In the last post, we talk about the function options coding style with C++ and Golang samples. Several days ago, one of my colleagues come to me talking about how to make it in the C with more intuitive way. Let’s have a look at this.
In >C++11 and Golang, function works as first citizen in the core language, which means you can operate them as a POD type, copy/move etc.
In C world, no closure concept, so programmer use some technics to implement such concepts.
#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)->member ) *__mptr = (ptr); \
(type *)( (char *)__mptr - offsetof(type,member) );})
struct Server {
char* addr;
int port;
int maxConn;
int timeoutInSec;
};
typedef void (*oper)(void* base, struct Server* srv);
struct OptionWithMaxConn {
int maxConn;
oper op;
};
void OptionOpsOnMaxConn(void* base, struct Server* srv) {
struct OptionWithMaxConn* p = container_of(base, struct OptionWithMaxConn, op);
srv->maxConn = p->maxConn;
}
struct OptionWithTimeout {
int timeoutInSec;
oper op;
};
void OptionOpsOnTimeout(void* base, struct Server* srv) {
struct OptionWithTimeout* p = container_of(base, struct OptionWithTimeout, op);
srv->timeoutInSec = p->timeoutInSec;
}
struct Server* NewServer(int count, ...) {
struct Server *srv = malloc(sizeof(struct Server));
srv->addr = "localhost";
srv->port = 1080;
srv->maxConn = 10;
va_list ap;
va_start (ap, count); /* Initialize the argument list. */
for (int i = 0; i < count; i++) {
oper* op = va_arg(ap, oper*); /* Get the next argument value. */
(*op)((void*)op, srv);
}
va_end (ap); /* Clean up. */
return srv;
}
int main(void)
{
struct OptionWithMaxConn conn = {
.maxConn = 100,
.op = OptionOpsOnMaxConn
};
struct OptionWithTimeout tmo = {
.timeoutInSec = 20,
.op = OptionOpsOnTimeout
};
struct Server* s = NewServer(2, &conn.op, &tmo.op);
printf("maxConn is %d, tmo is %d", s->maxConn, s->timeoutInSec);
}
Linux heavily use this technics in its source code, but the Cons is this techs was not suitable for the application code which need more flexible and be fast to do adaption with new requirements.
Clang has a library extension named runtime-rt beside the standard, this new language extension was first introduced by Apple who use this in its owned Object-C/C++ language(you can find the story why Apple introduce this new language extension, very interesting story), later Apple merge this change to the Clang as part of runtime-rt.
Block extension bring C with the possibility to write anonymous function, closure.
Then we can refactor the sample code above this this language extension support. github-gist-02
#include <stdio.h>
#include <stdlib.h>
#include <Block.h>
struct Server {
char* addr;
int port;
int maxConn;
int timeoutInSec;
};
typedef void (^Ops)(struct Server*);
Ops OptionWithMaxConn(int maxConn) {
return Block_copy( ^(struct Server* srv) {
srv->maxConn = maxConn;
});
}
Ops OptionWithTimeout(int timeoutInSec) {
return Block_copy( ^(struct Server* srv) {
srv->timeoutInSec = timeoutInSec;
});
}
struct Server* NewServer(int count, ...) {
struct Server *srv = malloc(sizeof(struct Server));
srv->addr = "localhost";
srv->port = 1080;
srv->maxConn = 10;
va_list ap;
va_start (ap, count); /* Initialize the argument list. */
for (int i = 0; i < count; i++) {
Ops* op = va_arg(ap, Ops*); /* Get the next argument value. */
(*op)(srv);
}
va_end (ap); /* Clean up. */
return srv;
}
int main(void)
{
Ops op1 = OptionWithMaxConn(1000);
Ops op2 = OptionWithTimeout(100);
struct Server* s = NewServer(2, &op1, &op2);
printf("maxConn is %d, tmo is %d", s->maxConn, s->timeoutInSec);
}
Much better, ha?! We all love C :-).
Next time, I will show how can we add a similar feature on the language C under Clang suite.
With population of customized SoC, many vendor add their own property language extension to their development SDK for better performance with hardware acceleration.