strncmp(NULL, "foo", 0) 定义明确吗？
Is strncmp(NULL, "foo", 0) well defined?
如果第三个参数为零，将 NULL 指针作为 strncmp 的参数是否安全？ IE。像这样的调用：
strncmp(NULL, "foo", 0);
来自the C strncmp documentation at cppreference.com：
The behavior is undefined when either lhs or rhs is the null pointer.
只需阅读文档即可。
C 标准说你不应该将 无效 指针传递给库函数，一般来说。
引用 C11，章节 §7.24.1，"String function conventions"，（强调我的）
Where an argument declared as size_t n specifies the length of the array for a
function, n can have the value zero on a call to that function. Unless explicitly stated
otherwise in the description of a particular function in this subclause, pointer arguments
on such a call shall still have valid values, as described in 7.1.4. On such a call, a
function that locates a character finds no occurrence, a function that compares two
character sequences returns zero, and a function that copies characters copies zero
characters.
而且我在 7.24.4.4 中没有看到任何具体提及（作为上述约束的例外），strncmp() 函数。
为 "invalid pointers" 添加上下文，引用 §7.1.4/p1，库函数的使用
[...] If an argument to a function has an invalid value (such as a value
outside the domain of the function, or a pointer outside the address space of the program,
or a null pointer, or a pointer to non-modifiable storage when the corresponding
parameter is not const-qualified) or a type (after promotion) not expected by a function
with variable number of arguments, the behavior is undefined. [...]
关于NULL，引用§7.19，<stddef.h>
NULL
which expands to an implementation-defined null pointer constant; [...]
如果第三个参数为零，将 NULL 指针作为 strncmp 的参数是否安全？ IE。像这样的调用：
strncmp(NULL, "foo", 0);
来自the C strncmp documentation at cppreference.com：
The behavior is undefined when either
lhsorrhsis the null pointer.
只需阅读文档即可。
C 标准说你不应该将 无效 指针传递给库函数，一般来说。
引用 C11，章节 §7.24.1，"String function conventions"，（强调我的）
Where an argument declared as
size_t nspecifies the length of the array for a function,ncan have the value zero on a call to that function. Unless explicitly stated otherwise in the description of a particular function in this subclause, pointer arguments on such a call shall still have valid values, as described in 7.1.4. On such a call, a function that locates a character finds no occurrence, a function that compares two character sequences returns zero, and a function that copies characters copies zero characters.
而且我在 7.24.4.4 中没有看到任何具体提及（作为上述约束的例外），strncmp() 函数。
为 "invalid pointers" 添加上下文，引用 §7.1.4/p1，库函数的使用
[...] If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer, or a pointer to non-modifiable storage when the corresponding parameter is not const-qualified) or a type (after promotion) not expected by a function with variable number of arguments, the behavior is undefined. [...]
关于NULL，引用§7.19，<stddef.h>
NULL
which expands to an implementation-defined null pointer constant; [...]