main_loop开始??每一次执行main_loop就是完成cmd读取、解析、执行的一次过程。
run_command分析cmd_tbl_s结构体??该结构体存储待执行的指令，包含参数数量限制，是否可重复，实现功能的函数的指针，usgae和相应help。
struct cmd_tbl_s {
char *name; /* Command Name */
int maxargs; /* maximum number of arguments */
int repeatable; /* autorepeat allowed? */
/* Implementation function */
// 实现的函数指针
int (*cmd)(struct cmd_tbl_s *, int, int, char *[]);
char *usage; /* Usage message (short) */
#ifdef CFG_LONGHELP
char *help; /* Help message (long) */
#endif
};
__u_boot_cmd_start与__u_boot_cmd_end。 __u_boot_cmd_start = .;
.u_boot_cmd : { *(.u_boot_cmd) }
__u_boot_cmd_end = .;#define Struct_Section __attribute__ ((unused,section (".u_boot_cmd")))
#define U_BOOT_CMD(name,maxargs,rep,cmd,usage,help) cmd_tbl_t __u_boot_cmd_##name Struct_Section = {#name, maxargs, rep, cmd, usage, help}??利用GCC编译器特性，为其附加段属性。
??最终效果类似于./common/command.c：
int
do_version (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
{
extern char version_string[];
printf ("\n%s\n", version_string);
return 0;
}
U_BOOT_CMD(
version, 1, 1, do_version,
"version - print monitor version\n",
NULL
);find_cmd/***************************************************************************
* find command table entry for a command
*/
cmd_tbl_t *find_cmd (const char *cmd)
{
cmd_tbl_t *cmdtp;
// 命令集数组首地址
cmd_tbl_t *cmdtp_temp = &__u_boot_cmd_start; /*Init value */
const char *p;
int len;
int n_found = 0;
/*
* Some commands allow length modifiers (like "cp.b");
* 对于可以带.的命令，只关心点之前的内容
* compare command name only until first dot.
*/
len = ((p = strchr(cmd, ‘.‘)) == NULL) ? strlen (cmd) : (p - cmd);
for (cmdtp = &__u_boot_cmd_start;
cmdtp != &__u_boot_cmd_end;
cmdtp++) {
if (strncmp (cmd, cmdtp->name, len) == 0) {
if (len == strlen (cmdtp->name))
return cmdtp; /* full match */
cmdtp_temp = cmdtp; /* abbreviated command ? */
n_found++;
}
}
if (n_found == 1) { /* exactly one match */
return cmdtp_temp;
}
return NULL; /* not found or ambiguous command */
}原文：https://www.cnblogs.com/0nism/p/12380574.html