Whats the addrlen field in recvfrom() used for? - c

Question

I'm using recvfrom in my program to get DGRAM data from a server I specify in src_addr. However, I'm not sure why I need to initialize and pass in addrlen.
I read the man page and I didn't really understand what it's getting at.
If src_addr is not NULL, and the underlying protocol provides the source address, this source address is filled in. When
src_addr is NULL, nothing is filled in; in this case, addrlen is not
used, and should also be NULL. The argument addrlen is a value-result argument, which the caller should initialize before the
call to the size of the buffer associated with src_addr, and
modified on return to indicate the actual size of the source address. The returned address is truncated if the buffer provided is too small;
in this case, addrlen will return a value greater than was supplied to the call.
I'm guessing that it's got something to do with src_addr being ipv4 or ipv6. Is this correct?
Thanks!

Answer 1

Maybe there is a missinterpretation from your side. Talking about:
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
struct sockaddr *src_addr, socklen_t *addrlen);
src_addr is not used to hand in the adress that you would like to listen to, but rather a storage location provided by you to get the actual source address handed out.
Thus if you set src_addr to NULL because youre not interested in the address at all, you don't have to care about addrlen as it won't get used anyway.
If on the other hand you want to be informed about the source address, you not only have to provide a storage location, but also tell how big the storage location you provided is.
Thats why you should initialize *addr_len to the buffer size you allocated.
After your call the value pointed to by addrlen will inform you about how much (if any) of the space you allocated to store the source address got actually filled with data.
About sizes
The whole hassle with struct sockaddr and passing sizes back and forth has to do with the fact that even thoug they're most heavily used in networking sockets were intended to be much more general concept.
Think about unix domain sockets as an an example as they are implemented via the filesystem they require an adressing scheme totaly different from that known from IP based networking. The type of sockaddr used here is:
struct sockaddr_un {
sa_family_t sun_family; /* AF_UNIX */
sun_path[UNIX_PATH_MAX]; /* pathname */
};
Compare this to the struct used in IP based networking:
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 */
};
it should be clear both don't have too much in common.
sockets were designed to be able to fit both cases.

Answer 2

ssize_t recvfrom(int socket, void *buffer, size_t length, int flags,
struct sockaddr *address, socklen_t *address_len);`
The address_len argument specifies the length of the address structure i.e. the number of bytes to use from the start address indicated at address(start address of memory location + number of bytes from the start address that hold the value)
The structure is defined in /usr/include/bits/socket.h
/* Structure describing a generic socket address. */
struct sockaddr
{
__SOCKADDR_COMMON (sa_); /* Common data: address family and length. */
char sa_data[14]; /* Address data. */
};
Thus the sa_data field holds the address data (start address of the data) whose length is indicated by the address_len argument.
... whenever a function says it takes a struct sockaddr* you can cast your
struct sockaddr_in*, struct sockaddr_in6*, or struct sockadd_storage*
to that type with ease and safety.
Therefore, as indicated in the man page and #WhozCraig in the comment to your question, this field is updated with the actual size when the method returns.
More information
recvfrom
Beej's Guide to Network Programming - struct sockaddr and pals