C：inet_ntop返回值的位置？

Question

I am reviewing the inet_ntop documentation. Typically, string builder operations in C return either a pointer to the end of the string, or an integer indicating the length of the string written.

The documentation of inet_ntop states quite ambiguously:

On success, inet_ntop() returns a non-null pointer to dst.  NULL is 
returned if there was an error, with the errno set to indicate the 
error.

There are two problems here:

1. Null string termination: it is not clear whether I need to add a null string delimiter throughout the memory allocation where inet_ntop is printing when compiling at order 3 or more

2. I do not know whether the output of the function returns a pointer to the last written character, or the first written character.

---

What does inet_ntop return?

Answer 1

It's quite clear to me, but perhaps you need to apply some "good will" to get it:

1. It's not a string in C if it isn't terminated, so of course the result is terminated.
2. dst is an input argument, even if it's a bit weirdly written "a pointer to dst " can't be "a pointer to dst plus something".

You can of course also read an implementation to see what's going on. The last statement for the "happy path" (no buffer overflow) for both IPv4 and v6 variants is:

return strcpy(dst, tmp);

Which instantly tells you that dst is receiving a terminated string, and that dst is returned.

Answer 2

它返回const char *并且由于要求dst参数是有效对象（不能将NULL作为dst传递），因此无需创建中间指针来返回函数的状态。

Answer 3

The inet_pton() documentation :

...

SYNOPSIS

 #include <arpa/inet.h> const char *inet_ntop(int af, const void *restrict src, char *restrict dst, socklen_t size); 

...

DESCRIPTION

The inet_ntop() function shall convert a numeric address into a text string suitable for presentation. The af argument shall specify the family of the address. This can be AF_INET or AF_INET6 . The src argument points to a buffer holding an IPv4 address if the af argument is AF_INET , or an IPv6 address if the af argument is AF_INET6 ; the address must be in network byte order. The dst argument points to a buffer where the function stores the resulting text string; it shall not be NULL . The size argument specifies the size of this buffer, which shall be large enough to hold the text string ( INET_ADDRSTRLEN characters for IPv4, INET6_ADDRSTRLEN characters for IPv6).

...

RETURN VALUE

The inet_ntop() function shall return a pointer to the buffer containing the text string if the conversion succeeds, and NULL otherwise, and set errno to indicate the error.

...

ERRORS

The inet_ntop() and inet_pton() functions shall fail if:

[ EAFNOSUPPORT ] The af argument is invalid.

[ ENOSPC ] The size of the inet_ntop() result buffer is inadequate.

And the definition of a "string" :

3.92 Character String

A contiguous sequence of characters terminated by and including the first null byte.

Given that, the answer to

1. Null string termination: it is not clear whether I need to add a null string delimiter throughout the memory allocation where inet_ntop is printing when compiling at order 3 or more

should be clear. A string by definition includes the null byte. inet_pton() returns a pointer to a buffer that contains such a string. No addition of a null byte is needed.

And the answer to

2. I do not know whether the output of the function returns a pointer to the last written character, or the first written character.

is given by the C standard in 6.3.2.3 Pointers , paragraph 7 :

... When a pointer to an object is converted to a pointer to a character type, the result points to the lowest addressed byte of the object.

So, the "pointer to the buffer containing the text string if the conversion succeeds" returned by inet_ntop() points to the first byte in the string.