static inline char * __attribute__((warn_unused_result)) __ul_returns_nonnull
xstrndup(const char *str, size_t size)
{
        char *ret;

        if (!str)
                return NULL;

// 这个是什么错误    ret = strndup(str, size);

        if (!ret)
                err(XALLOC_EXIT_CODE, "cannot duplicate string");
        return ret;
}



gcc -c hexdump.c

In file included from hexdump.c:59:0:

xalloc.h: In function 'xstrndup':
xalloc.h:83:15: warning: incompatible implicit declaration of built-in function
'strndup' [enabled by default]
         ret = strndup(str, size);

strndup 定义在哪里？你#include它声明所在的头(头文件)了吗？