thomasloven.com/pages/2013-12-12-VFS-syscalls.md

title: "VFS syscalls"
subtitle: "...so I put wrappers 'round your wrappers..."
tags: [osdev]

[Last time](/blog/2013/12/Virtual-File-System2/) I started to rewrite
the VFS layer of my hobby kernel - again. This time I'll take a look at
the system call couplings.

Since a [while](/blog/2013/08/Catching-Up/), I've had a cross compiler
and [newlib](http://sourceware.org/newlib) for my kernel, which means I
have some basic syscall interfaces to start from.

Newlib requires the following syscalls:

    :::c
    int close(int file)
    int fstat(int file, struct stat *st)
    int isatty(int file)
    int link(char *old, char *new)
    int lseek(int file, int ptr, int dir)
    int open(const char *name, int flags, int mode)
    int read(int file, char *ptr, int len)
    int stat(const char *file, struct stat *st)
    int unlink(char *name)
    int write(int file, char *ptr, int len)

###open and close
Everything starts with `open`, so let's look at that first.

In order to keep track of the files that are opened by a process, we
need a new data structure, though; the _file descriptor_.

    :::c
    typedef struct
    {
        INODE ino;
        uint32_t offset;
        uint32_t flags;
        uint32_t users;
    } file_desc_t;

The file descriptor keeps track of our position in the file as well as
the mode it was opened in. File descriptors can also be shared between
processes (after a `fork()` for example), and it therefore has a use
counter. Two macros are used to manipulate the use counter

    :::c
    #define fd_get(fd) { (fd)->users++ }
    #define fd_put(fd) { (fd)->users--; if(!(fd)->users)free(fd) }

Each process descriptor has an array of pointers to file descriptors

    :::c
    file_desc_t *fd[NUM_FILEDES];

`open` starts by finding a free file descriptor. It then finds the file,
opens the file and returns the index of the file descriptor it used:

    :::c
    int open(const char *name, int flags, int mode)
    {
        int fd;

        // Find unused file descriptor
        process_t *p = current->proc;
        int i;
        for(i=0; i < NUM_FILEDES; i++)
        {
            if(p->fd[i])
                continue;
            fd = i;
            p->fd[fd] = calloc(1, sizeof(file_desc_t));
            fd_get(p->fd[fd]);
            break;
        }

        // Find file
        INODE ino = vfs_namei(name);
        // Open file
        vfs_open(name, flags);

        // Setup file descriptor
        p->fd[fd]->ino = ino;
        p->fd[fd]->offset = 0;
        p->fd[fd]->flags = flags;

        return fd;
    }

I stripped away all of the sanity checking and error handling code here.
With that code, the function is more than twice as long.

`close` is even easier:

    :::c
    int close(int file)
    {
        int retval = vfs_close(p->fd[file]->ino);
        if(!p->fd[file]->ino->parent)
            free(p->fd[file]->ino);
        fd_put(p->fd[file]);
        p->fd[file] = 0;

        return retval;
    }

I always check if an inode has a parent before freeing it. If it has a
parent, it's part of the vfs mount tree, and should be kept around.

###read and write
Next, let's look at read.
It's actually really simple (excluding sanity checking and error
handling):

    :::c
    int read(int file, char *ptr, int len)
    {
        process_t *p = current->proc;
        INODE node = p->fd[file]->ino;
        int ret = vfs_read(node, ptr, len, p->fd[file]->offset);
        p->fd[file]->offset += ret;
        return ret;
    }

Write is pretty much the same:

    :::c
    int write(int file, char *ptr, int len)
    {
        process_t *p = current->proc;
        INODE node = p->fd[file]->ino;
        int ret = vfs_write(node, ptr, len, p->fd[file]->offset);
        p->fd[file]->offset += ret;
        return ret;
    }

###stat, fstat and isatty
`fstat` and `isatty` just passes on the information to the corresponding
vfs functions:

    :::c
    int fstat(int file, struct stat *st)
    {
        process_t *p = current->proc;
        INODE node = p->fd[file]->ino;
        return vfs_fstat(node, st);
    }

    :::c
    int isatty(int file)
    {
        process_t *p = current->proc;
        INODE node = p->fd[file]->ino;
        return vfs_isatty(node);
    }

`stat` performs a `namei` lookup to get the node instead of taking it
from the process' file descriptor table.

    :::c
    int stat(const char *file, struct stat *st)
    {
        INODE node = vfs_namei(file);
        int retval = vfs_fstat(node, st);
        if(!node->parent)
            free(node);
        return retval;
    }

###lseek
The final function I'll look at now is `lseek` which sets the current
position in the file:

    :::c
    int lseek(int file, int ptr, int dir)
    {
        process_t *p = current->proc;
        if(dir == SEEK_SET)
        {
            p->fd[file]->offset = ptr;
        }
        if(dir == SEEK_CUR)
        {
            p->fd[file]->offset += ptr;
        }
        if(dir == SEEK_END)
        {
            p->fd[file]->offset = p->fd[file]->ino->length + ptr;
        }
        return p->fd[file]->offset;
    }

I'll leave `link` and `unlink` for now, and come back to them when
I need them (i.e. I wish to implement a user writeable filesystem).

In the next post, we'll mount an actual filesystem.