3.6 inet_aton,
inet_addr, and inet_ntoa Functions
We will describe two groups of address
conversion functions in this section and the next. They convert
Internet addresses between ASCII strings (what humans prefer to
use) and network byte ordered binary values (values that are stored
in socket address structures).
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inet_aton, inet_ntoa, and
inet_addr convert an IPv4 address from a dotted-decimal
string (e.g., "206.168.112.96") to its 32-bit network byte
ordered binary value. You will probably encounter these functions
in lots of existing code.
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The newer
functions, inet_pton and inet_ntop, handle both
IPv4 and IPv6 addresses. We describe these two functions in the
next section and use them throughout the text.
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#include <arpa/inet.h>
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int inet_aton(const char *strptr, struct in_addr *addrptr);
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Returns: 1 if string was valid, 0 on error
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in_addr_t inet_addr(const char
*strptr);
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Returns: 32-bit binary network byte ordered IPv4
address; INADDR_NONE if error
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char *inet_ntoa(struct in_addr
inaddr);
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Returns: pointer to dotted-decimal string
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The first of these, inet_aton, converts
the C character string pointed to by strptr into its 32-bit binary network byte
ordered value, which is stored through the pointer addrptr. If successful, 1 is returned;
otherwise, 0 is returned.
An undocumented feature of inet_aton is
that if addrptr is a null pointer,
the function still performs its validation of the input string but
does not store any result.
inet_addr does the same conversion,
returning the 32-bit binary network byte ordered value as the
return value. The problem with this function is that all
232 possible binary values are valid IP addresses
(0.0.0.0 through 255.255.255.255), but the function returns the
constant INADDR_NONE (typically 32 one-bits) on an error.
This means the dotted-decimal string 255.255.255.255 (the IPv4
limited broadcast address, Section 20.2)
cannot be handled by this function since its binary value appears
to indicate failure of the function.
A potential problem with inet_addr is
that some man pages state that it returns 鈥? on an error, instead
of INADDR_NONE. This can lead to problems, depending on
the C compiler, when comparing the return value of the function (an
unsigned value) to a negative constant.
Today, inet_addr is deprecated and any
new code should use inet_aton instead. Better still is to
use the newer functions described in the next section, which handle
both IPv4 and IPv6.
The inet_ntoa function converts a
32-bit binary network byte ordered IPv4 address into its
corresponding dotted-decimal string. The string pointed to by the
return value of the function resides in static memory. This means
the function is not reentrant, which we will discuss in Section 11.18.
Finally, notice that this function takes a structure as its
argument, not a pointer to a structure.
Functions that take actual structures as
arguments are rare. It is more common to pass a pointer to the
structure.
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