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).