB file to act as my file system. From there I will write and read files into blocks of 512 bytes each. My question how I can write a struct to a file? Does fwrite allow me to do this? For example:

struct FATEntry entry1;
strcpy(entry1.name, "abc");
entry1.pos = 3;
entry1.size = 10;
entry1.mtime = 100;
cout << entry1.name;

file = fopen("filesys", "w");
fwrite(&entry1,sizeof(entry1),1,file);
fclose(file);

Will that store the struct in bytes? How do I read from this? I'm having trouble understanding what I will get back when I use fread

Answer 1

Will that store the struct in bytes?

• yes. in C++ you will need to explicitly cast &entry1 to (void*)

How do I read from this?

• fread((void*)&entry1,sizeof(entry1),1,file);

(but don't forget "r" flag to fopen())

The real problem in your case is that the struct will probably be padded by the compiler, for efficient access. so you will have to use __attribute__((packed)) if you are using gcc.

[EDIT] code sample (C, not C++):

struct FATEntry entry1 { "abc", 3, 10, 100 };
FILE* file1 = fopen("filesys", "wb");
fwrite(&entry1, sizeof(struct FATEntry), 1, file1);
fclose(file1)

struct FATEntry entry2 { "", 0, 0, 0 };
FILE* file2 = fopen("filesys", "rb");
fread(&entry2, sizeof(struct FATEntry), 1, file2;
fclose(file2)

You can now check that you read what you have written earlier:

assert(memcmp(&entry1, &entry2, sizeof(struct FATEntry))==0);

the assert will fail if the read or the write did not succeed (I didn't check for this).