Chapter 27. Sockets

If a network stack is present, then the FILEIO infrastructure also provides access to the standard BSD socket calls.

The netstack table contains entries which describe the network protocol stacks that are in the system image. Each resident stack should export an entry to this table using the NSTAB_ENTRY() macro.

Each table entry has the following structure:

struct cyg_nstab_entry
{
    cyg_bool            valid;          // true if stack initialized
    cyg_uint32          syncmode;       // synchronization protocol
    char                *name;          // stack name
    char                *devname;       // hardware device name
    CYG_ADDRWORD        data;           // private data value

    int     (*init)( cyg_nstab_entry *nste );
    int     (*socket)( cyg_nstab_entry *nste, int domain, int type,
		       int protocol, cyg_file *file );
};

This table is analogous to a combination of the filesystem and mount tables.

The valid field is set true if the stack's init() function returned successfully and the syncmode field contains the CYG_SYNCMODE_SOCK_* bits described above.

The name field contains the name of the protocol stack.

The devname field names the device that the stack is using. This may reference a device under "/dev", or may be a name that is only meaningful to the stack itself.

The init() function pointer is called during system initialization to start the protocol stack running. If it returns non-zero the valid field is set false and the stack will be ignored subsequently.

The socket() function is called to attempt to create a socket in the stack. When the socket() API function is called the netstack table is scanned and for each valid entry the socket() function pointer is called. If this returns non-zero then the scan continues to the next valid stack, or terminates with an error if the end of the table is reached.

The result of a successful socket call is an initialized file object with the f_xops field pointing to the following structure:

struct cyg_sock_ops
{
    int (*bind)      ( cyg_file *fp, const sockaddr *sa, socklen_t len );
    int (*connect)   ( cyg_file *fp, const sockaddr *sa, socklen_t len );
    int (*accept)    ( cyg_file *fp, cyg_file *new_fp,
                       struct sockaddr *name, socklen_t *anamelen );
    int (*listen)    ( cyg_file *fp, int len );
    int (*getname)   ( cyg_file *fp, sockaddr *sa, socklen_t *len, int peer );
    int (*shutdown)  ( cyg_file *fp, int flags );
    int (*getsockopt)( cyg_file *fp, int level, int optname,
                       void *optval, socklen_t *optlen);
    int (*setsockopt)( cyg_file *fp, int level, int optname,
                       const void *optval, socklen_t optlen);
    int (*sendmsg)   ( cyg_file *fp, const struct msghdr *m,
                       int flags, ssize_t *retsize );
    int (*recvmsg)   ( cyg_file *fp, struct msghdr *m,
                       socklen_t *namelen, ssize_t *retsize );
};

It should be obvious from the names of these functions which API calls they provide support for. The getname() function pointer provides support for both getsockname() and getpeername() while the sendmsg() and recvmsg() function pointers provide support for send(), sendto(), sendmsg(), recv(), recvfrom() and recvmsg() as appropriate.

2017-02-09
Documentation license for this page: Open Publication License