Cmdbuf, parsecmd, respondcmderror, lookupcmd – control message parsing


#include <u.h> #include <libc.h> #include <fcall.h> #include <thread.h> #include <9p.h>

typedef struct Cmdbuf { char *buf; char **f; int nf; } Cmdbuf;

typedef struct Cmdtab { int index; char *cmd; int narg; };

Cmdbuf *parsecmd(char *p, int n) Cmdtab *lookupcmd(Cmdbuf *cb, Cmdtab *tab, int ntab) void respondcmderror(Req *r, Cmdbuf *cb, char *fmt, ...)


These data structures and functions provide parsing of textual control messages.

Parsecmd treats the n bytes at p (which need not be NUL-terminated) as a UTF string and splits it using tokenize (see getfields(2)). It returns a Cmdbuf structure holding pointers to each field in the message. It is the caller’s responsibility to free this structure when it is no longer needed.

Lookupcmd walks through the array ctab, which has ntab entries, looking for the first Cmdtab that matches the parsed command. (If the parsed command is empty, lookupcmd returns nil immediately.) A Cmdtab matches the command if cmd is equal to cb->f[0] or if cmd is *. Once a matching Cmdtab has been found, if narg is not zero, then the parsed command must have exactly narg fields (including the command string itself). If the command has the wrong number of arguments, lookupcmd returns nil. Otherwise, it returns a pointer to the Cmdtab entry. If lookupcmd does not find a matching command at all, it returns nil. Whenever lookupcmd returns nil, it sets the system error string.

Respondcmderror responds to request r with an error of the form ‘fmt: cmd,’ where fmt is the formatted string and cmd is a reconstruction of the parsed command. Fmt is often simply %r .


This interface is not used in any distributed 9P servers. It was lifted from the Plan 9 kernel. Almost any kernel driver (/sys/src/9/*/dev*.c) is a good example.