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In C, using a previously undeclared function constitutes an implicit declaration of the function. In an implicit declaration, the return type is int if
I recall correctly. Now, GCC has built-in definitions for some standard functions. If an implicit declaration does not match the built-in definition, you get this warning.
To fix the problem, you have to declare the functions before using them; normally you do this by including the appropriate header. I recommend not to use the -fno-builtin-* flags if possible.
Instead of stdlib.h, you should try
#include <string.h>
That's where strcpy and strncpy are
defined, at least according to the strcpy(2) man page.
The exit function
is defined in stdlib.h, though, so I don't know what's going on there.
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说是要包含 #include <string.h> 没有问题了……我测试果然如此！！！