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Introduction to Socket Details

Unix Network Programming

II - Unit

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Socket Address Structure

Most socket functions (Posix C lib) require apointer to a socket address struct as an

argument Each supported protocol suite defines its own

socket address struct Names begin with sockaddr_ End with a unique suffix for each protocol

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IPv4 Socket Address Struct

/usr/include/netinet/in.h [#include ]/* Internet address. */struct in_addr

{in_addr_t s_addr;

};

/* Structure describing an Internet socket address. */struct sockaddr_in

{ __SOCKADDR_COMMON (sin_);

in_port_t sin_port; /* Port number. */struct in_addr sin_addr; /* Internet address. */uint8_t sin_len; /* length of structure */sa_family_t sin_family; /* 32 bit IPV4 address */char sin_zero[8];

};

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Generic socket Address Struct

int bind(int, struct sockaddr *,sock_len_t);

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IPv4 Socket Address Structure

IPv4 address and TCP or UDP port number

are always stored in network byte order(discussed later)

An IPv4 socket address structure, commonly

called an Internet socket address structureis named sockaddr_in and is defined byincluding header.

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Generic Socket Address Structure

A socket address structure is always passedby reference when passed as an argument

Any socket function must deal with structsfrom any of the supported protocol families Therefore they require a generic struct Defined in (actually pulled out of

on our system/* Structure describing a generic socket address. */struct sockaddr

{ __SOCKADDR_COMMON (sa_); /* Common data: address family and length. */char sa_data[14]; /* Address data. */

};

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Generic Socket Address Struct

Socket functions are then defined as taking apointer to the generic socket address

structure:int bind(int, struct sockaddr *, socklen_t);

To call bind, for example:struct sockaddr_in serv; /* IPv4 socket address structure */

/* fill in serv{} */bind(sockfd, (struct sockaddr *) &serv, sizeof(serv));

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IPv6 Socket Address Structure

IPv6 socket address is also defined in :/* IPv6 address */struct in6_addr

{uint8_t s6_addr[16];

};#define SIN6_LEN /* required for compile test */

/* Ditto, for IPv6. */struct sockaddr_in6

{__SOCKADDR_COMMON (sin6_);in_port_t sin6_port; /* Transport layer port # */uint32_t sin6_flowinfo; /* IPv6 flow information */struct in6_addr sin6_addr; /* IPv6 address */

uint32_t sin6_scope_id; /* set of interfaces for a scope, IPv6 scope-id*/};

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New Generic Socket Address

IPv6 defined a new generic socket addressstructure:

struct sockaddr_storage{

__SOCKADDR_COMMON (ss_); /* Address family, etc. */__ss_aligntype __ss_align; /* Force desired alignment. */char __ss_padding[_SS_PADSIZE];

};

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Comparison of Socket Address

Structures

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Byte Ordering Functions

Consider a 16-bit integer that is made up oftwo bytes.

There are 2 ways to store the two bytes inmemory: With the low-order byte at the starting address

(little-endian) With the high-order byte at the starting address

(big-endian) Show figure 3.9, pg. 77

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Network Byte Order

We must deal with these byte orderingdifferences as network programmers

Networking protocols must specify a networkbyte order

IP uses big-endian byte ordering

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Value Result Arguments

When a socket address structure is passed toany socket function, it is always passed byreference, i.e a pointer to the structure ispassed.

The length of the structure is also passed asan argument

Length is passed depends on which directionthe structure is being passedi.e from the process to kernel or vice versa.

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Value-Result Argument

Three functions, bind ,connect and sento pass asocket address structure from the process to the

kernel One of the argument to these 3 functions is the

pointer to the socket address structure

Another argument is the integer size of thestructure.struct sockaddr_in serv;Connect(sockfd,(SA *) &serv, sizeof(serv));

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Value -Result Arguments

bind, connect, sento

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Value-Result Arguments Kernel to the process, size can change accept , recvfrom , getsockname ,

g etpeername

Fixed size data structure Kernel informs howmuch information was

stored

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Size is both a value when the function iscalled( it tells the kernel the size of the

structure so that kernel does not write pastthe end of the structure when filling it) A result when the function returns (it tells the

process how much information the kernelactually stored in the structure)

This type of argument is called a value-resultargument

Value-Result Arguments

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Byte Manipulation Functions

2 groups of functions operate on multibytefields

#include void bzero (void *dest, size_t nbytes);void bcopy (const void *src, void *dest, size_t nbytes);int bcmp (const void *ptr1, const void *ptr2, size_t nbytes);

void* memset (void *dest, int c, size_t len);void* memcpy (void *dest, const void *src, size_t nbytes);int memcmp(const void *ptr1, const void *ptr2, size_t nbytes);

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The functions beginning with str (forstring),defined by including the

header Whose names begin with b (for byte) Names begin with mem (for memory) bzero sets the specified number of bytes to 0

in the destination . We often use this function to initialize a

socket address structure to 0

Byte Manipulation Functions

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bcopy moves the specified number of bytesfrom source to the destination.

bcmp compares two arbitrary bytes strings .returns zero if two byte strings are

identicalreturns nonzero if two byte strings

are not equal memset sets the specified number of bytes

to the value c in the destination

Byte Manipulation Functions

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memcpy is similar to bcopy, but the order ofthe two pointer arguments is swapped

bcopy correctly handles overlappingfields, while the behavior of memcpy isundefined if the source and destination

overlap memcmp compares two arbitrary byte

strings and return 0 or less than o,

Byte Manipulation Functions