The bind() function accepts a pointer to a sockaddr, but in all examples I've seen, a sockaddr_in structure is used instead, and is cast to sockaddr:
struct sockaddr_in name;
...
if (bind (sock, (struct sockaddr *) &name, sizeof (name)) < 0)
...
I can't wrap my head around why is a sockaddr_in struct used. Why not just prepare and pass a sockaddr?
Is it just convention?
No, it's not just convention.
sockaddr is a generic descriptor for any kind of socket operation, whereas sockaddr_in is a struct specific to IP-based communication (IIRC, "in" stands for "InterNet"). As far as I know, this is a kind of "polymorphism" : the bind() function pretends to take a struct sockaddr *, but in fact, it will assume that the appropriate type of structure is passed in; i. e. one that corresponds to the type of socket you give it as the first argument.
I don't know if its very much relevant for this question, but I would like to provide some extra info which may make the typecaste more understandable as many people who haven't spent much time with C get confused seeing such a typecaste.
I use macOS, so I am taking examples based on header files from my system.
struct sockaddr is defined as follows:
struct sockaddr {
__uint8_t sa_len; /* total length */
sa_family_t sa_family; /* [XSI] address family */
char sa_data[14]; /* [XSI] addr value (actually larger) */
};
struct sockaddr_in is defined as follows:
struct sockaddr_in {
__uint8_t sin_len;
sa_family_t sin_family;
in_port_t sin_port;
struct in_addr sin_addr;
char sin_zero[8];
};
Starting from the very basics, a pointer just contains an address. So struct sockaddr * and struct sockaddr_in * are pretty much the same. They both just store an address. Only relevant difference is how compiler treats their objects.
So when you say (struct sockaddr *) &name, you are just tricking the compiler and telling it that this address points to a struct sockaddr type.
So let's say the pointer is pointing to a location 1000. If the struct sockaddr * stores this address, it will consider memory from 1000 to sizeof(struct sockaddr) possessing the members as per the structure definition. If struct sockaddr_in * stores the same address it will consider memory from 1000 to sizeof(struct sockaddr_in).
When you typecasted that pointer, it will consider the same sequence of bytes upto sizeof(struct sockaddr).
struct sockaddr *a = &name; // consider &name = 1000
Now if I access a->sa_len, the compiler would access from location 1000 to sizeof(__uint8_t) which is same bytes size as in case of sockaddr_in. So this should access the same sequence of bytes.
Same pattern is for sa_family.
After that there is a 14 byte character array in struct sockaddr which stores data from in_port_t sin_port (typedef'd 16 bit unsigned integer = 2 bytes ) , struct in_addr sin_addr (simply a 32 bit ipv4 address = 4 bytes) and char sin_zero[8](8 bytes). These 3 add up to make 14 bytes.
Now these three are stored in this 14 bytes character array and we can access any of these three by accessing appropriate indices and typecasting them again.
user529758's answer already explains the reason to do this.
This is because bind can bind other types of sockets than IP sockets, for instance Unix domain sockets, which have sockaddr_un as their type. The address for an AF_INET socket has the host and port as their address, whereas an AF_UNIX socket has a filesystem path.
Related
I'm learning HTTP protocol following a tutorial which gives an understandable piece of code and here's part of it.
struct sockaddr_in address;
...
address.sin_family = AF_INET;
address.sin_addr.s_addr = INADDR_ANY;
address.sin_port = htons( PORT );
memset(address.sin_zero, '\0', sizeof address.sin_zero);
if (bind(server_fd, (struct sockaddr *)&address, sizeof(address))<0)
{
perror("In bind");
exit(EXIT_FAILURE);
}
The example code works well, although I don't understand the some kind of transfer between two structs.
the definition of struct sockaddr_in in <netinet/in.h> is
struct sockaddr_in {
__uint8_t sin_len;
sa_family_t sin_family;
in_port_t sin_port;
struct in_addr sin_addr;
char sin_zero[8];
};
the definition of struct sockaddr in <sys/socket.h> is
struct sockaddr {
__uint8_t sa_len; /* total length */
sa_family_t sa_family; /* [XSI] address family */
char sa_data[14]; /* [XSI] addr value (actually larger) */
};
They have different structures, how the "transfer/casting" works there?
I don't understand the some kind of transfer between two structs.
There is no data transfer between different structs, nor any conversion of structure objects. In bind(server_fd, (struct sockaddr *)&address, sizeof(address)), a pointer to a struct is converted to a different object pointer type. This is explicitly allowed by C.
The C language specification does not define any behavior for accessing the struct via the converted pointer. Any attempt to do so would violate the strict aliasing rule, but that's not your problem. The example you presented demonstrates an utterly standard usage idiom for the bind() function, for which it was designed. Therefore, you can rely on the bind() implementation to do the right thing with it, by whatever magic is required.
Conceptually, though, you can observe that the first two members of struct sockaddr and struct sockaddr_in have the same data types. You could imagine, then, that bind is able to access those two members via the converted pointer, despite it constituting a strict-aliasing violation. Although C does not define behavior for that, POSIX implicitly requires that it work in at least this case. Having then done that, the second of those members indicates the address family, by which bind() can invoke the appropriate behavior for the address's actual type.
That is a variation on C-style polymorphism. It is helped out by the third bind argument, the size of the address object, which enables bind() to copy the address object without knowing its true effective data type.
These structure types and the bind() API could have been defined a bit differently to avoid the implied strict-aliasing violation, but that wasn't necessary in early C, where member names corresponded directly to offsets from the beginning of the structure. And where those names were global, which is why you see the sin_ and sa_ prefixes in those member names, and similar in many other structure types provided by the system. Nowadays, it's best to just accept that that's how bind() is used, and it's up to the system to provide a bind() implementation that accommodates it.
The casting works.
Looking at the two structures:
struct sockaddr_in {
__uint8_t sin_len;
sa_family_t sin_family;
in_port_t sin_port;
struct in_addr sin_addr;
char sin_zero[8];
};
struct sockaddr {
__uint8_t sa_len; /* total length */
sa_family_t sa_family; /* [XSI] address family */
char sa_data[14]; /* [XSI] addr value (actually larger) */
};
First two members, sin_len and sa_len, sin_family and sa_family will not be problematic as those are of the same data type. The padding for sa_family_t works exactly the same on both ends.
Looking at the reference,
in_port_t Equivalent to the type uint16_t as described in <inttypes.h>
in_addr_t Equivalent to the type uint32_t as described in <inttypes.h>
For windows, struct in_addr looks like below:
struct in_addr {
union {
struct {
u_char s_b1;
u_char s_b2;
u_char s_b3;
u_char s_b4;
} S_un_b;
struct {
u_short s_w1;
u_short s_w2;
} S_un_w;
u_long S_addr;
} S_un;
};
and that for a linux is:
struct in_addr {
uint32_t s_addr; /* address in network byte order */
};
The whole confusion you might have is because of how the contents align. However, it is a well-thought historic design. It is intended to accommodate implementation-dependent aspects in the design.
When I Secondly, implementation-dependent -- it refers to the fact that implementation of in_addr_t is not consistent across all systems, as seen above.
In a nutshell, this entire magic works, because of the 2 things: The exact size and padding nature of the first two members and then lastly the data type of sa_data[14] is char, or more precisely an array of a 1-byte data-type. This design trick with union inside a struct has been widely used.
Unix Network Programming Volume 1 states:
The reason the sin_addr member is a structure, and not just an in_addr_t, is historical. Earlier releases (4.2BSD) defined the in_addr structure as a union of various structures, to allow access to each of the 4 bytes and to both of the 16-bit values contained within the 32-bit IPv4 address. This was used with class A, B, and C addresses to fetch the appropriate bytes of the address. But with the advent of subnetting and then the disappearance of the various address classes with classless addressing, the need for the union disappeared. Most systems today have done away with the union and just define in_addr as a structure with a single in_addr_t member.
Not what you asked for, but good to know:
The same header states:
The sockaddr_in structure is used to store addresses for the Internet address family. Values of this type shall be cast by applications to struct sockaddr for use with socket functions.
So, sockaddr_in is a struct specific to IP-based communication and sockaddr is more of a generic structure for socket operations.
Just a try:
#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
int main(void)
{
printf("sizeof(struct sockaddr_in) = %zu bytes\n", sizeof(struct sockaddr_in));
printf("sizeof(struct sockaddr) = %zu bytes\n", sizeof(struct sockaddr));
return 0;
}
Prints:
sizeof(struct sockaddr_in) = 16 bytes
sizeof(struct sockaddr) = 16 bytes
I think this cast breaks the strict aliasing rule and then is undefined behaviour if the bind function dereferences the pointer.
In practice the code assumes that all fields of struct sockaddr_in are contiguous so you can access a buffer of bytes either as a struct sockaddr_in or as a struct sockaddr equivalently. But the fields of a structure are not guaranteed to be contiguous. If in_port_tis two bytes long for example, there may very well be a hole between sin_portand sin_addr with a 32 bytes machine compiler because it may want to align sin_addr field on 32 bytes address.
This way of coding is frequent when you develop a communication interface driver: you receive a buffer of bytes that need to be interpreted as a data structure (like: first byte is an adress, following bytes are a length, etc...). Casting from a structure to another one avoids to copy data.
Note that usually compilers provide non-standard-C ways to guarantee that all fields of structures are contigiuous. For example with gcc it is __attribute__((packed))
Now, to answer to your question: provided the structures are packed and there is no undefined behaviour, the cast basically does nothing. sa_data will be the array of bytes located after the field sin_family. So this array will consist of sin_port, followed by sin_addr followed by the array sin_zero.
EDIT: I compiled tje following structures on STM32H7 (ARM cortex M7, 32 bits architecture) with arm-none-eabi-gcc:
struct in_addr {
uint32_t s_addr;
};
struct sockaddr_in {
uint8_t sin_len;
uint16_t sin_family;
uint16_t sin_port;
struct in_addr sin_addr;
char sin_zero[8];
};
struct sockaddr {
uint8_t sa_len;
uint16_t sa_family;
char sin_zero[14];
};
The size of sockaddr_in is 20.
The size of sockaddr is 18.
Note that if sa_family_t is of type char and not short, due to alignment, both structures are same size.
int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
The actual structure passed for the addr argument will depend on the address family. The sockaddr structure is defined as something like:
struct sockaddr {
sa_family_t sa_family;
char sa_data[14];
}
So for an IPv4 address (AF_INET), the actual struct that will be passed is this:
/* Source http://linux.die.net/man/7/ip */
struct sockaddr_in {
sa_family_t sin_family; /* address family: AF_INET */
in_port_t sin_port; /* port in network byte order */
struct in_addr sin_addr; /* internet address */
};
/* Internet address. */
struct in_addr {
uint32_t s_addr; /* address in network byte order */
};
Does the bind code read the sockaddr.sa_family value and depending on the value it finds, it will then cast the sockaddr struct into the appropriate struct such as sockaddr_in?
Why is the sa_data set to 14 characters? If I understand correct, the sa_data field is just a field that will have large enough memory space to fit all address family types? Presumably the original designers anticipated that 14 characters would be wide enough to fit all future types.
According to the glibc manual:
The length 14 of sa_data is essentially arbitrary.
And the FreeBSD developers handbook mentions the following:
Please note the vagueness with which the sa_data field is declared,
just as an array of 14 bytes, with the comment hinting there can be
more than 14 of them.
This vagueness is quite deliberate. Sockets is a very powerful
interface. While most people perhaps think of it as nothing more than
the Internet interface—and most applications probably use it for that
nowadays—sockets can be used for just about any kind of interprocess
communications, of which the Internet (or, more precisely, IP) is only
one.
Yes, the sa_family field is used to recognize how to treat the struct passed (which is cast to struct sockaddr* in a call to bind). You can read more about how it works also in a FreeBSD developers handbook.
And actually there are "polymorphic" (sub)types of sockaddr, in which sa_data contains more than 16 bytes, for example:
struct sockaddr_un {
sa_family_t sun_family; /* AF_UNIX */
char sun_path[108]; /* pathname */
};
The sockaddr struct is used as a tagged union. By reading the sa_family field it can be cast to a struct of the proper form.
The 14 bytes is arbitrary. It's big enough to hold IPv4 addresses, but not big enough to hold IPv6 addresses. There is also a sockaddr_storage struct which is big enough for both. Reading the Microsoft docs on SOCKADDR_STORAGE, it comes in at 128 bytes, so much larger than needed for IPv6. Checking some Linux headers, it seems to be at least that large there as well.
For reference, the IPv6 struct is:
struct sockaddr_in6 {
u_int16_t sin6_family; // address family, AF_INET6
u_int16_t sin6_port; // port number, Network Byte Order
u_int32_t sin6_flowinfo; // IPv6 flow information
struct in6_addr sin6_addr; // IPv6 address
u_int32_t sin6_scope_id; // Scope ID
};
struct in6_addr {
unsigned char s6_addr[16]; // IPv6 address
};
As you can see, the 16 byte s6_addr field is already bigger than the 14 byte sa_data field on it's own. Total size after the sa_family field is 26 bytes.
In functions as connect() the last parameter is the size of the sockaddr_in structure.
All well, but sin_zero is unused (the system doesn't "pop sin_zero" as I understood in stackoverflow.com/questions/28280581/how-kernels-recognize-sin-zero-sockaddr-in-structure-pushed), so for what the functions need this parameter?
I.e. asm or when C is compiled to asm, the RET of connect() must specify the number of bytes to delete (RET n), bytes that are the connect() arguments. Why n is a fixed number (don't change I specify the sin_zero or no) if I can push or no sin_zero in asm and assign value to sin_zero in C. And if I create a program in asm and I don't push sin_zero to the stack, the programs works perfectly... but n is the same number!
A sockaddr_in is a structure containing an internet address. This structure is defined in <netinet/in.h>. This is server/client address struct sockaddr_in serv_addr, cli_addr; Here is the definition:
struct sockaddr_in {
short sin_family;
u_short sin_port;
struct in_addr sin_addr;
char sin_zero[8];
};
sin_family – use AF_INET
sin_port – port number (in network byte order => use htons(port))
sin_addr – Internet address described by struct in_addr
struct in_addr {
unsigned long s_addr;
};
Set s_addr to INADDR_ANY => local internet address.
sin_zero[] – set to 0 with bzero() or memset(). Padding to make structure the same size as SOCKADDR.
bind() example
int mysock,err;
struct sockaddr_in myaddr;
mysock = socket(AF_INET,SOCK_STREAM,0);
myaddr.sin_family = AF_INET;
myaddr.sin_port = htons( portnum );
myaddr.sin_addr.s_addr = INADDR_ANY;
bzero(&(myaddr.sin_zero),sizeof(myaddr.sin_zero));
err = bind(mysock, (struct sockaddr *) &myaddr, sizeof(myaddr));
I have found some info about sin_zero (Unix network programming chapter 3.2)
The POSIX specification requires only three members in the structure: sin_family, sin_addr, and sin_port. It is acceptable for a POSIX-compliant implementation to define additional structure members, and this is normal for an Internet socket address structure. Almost all implementations add the sin_zero member so that all socket address structures are at least 16 bytes in size.
And the definition of sin_zero
unsigned char __pad[__SOCK_SIZE__ - sizeof(short int)
- sizeof(unsigned short int) - sizeof(struct in_addr)];
};
#define sin_zero __pad
Most of the net code does not use sockaddr_in, it uses sockaddr. When you use a function like sendto, you must explicitly cast sockaddr_in, or whatever address your using, to sockaddr. sockaddr_in is the same size as sockaddr, but internally the sizes are the same because of a slight hack.
That hack is sin_zero. Really the length of useful data in sockaddr_in is shorter than sockaddr. But the difference is padded in sockaddr_in using a small buffer; that buffer is sin_zero.
On some architectures, it wont cause any problems not clearing sin_zero. But on other architectures it might. Its required by specification to clear sin_zero, so you must do this if you intend your code to be bug free for now and in the future.
Please, have a look at page.
I think, kernel uses size parameter to see your sockaddr is ipv6 address or not because ipv6 address size is bigger than other net address in linux and unix.
Maybe it's the architecture issue, on different architecture like 64-bit / 32-bit size of primitive declared in the sockaddr_in will be different. That's why it may be a parameter declared in the connect() function.
getaddrinfo() fills struct addrinfo where actual packed address stored in struct sockaddr *ai_addr field. struct sockaddr has field char *sa_data with actual binary address representation. I want to copy this sa_data to another variable using memcpy(), for this I need to know actual length of this field.
I thinked ai_addrlen is what I need. But in fact it returns 16 for IPv4 and 28 for IPv6. Too much I think. So, what is the most correct way to get sa_data length? May be length = sa_family == AF_INET ? 4 : 16 or length = ai_addrlen - 12?
The getaddrinfo() function returns a linked list consisting of struct addrinfo records. Each of those records includes a pointer to struct sockaddr which is a polymorphic type, behind which there is actually a struct sockaddr_in or struct sockaddr_in6. The ai_addrlen member tells you how large is that structure, i.e. returns either sizeof (struct sockaddr_in) or sizeof (struct sockaddr_in6).
Those structures contain the binary IP address, the port number (for the transport protocol like TCP or UDP), the family (repeated in struct addrinfo as well) and other data (flowinfo and scope_id for IPv6, when looking at linux docs).
The actual binary address is inside the struct sockaddr_in and strict sockaddr_in6 as struct in_addr sin_addr and struct in6_addr sin6_addr. You can use those fields or types to determine the size.
length = sa_family == AF_INET ? sizeof (struct in_addr) : sizeof (struct in6_addr)
As those are known numbers, you can of course use what you have already in your question.
length = sa_family == AF_INET ? 4 : 16
But you also tried to do arithmetics on ai_addrlen and that is not a good idea, as the types can vary by operating system and there's no logic in that anyway. I can add more details if you had a specific use case or specific code.
I'm looking at functions such as connect() and bind() in C sockets and notice that they take a pointer to a sockaddr struct. I've been reading and to make your application AF-Independent, it is useful to use the sockaddr_storage struct pointer and cast it to a sockaddr pointer because of all the extra space it has for larger addresses.
What I am wondering is how functions like connect() and bind() that ask for a sockaddr pointer go about accessing the data from a pointer that points at a larger structure than the one it is expecting. Sure, you pass it the size of the structure you are providing it, but what is the actual syntax that the functions use to get the IP Address off the pointers to larger structures that you have cast to struct *sockaddr?
It's probably because I come from OOP languages, but it seems like kind of a hack and a bit messy.
Functions that expect a pointer to struct sockaddr probably typecast the pointer you send them to sockaddr when you send them a pointer to struct sockaddr_storage. In that way, they access it as if it was a struct sockaddr.
struct sockaddr_storage is designed to fit in both a struct sockaddr_in and struct sockaddr_in6
You don't create your own struct sockaddr, you usually create a struct sockaddr_in or a struct sockaddr_in6 depending on what IP version you're using. In order to avoid trying to know what IP version you will be using, you can use a struct sockaddr_storage which can hold either. This will in turn be typecasted to struct sockaddr by the connect(), bind(), etc functions and accessed that way.
You can see all of these structs below (the padding is implementation specific, for alignment purposes):
struct sockaddr {
unsigned short sa_family; // address family, AF_xxx
char sa_data[14]; // 14 bytes of protocol address
};
struct sockaddr_in {
short sin_family; // e.g. AF_INET, AF_INET6
unsigned short sin_port; // e.g. htons(3490)
struct in_addr sin_addr; // see struct in_addr, below
char sin_zero[8]; // zero this if you want to
};
struct sockaddr_in6 {
u_int16_t sin6_family; // address family, AF_INET6
u_int16_t sin6_port; // port number, Network Byte Order
u_int32_t sin6_flowinfo; // IPv6 flow information
struct in6_addr sin6_addr; // IPv6 address
u_int32_t sin6_scope_id; // Scope ID
};
struct sockaddr_storage {
sa_family_t ss_family; // address family
// all this is padding, implementation specific, ignore it:
char __ss_pad1[_SS_PAD1SIZE];
int64_t __ss_align;
char __ss_pad2[_SS_PAD2SIZE];
};
So as you can see, if the function expects an IPv4 address, it will just read the first 4 bytes (because it assumes the struct is of type struct sockaddr. Otherwise it will read the full 16 bytes for IPv6).
In C++ classes with at least one virtual function are given a TAG. That tag allows you to dynamic_cast<>() to any of the classes your class derives from and vice versa. The TAG is what allows dynamic_cast<>() to work. More or less, this can be a number or a string...
In C we are limited to structures. However, structures can also be assigned a TAG. In fact, if you look at all the structures that theprole posted in his answer, you will notice that they all start with 2 bytes (an unsigned short) which represents what we call the family of the address. This defines exactly what the structure is and thus its size, fields, etc.
Therefore you can do something like this:
int bind(int fd, struct sockaddr *in, socklen_t len)
{
switch(in->sa_family)
{
case AF_INET:
if(len < sizeof(struct sockaddr_in))
{
errno = EINVAL; // wrong size
return -1;
}
{
struct sockaddr_in *p = (struct sockaddr_in *) in;
...
}
break;
case AF_INET6:
if(len < sizeof(struct sockaddr_in6))
{
errno = EINVAL; // wrong size
return -1;
}
{
struct sockaddr_in6 *p = (struct sockaddr_in6 *) in;
...
}
break;
[...other cases...]
default:
errno = EINVAL; // family not supported
return -1;
}
}
As you can see, the function can check the len parameter to make sure that the length is enough to fit the expected structure and therefore they can reinterpret_cast<>() (as it would be called in C++) your pointer. Whether the data is correct in the structure is up to the caller. There is not much choice on that end. These functions are expected to verify all sorts of things before it uses the data and return -1 and errno whenever a problem is found.
So in effect, you have a struct sockaddr_in or struct sockaddr_in6 that you (reinterpret) cast to a struct sockaddr and the bind() function (and others) cast that pointer back to a struct sockaddr_in or struct sockaddr_in6 after they checked the sa_family member and verified the size.