浅析getenv库函数背后的故事

浅析getenv库函数背后的故事

pc执行evn指令,手机使用adb shell执行printenv指令.
PWD=/home/luther
所以
const char *val = getenv("PWD");获取"PWD"环境变量对应的数值,所以val指向"/home/luther"字符串,失败返回NULL.
看看getenv库函数的具体实现：
=====================================================================
char *
getenv(const char *name)
{
    int offset;

    return (__findenv(name, &offset));
}

char *
__findenv(const char *name, int *offset)
{
    extern char **environ;
    int len, i;
    const char *np;
    char **p, *cp;

    if (name == NULL || environ == NULL)
        return (NULL);
    for (np = name; *np && *np != '='; ++np)
        ;
    len = np - name;
    for (p = environ; (cp = *p) != NULL; ++p) {
//environ为glibc库的环境变量指针数组,可以使用env指令显示environ变量指向的所有数值[luther.gliethttp].
        for (np = name, i = len; i && *cp; i--)
            if (*cp++ != *np++)
                break;
        if (i == 0 && *cp++ == '=') {
            *offset = p - environ;
            return (cp);//ok,这里cp指针指向"/home/luther"了.
    }
  }
    return (NULL);
}