Considerations for using variable length parameters in C language

The variable length parameter in C language greatly facilitates our programming, and it is also very easy to use inadvertently resulting in its hidden errors. Taking this function as an example, a segment error occurs randomly after a period of time, and the position of the error is stable in the vsprintf () function.

..................

A_listap;

Va_start (Ap,cmd);

..................

rep= (redisreply *) Redisvcommand (RC, CMD, AP);

VSPRINTF (Str,cmd, AP);

..................

Va_end (AP);

To delve into why, we downloaded the Redis source code and the glibc source code. See the implementation of Redisvcommand in the Redis source code:

Void*redisvcommand (Rediscontext *c, const char *format, va_list AP) {

if (Redisvappendcommand (c,format,ap)! = REDIS_OK)

Returnnull;

Return__redisblockforreply (c);

}

It mainly calls the Redisvappendcommand:

Intredisvappendcommand (Rediscontext *c, const char *format, va_list AP) {

Char*cmd;

Intlen;

len= Redisvformatcommand (&CMD,FORMAT,AP);

if (len = =-1) {

__redisseterror (C,redis_err_oom, "outof memory");

Returnredis_err;

}else if (len = =-2) {

__redisseterror (C,redis_err_other, "Invalidformat string");

Returnredis_err;

}

if (__redisappendcommand (c,cmd,len)! = REDIS_OK) {

Free (cmd);

Returnredis_err;

}

Free (cmd);

RETURNREDIS_OK;

}

In the Redisvappendcommand () function, Va_arg is used, such as the following part of the code:

The Newarg so it can checked even if it is nottouched. */

257 newarg = Curarg;

258

259 switch (c[1]) {

260 case ' s ':

261 arg = va_arg (ap,char*);

262 size = strlen (ARG);

263 if (Size > 0)

Newarg = Sdscatlen (curarg,arg,size);

265 break;

266 case ' B ':

267 arg = Va_arg (ap,char*);

268 size = Va_arg (ap,size_t);

269 if (Size > 0)

Newarg = Sdscatlen (curarg,arg,size);

271 break;

272 case '% ':

273 Newarg = Sdscat (curarg, "%");

274 break;

275 Default:

At first glance, the AP passes in the form parameter, will not change, but look closely va_arg's help document can see, in fact each call Va_arg () will change the value of the AP:

Va_arg ()

Theva_arg () macro expands to an expression of the type and valueof the next argument in the call.  The argument AP is the va_list ap initialized by Va_start (). Each call to Va_arg () modifies APS sothat the next call returns the next argument. The argument Typeis a

TypeName specified So, the type of a pointer to an object, hasthe specified type can be obtained simply by adding A * to type.

Thefirst use of the va_arg () macro after that of the Va_start () macroreturns the argument after last. Successive invocations return the

Valuesof the remaining arguments.

The AP is also passed in as a pointer pointer, so the variable-length parameter AP in the upper-call function changes, causing a segment error in the use of the variable-length parameter after it is encountered. Because the front has been traversed again, the AP to the end.

With this in mind, for a function to invoke the use of multiple variable-length parameters, a safe use is to assign a variable-length parameter va_list to each of the called functions individually. Accordingly, the above code should be rewritten like this:

A_listap;

Va_listaq;

Va_start (Ap,cmd);

Va_copy (AQ,AP);

.........

rep= (redisreply *) Redisvcommand (conn, CMD, AP);

vsprintf (Str,cmd, aq);

Va_end (AP);

Va_end (aq);

.........

This article is from the "Store Chef" blog, so be sure to keep this source http://xiamachao.blog.51cto.com/10580956/1951621

Considerations for using variable length parameters in C language