（二）wireguard-tools源码解读（1)wg.c

• 源码解读
• 备注：函数指针

// SPDX-License-Identifier: GPL-2.0


#include 
#include 
#include 

#include "subcommands.h"
#include "version.h"

const char *PROG_NAME;

static const struct {
	const char *subcommand;
	int (*function)(int, const char**);
	const char *description;
} subcommands[] = {
	{ "show", show_main, "Shows the current configuration and device information" },
	{ "showconf", showconf_main, "Shows the current configuration of a given WireGuard interface, for use with `setconf'" },
	{ "set", set_main, "Change the current configuration, add peers, remove peers, or change peers" },
	{ "setconf", setconf_main, "Applies a configuration file to a WireGuard interface" },
	{ "addconf", setconf_main, "Appends a configuration file to a WireGuard interface" },
	{ "syncconf", setconf_main, "Synchronizes a configuration file to a WireGuard interface" },
	{ "genkey", genkey_main, "Generates a new private key and writes it to stdout" },
	{ "genpsk", genkey_main, "Generates a new preshared key and writes it to stdout" },
	{ "pubkey", pubkey_main, "Reads a private key from stdin and writes a public key to stdout" }
};

static void show_usage(FILE *file)
{
	fprintf(file, "Usage: %s  []nn", PROG_NAME);
	fprintf(file, "Available subcommands:n");
	for (size_t i = 0; i < sizeof(subcommands) / sizeof(subcommands[0]); ++i)
		fprintf(file, "  %s: %sn", subcommands[i].subcommand, subcommands[i].description);
	fprintf(file, "You may pass `--help' to any of these subcommands to view usage.n");
}

int main(int argc, const char *argv[])
{
	
	PROG_NAME = argv[0];
    
	if (argc == 2 && (!strcmp(argv[1], "-v") || !strcmp(argv[1], "--version") || !strcmp(argv[1], "version"))) {
		printf("wireguard-tools v%s - https://git.zx2c4.com/wireguard-tools/n", WIREGUARD_TOOLS_VERSION);
		return 0;
	}
	if (argc == 2 && (!strcmp(argv[1], "-h") || !strcmp(argv[1], "--help") || !strcmp(argv[1], "help"))) {
		show_usage(stdout);
		return 0;
	}

	if (argc == 1) {
		static const char *new_argv[] = { "show", NULL };
		return show_main(1, new_argv);
	}

	for (size_t i = 0; i < sizeof(subcommands) / sizeof(subcommands[0]); ++i) {
		if (!strcmp(argv[1], subcommands[i].subcommand))
			return subcommands[i].function(argc - 1, argv + 1);
	}

	fprintf(stderr, "Invalid subcommand: `%s'n", argv[1]);
	show_usage(stderr);
	return 1;
}

关于函数指针的用法，本人认为是C语言的经典精华部分，在各大高性能软件中，都会看到类似的用法，尤其是在Nginx的源码中，更是用到极处。
比如在nginx源码中：typedef size_t (*ngx_log_handler_pt) (void *ctx, char *buf, size_t len);
举个最直接简单的例子吧，

#include 
using namespace std;
typedef int (*Pointer) (char, char);
 
int ss(char a, char b)
{
    cout<<"in func ss()"<
    cout<<"in func bb()"<
    Pointer p;
    p = ss;
    p('a','b');
    p = bb;
    p('a', 'b');
    return 0;
}

通过p来调用两个函数。
运行结果：

in func ss()
a
b
in func bb()
b
a

当然了，这种函数指针，跟静态数组以及偏移量结合起来使用，不但能简洁源代码，并且能大大提升程序的运行效率。