getopt是linux下解析命令行参数的api。以linux内核代码的一个例子来说明：
static void cmdline(int argc, char *argv[])
{
int opt;
progname = basename(argv[0]);
while ((opt = getopt(argc, argv, "+lci:m:")) != -1) {
switch (opt) {
case ‘l‘:
if (mode)
print_wrong_arg_exit();
mode = list;
break;
case ‘i‘:
/* only allow -i with -m or no option */
if (mode && mode != show)
print_wrong_arg_exit();
interval = atoi(optarg);
break;
case ‘m‘:
if (mode)
print_wrong_arg_exit();
mode = show;
show_monitors_param = optarg;
break;
case ‘c‘:
wake_cpus = 1;
break;
default:
print_wrong_arg_exit();
}
}
if (!mode)
mode = show_all;
}
getopt仅支持"-v"类型的带参以及不带参数的命令行参数解析，包含"--v"一对短横线的复杂命令行参数请使用其他api。
函数原型
int getopt(int argc, char * const argv[],
const char *optstring);
argc和argv都懂得，optstring告诉getopt需要哪些命令以及哪些命令需要参数，以"+lci:m:"为例，带"+"的参数不多见忽略掉，‘l‘和‘c‘是不带参数的命令选项，‘i:‘和‘m:‘是带参数的命令选项，两者通过‘:‘进行区分。getopt获取命令以后参数保存在全局变量optarg中。
包括optarg在内，getopt使用三个全局变量来保存状态(该函数是线程非安全的，多线程使用需要自己加保护)。
optarg，保存命令的参数，如果有的话。
optind，下一次getopt时指向的argv指针的索引 。
optopt，最后一个已知选项。
optind用来获取除了命令行命令和参数以外的其他内容，比如命令行的最后添加一个默认参数配置文件。
上面的例子是一个直观的解析用法，再来一个推荐的解析方式。
while ((opt = getopt(argc, argv, "+lci:m:")) != -1) {
switch (opt) {
case ‘A‘:
/* enables "shutdown" command */
settings.shutdown_command = true;
break;
case ‘a‘:
/* access for unix domain socket, as octal mask (like chmod)*/
settings.access= strtol(optarg,NULL,8);
break;
default:
break;
}
settings是一个入参结构体，对应入参的命令和参数
struct settings {
size_t maxbytes;
int maxconns;
int port;
int udpport;
char *inter;
int verbose;
rel_time_t oldest_live; /* ignore existing items older than this */
uint64_t oldest_cas; /* ignore existing items with CAS values lower than this */
int evict_to_free;
char *socketpath; /* path to unix socket if using local socket */
int access; /* access mask (a la chmod) for unix domain socket */
double factor; /* chunk size growth factor */
int chunk_size;
int num_threads; /* number of worker (without dispatcher) libevent threads to run */
int num_threads_per_udp; /* number of worker threads serving each udp socket */
char prefix_delimiter; /* character that marks a key prefix (for stats) */
int detail_enabled; /* nonzero if we‘re collecting detailed stats */
int reqs_per_event; /* Maximum number of io to process on each
io-event. */
bool use_cas;
enum protocol binding_protocol;
int backlog;
int item_size_max; /* Maximum item size, and upper end for slabs */
bool sasl; /* SASL on/off */
bool maxconns_fast; /* Whether or not to early close connections */
bool lru_crawler; /* Whether or not to enable the autocrawler thread */
bool lru_maintainer_thread; /* LRU maintainer background thread */
bool slab_reassign; /* Whether or not slab reassignment is allowed */
int slab_automove; /* Whether or not to automatically move slabs */
int hashpower_init; /* Starting hash power level */
bool shutdown_command; /* allow shutdown command */
int tail_repair_time; /* LRU tail refcount leak repair time */
bool flush_enabled; /* flush_all enabled */
char *hash_algorithm; /* Hash algorithm in use */
int lru_crawler_sleep; /* Microsecond sleep between items */
uint32_t lru_crawler_tocrawl; /* Number of items to crawl per run */
int hot_lru_pct; /* percentage of slab space for HOT_LRU */
int warm_lru_pct; /* percentage of slab space for WARM_LRU */
int crawls_persleep; /* Number of LRU crawls to run before sleeping */
bool expirezero_does_not_evict; /* exptime == 0 goes into NOEXP_LRU */
};
settings封装了可能有的各个命令及参数。把settings定义为全局变量可在整个程序访问。