frame(l)

NAME
frame - simple message protocol for byte streams

SYNOPSIS

# include <begemot.h>

int frame_write(int fd, void *hdr, u_int hdr_len,
void *arg...)

int frame_writev(int fd, void *hdr, u_int hdr_len,
struct iovec *vec, u_int veclen)

int frame_read(int fd, void *hdr, u_int hdr_len,
void **parg, u_int *plen)

int framefd_write(int fd, void *hdr, u_int hdr_len,
void *arg...)

int framefd_writev(int fd, void *hdr, u_int hdr_len,
struct iovec *vec, u_int veclen)

int frame_read(int fd, void *hdr, u_int hdr_len,
void **parg, u_int *plen)

DESCRIPTION
Most reliable protocols available on UNIX systems are byte stream protocols. On the other hand there are many cases, where you need to transfer messages. A simple way to do this is to prepend the message length to the message and transfer it on the byte stream. The frame_* functions are designed to simplify this task for most cases. They assume, that you want to transfer messages, which consist of a fixed length header and a variable length part. The fixed header consists of at least four bytes, which contain the variable parts length in network byte order. For the framefd_* functions the header consists of 8 bytes: 4 bytes for the length and 4 bytes for a file descriptor. Of course, the fixed header can be longer than this minimum.

There are two pairs of functions. The first pair frame_read and frame_write can be used on any reliable byte stream (pipes, UNIX domain sockets, TCP sockets, ...). The framefd_read and framefd_write functions are specifically designed for UNIX domain sockets -- additionally to messages they allow the optional transfer of a file descriptor with each message.

The parameter fd is the file descriptor on which the transfer should take place. hdr is a pointer to the fixed size header, which is of size hdr_len. hdr_len must be either 4 or 8 bytes minimum. The first four bytes are set to the length of the variable part in the write functions and contain the length of this part if the read functions return. The second four bytes of the header must contain the file descriptor or -1 for the framefd_write function. It will contain the received file descriptor, -1 if there was no file descriptor or -2, if the writing side indicated, that it will send a file descriptor, but did not send one. The latter indicates an error in the protocol. A file descriptor, that is received, but not expected is closed.

arg is the start of a variable argument list. This list should consist of pairs of void * pointers and u_int sizes, each pair describing a piece of the variable message part. The list must be terminated by a NULL pointer. If no variable part is to be transmitted, the first pointer should simply be NULL.

parg on the other hand should point to a pointer which points to a buffer for the variable length part. The size of the buffer is indicated by the number pointed to by plen. If the indicated buffer size is too small for the received message's variable part it will be reallocated by calling xrealloc(l). The actual length of the variable part is indicated in the first four bytes of the header. This mechanism tries to minimize buffer allocations and reallocations. There is no need to reallocate anything once you got the biggest message.

An example on how to use the functions can be found at the end of frame.c.

The functions with the v suffix take a pointer to a struct iovec and an associated length instead of a variable argument list. To use these functions you need to

# include <sys/uio.h>

RETURN VALUES
The write functions return the return value of the corresponding system call. Look under sendmsg(2) and writev(2).

If frame_read is unable to receive the fixed size header, or the header is too small, it returns the value from the recvmsg(2) or readv(2) system call. If the corresponding call for the variable part returns an error or EOF, the system call's value is returned (0 or -1). If the calls for both the header and the variable part are ok, the sum of the sizes of both received parts is returned. This means, that you should check, that the value returned from the read functions is equal to the sum of hdr_len and the first four bytes of the header.

SEE ALSO
sendmsg(2),writev(2) recvmsg(2),readv(2)

BUGS
The FreeBSD kernel crashes, if you use framefd_write on the writing side and frame_read on the reading side. The crash occures in so_flush in both the 2.X and 3.0 kernels.

You cannot transfer file descriptors over pipes in FreeBSD 3.0 anymore, because the pipe implementation is not based on socketpair(2) as it was the case in all earlier BSDs.

Due to a bug in the Solaris 2.5 socket emulation it is not possible to mix framefd_write with frame_read.

The write functions impose a limit on the length of the argument list and the uio vector. This limit is defined in frame.c and is currently defined to be 100.

AUTHOR
Hartmut Brandt, harti@freebsd.org