Fcall, convS2M, convD2M, convM2S, convM2D, fcallfmt, dirfmt, dirmodefmt, read9pmsg, statcheck, sizeS2M, sizeD2M – interface to Plan 9 File protocol

#include <u.h>
#include <libc.h>
#include <fcall.h>
uint convS2M(Fcall *f, uchar *ap, uint nap)
uint convD2M(Dir *d, uchar *ap, uint nap)
uint convM2S(uchar *ap, uint nap, Fcall *f)
uint convM2D(uchar *ap, uint nap, Dir *d, char *strs)
int dirfmt(Fmt*)
int fcallfmt(Fmt*)
int dirmodefmt(Fmt*)
int read9pmsg(int fd, uchar *buf, uint nbuf)
int statcheck(uchar *buf, uint nbuf)
uint sizeS2M(Fcall *f)
uint sizeD2M(Dir *d)

These routines convert messages in the machine-independent format of the Plan 9 file protocol, 9P, to and from a more convenient form, an Fcall structure:
#define MAXWELEM 16
struct Fcall
uchar type;
u32int     fid;
ushort     tag;
union {
struct {
u32int msize;            /* Tversion, Rversion */
char    *version;          /* Tversion, Rversion */
struct {
ushort oldtag;           /* Tflush */
struct {
char    *ename;           /* Rerror */
struct {
Qid     qid;             /* Rattach, Ropen, Rcreate */
u32int iounit;           /* Ropen, Rcreate */
struct {
Qid     aqid;             /* Rauth */
struct {
u32int afid;             /* Tauth, Tattach */
char    *uname;           /* Tauth, Tattach */
char    *aname;           /* Tauth, Tattach */
struct {
u32int perm;             /* Tcreate */
char    *name;            /* Tcreate */
uchar    mode;             /* Tcreate, Topen */
struct {
u32int newfid;           /* Twalk */
ushort nwname;           /* Twalk */
char    *wname[MAXWELEM];    /* Twalk */
struct {
ushort nwqid;            /* Rwalk */
Qid     wqid[MAXWELEM];     /* Rwalk */
struct {
vlong    offset;           /* Tread, Twrite */
u32int count;            /* Tread, Twrite, Rread */
char    *data;            /* Twrite, Rread */
struct {
ushort nstat;            /* Twstat, Rstat */
uchar    *stat;            /* Twstat, Rstat */
} Fcall;
/* these are implemented as macros */
uchar      GBIT8(uchar*)
ushort     GBIT16(uchar*)
ulong      GBIT32(uchar*)
vlong      GBIT64(uchar*)
void       PBIT8(uchar*, uchar)
void       PBIT16(uchar*, ushort)
void       PBIT32(uchar*, ulong)
void       PBIT64(uchar*, vlong)
#define    BIT8SZ      1
#define    BIT16SZ     2
#define    BIT32SZ     4
#define    BIT64SZ     8
This structure is defined in <fcall.h>. See section 5 for a full description of 9P messages and their encoding. For all message types, the type field of an Fcall holds one of Tversion, Rversion, Tattach, Rattach, etc. (defined in an enumerated type in <fcall.h>). Fid is used by most messages, and tag is used by all messages. The other fields are used selectively by the message types given in comments.
ConvM2S takes a 9P message at ap of length nap, and uses it to fill in Fcall structure f. If the passed message including any data for Twrite and Rread messages is formatted properly, the return value is the number of bytes the message occupied in the buffer ap, which will always be less than or equal to nap; otherwise it is 0. For Twrite and Tread messages, data is set to a pointer into the argument message, not a copy.
ConvS2M does the reverse conversion, turning f into a message starting at ap. The length of the resulting message is returned. For Twrite and Rread messages, count bytes starting at data are copied into the message.
The constant IOHDRSZ is a suitable amount of buffer to reserve for storing the 9P header; the data portion of a Twrite or Rread will be no more than the buffer size negotiated in the Tversion/Rversion exchange, minus IOHDRSZ.
The routine sizeS2M returns the number of bytes required to store the machine-independent representation of the Fcall structure f, including its initial 32-bit size field. In other words, it reports the number of bytes produced by a successful call to convS2M.
Another structure is Dir, used by the routines described in stat(3). ConvM2D converts the machine-independent form starting at ap into d and returns the length of the machine-independent encoding. The strings in the returned Dir structure are stored at successive locations starting at strs. Usually strs will point to storage immediately after the Dir itself. It can also be a nil pointer, in which case the string pointers in the returned Dir are all nil; however, the return value still includes their length.
ConvD2M does the reverse translation, also returning the length of the encoding. If the buffer is too short, the return value will be BIT16SZ and the correct size will be returned in the first BIT16SZ bytes. (If the buffer is less that BIT16SZ, the return value is zero; therefore a correct test for complete packing of the message is that the return value is greater than BIT16SZ). The macro GBIT16 can be used to extract the correct value. The related macros with different sizes retrieve the corresponding-sized quantities. PBIT16 and its brethren place values in messages. With the exception of handling short buffers in convD2M, these macros are not usually needed except by internal routines.
Analogous to sizeS2M, sizeD2M returns the number of bytes required to store the machine-independent representation of the Dir structure d, including its initial 16-bit size field.
The routine statcheck checks whether the nbuf bytes of buf contain a validly formatted machine-independent Dir entry suitable as an argument, for example, for the wstat (see stat(3)) system call. It checks that the sizes of all the elements of the the entry sum to exactly nbuf, which is a simple but effective test of validity. Nbuf and buf should include the second two-byte (16-bit) length field that precedes the entry when formatted in a 9P message (see stat(9p)); in other words, nbuf is 2 plus the sum of the sizes of the entry itself. Statcheck also verifies that the length field has the correct value (that is, nbuf−2). It returns 0 for a valid entry and −1 for an incorrectly formatted entry.
Dirfmt, fcallfmt, and dirmodefmt are formatting routines, suitable for fmtinstall(3). They convert Dir*, Fcall*, and long values into string representations of the directory buffer, Fcall buffer, or file mode value. Fcallfmt assumes that dirfmt has been installed with format letter D and dirmodefmt with format letter M.
Read9pmsg calls read(3) multiple times, if necessary, to read an entire 9P message into buf. The return value is 0 for end of file, or -1 for error; it does not return partial messages.


intro(3), 9p(3), stat(3), intro(9p)

Space Glenda