inet(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

INET(3)                 Linux Programmer's Manual                INET(3)

NAME         top

       inet_aton, inet_addr, inet_network, inet_ntoa, inet_makeaddr,
       inet_lnaof, inet_netof - Internet address manipulation routines

SYNOPSIS         top

       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <arpa/inet.h>

       int inet_aton(const char *cp, struct in_addr *inp);

       in_addr_t inet_addr(const char *cp);
       in_addr_t inet_network(const char *cp);

       char *inet_ntoa(struct in_addr in);

       struct in_addr inet_makeaddr(in_addr_t net, in_addr_t host);

       in_addr_t inet_lnaof(struct in_addr in);
       in_addr_t inet_netof(struct in_addr in);

   Feature Test Macro Requirements for glibc (see
   feature_test_macros(7)):

       inet_aton(), inet_ntoa():
           Since glibc 2.19:
               _DEFAULT_SOURCE
           In glibc up to and including 2.19:
               _BSD_SOURCE || _BSD_SOURCE

DESCRIPTION         top

       inet_aton() converts the Internet host address cp from the IPv4
       numbers-and-dots notation into binary form (in network byte
       order) and stores it in the structure that inp points to.
       inet_aton() returns nonzero if the address is valid, zero if not.
       The address supplied in cp can have one of the following forms:

       a.b.c.d
              Each of the four numeric parts specifies a byte of the
              address; the bytes are assigned in left-to-right order to
              produce the binary address.

       a.b.c  Parts a and b specify the first two bytes of the binary
              address.  Part c is interpreted as a 16-bit value that
              defines the rightmost two bytes of the binary address.
              This notation is suitable for specifying (outmoded) Class
              B network addresses.

       a.b    Part a specifies the first byte of the binary address.
              Part b is interpreted as a 24-bit value that defines the
              rightmost three bytes of the binary address.  This
              notation is suitable for specifying (outmoded) Class A
              network addresses.

       a      The value a is interpreted as a 32-bit value that is
              stored directly into the binary address without any byte
              rearrangement.

       In all of the above forms, components of the dotted address can
       be specified in decimal, octal (with a leading 0), or
       hexadecimal, with a leading 0X).  Addresses in any of these forms
       are collectively termed IPV4 numbers-and-dots notation.  The form
       that uses exactly four decimal numbers is referred to as IPv4
       dotted-decimal notation (or sometimes: IPv4 dotted-quad
       notation).

       inet_aton() returns 1 if the supplied string was successfully
       interpreted, or 0 if the string is invalid (errno is not set on
       error).

       The inet_addr() function converts the Internet host address cp
       from IPv4 numbers-and-dots notation into binary data in network
       byte order.  If the input is invalid, INADDR_NONE (usually -1) is
       returned.  Use of this function is problematic because -1 is a
       valid address (255.255.255.255).  Avoid its use in favor of
       inet_aton(), inet_pton(3), or getaddrinfo(3), which provide a
       cleaner way to indicate error return.

       The inet_network() function converts cp, a string in IPv4
       numbers-and-dots notation, into a number in host byte order
       suitable for use as an Internet network address.  On success, the
       converted address is returned.  If the input is invalid, -1 is
       returned.

       The inet_ntoa() function converts the Internet host address in,
       given in network byte order, to a string in IPv4 dotted-decimal
       notation.  The string is returned in a statically allocated
       buffer, which subsequent calls will overwrite.

       The inet_lnaof() function returns the local network address part
       of the Internet address in.  The returned value is in host byte
       order.

       The inet_netof() function returns the network number part of the
       Internet address in.  The returned value is in host byte order.

       The inet_makeaddr() function is the converse of inet_netof() and
       inet_lnaof().  It returns an Internet host address in network
       byte order, created by combining the network number net with the
       local address host, both in host byte order.

       The structure in_addr as used in inet_ntoa(), inet_makeaddr(),
       inet_lnaof(), and inet_netof() is defined in <netinet/in.h> as:

           typedef uint32_t in_addr_t;

           struct in_addr {
               in_addr_t s_addr;
           };

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌───────────────────────────────┬───────────────┬────────────────┐
       │Interface                      Attribute     Value          │
       ├───────────────────────────────┼───────────────┼────────────────┤
       │inet_aton(), inet_addr(),      │ Thread safety │ MT-Safe locale │
       │inet_network(), inet_ntoa()    │               │                │
       ├───────────────────────────────┼───────────────┼────────────────┤
       │inet_makeaddr(), inet_lnaof(), │ Thread safety │ MT-Safe        │
       │inet_netof()                   │               │                │
       └───────────────────────────────┴───────────────┴────────────────┘

CONFORMING TO         top

       inet_addr(), inet_ntoa(): POSIX.1-2001, POSIX.1-2008, 4.3BSD.

       inet_aton() is not specified in POSIX.1, but is available on most
       systems.

NOTES         top

       On x86 architectures, the host byte order is Least Significant
       Byte first (little endian), whereas the network byte order, as
       used on the Internet, is Most Significant Byte first (big
       endian).

       inet_lnaof(), inet_netof(), and inet_makeaddr() are legacy
       functions that assume they are dealing with classful network
       addresses.  Classful networking divides IPv4 network addresses
       into host and network components at byte boundaries, as follows:

       Class A
              This address type is indicated by the value 0 in the most
              significant bit of the (network byte ordered) address.
              The network address is contained in the most significant
              byte, and the host address occupies the remaining three
              bytes.

       Class B
              This address type is indicated by the binary value 10 in
              the most significant two bits of the address.  The network
              address is contained in the two most significant bytes,
              and the host address occupies the remaining two bytes.

       Class C
              This address type is indicated by the binary value 110 in
              the most significant three bits of the address.  The
              network address is contained in the three most significant
              bytes, and the host address occupies the remaining byte.

       Classful network addresses are now obsolete, having been
       superseded by Classless Inter-Domain Routing (CIDR), which
       divides addresses into network and host components at arbitrary
       bit (rather than byte) boundaries.

EXAMPLES         top

       An example of the use of inet_aton() and inet_ntoa() is shown
       below.  Here are some example runs:

           $ ./a.out 226.000.000.037      # Last byte is in octal
           226.0.0.31
           $ ./a.out 0x7f.1               # First byte is in hex
           127.0.0.1

   Program source

       #define _BSD_SOURCE
       #include <arpa/inet.h>
       #include <stdio.h>
       #include <stdlib.h>

       int
       main(int argc, char *argv[])
       {
           struct in_addr addr;

           if (argc != 2) {
               fprintf(stderr, "%s <dotted-address>\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           if (inet_aton(argv[1], &addr) == 0) {
               fprintf(stderr, "Invalid address\n");
               exit(EXIT_FAILURE);
           }

           printf("%s\n", inet_ntoa(addr));
           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

       byteorder(3), getaddrinfo(3), gethostbyname(3), getnameinfo(3),
       getnetent(3), inet_net_pton(3), inet_ntop(3), inet_pton(3),
       hosts(5), networks(5)

COLOPHON         top

       This page is part of release 5.11 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

GNU                            2021-03-22                        INET(3)

Pages that refer to this page: getaddrinfo(3)getaddrinfo_a(3)gethostbyname(3)inet_net_pton(3)inet_ntop(3)inet_pton(3)networks(5)ip(7)