Understanding Socket Communication Buffer Size

Short Answer
The buffer size should be 2048 bytes

Long answer
The maximum size of UDP packets on Ethernet can be determined by the MTU. The maximum MTU value can be 1500 bytes or 1492 or 1454 when PPPoE is included.Therefore, it is safe to set the maximum number of bytes that can be transferred at one time to 1454.

The buffer size should be greater than 1454, so any number is fine.Extremely speaking, it's also ants that have a 1 gigabyte buffer and use only 1454 bytes.It's meaningless, but it's safe to have a power size of 2 to fit well into the CPU's L1/L2/L3 cache boundary (in the hope that the compiler and execution environment will put it on), so it's enough to use it as 2048, which should be 1454 or higher is 2048.

If all equipment is jumbo-framed on the local intranet, you can increase the MTU, but the best value depends on the equipment, so the best value depends on the field.The software creator should not worry about it and adopt MTU=1454 bytes.

If UDP is implemented over Ethernet, UDP can only succeed or fail, so "how much buffer did you send?" is meaningless.From the receiver's point of view, the transfer is either successful or unsuccessful.sendto() gets exactly the number of bytes sent by sendto() and nothing (not even known if it succeeds or fails)

The question is ambiguous, so the interpretation may be different.
If possible, it would be better to describe the situation in detail

UDP packet size and maximum message size are determined at the design stage (originally)

If you don't know the structure (details) of the sending program, you can check the contents of the sending.

• Check with tools such as tcpdump, Wireshark, and eBPF
• You can do it with the user app for now.
  1. Flags for recv, recvfrom, recvmsg:MSG_PEEK, MSG_TRUNC show the packet length
  2. Insufficient Buffer Expands
  3. (without MSG_PEEK) Actual Load

As other solutions show, the design of the receive buffer size is

• Size values are relatively small to match hardware and network realities to a maximum of 2 powers
• If you can consider the sender, you can send it up to MTU-30 bytes (various header lengths)

Also note that recent operating systems including Ubuntu may not be able to send (as is) if the MTU is exceeded.

Note: udp-IPv4 User Datagram Protocol

By default, UDP on Linux performs Path MTU Discovery. In other words, the kernel records the maximum transmission unit (MTU) for a specific destination IP address and returns EMSGSIZE if UDP packet write exceeds the MTU. Applications should reduce the packet size if EMSGSIZE is returned. You can also disable Path MTU Discovery using the socket option IP_MTU_DISCOVER or the /proc/sys/net/ipv4/ip_no_pmtu_disc file (see ip(7) for more information). If Path MTU Discovery is disabled, UDP fragments the packet if the packet size is larger than the interface MTU. However, disabling Path MTU Discovery is not recommended for performance and reliability reasons.

Add

About the code you used as a reference

It doesn't look like a program to refer to.

• recvfrom() argument from_addr, sin_size has not been initialized (sin_size is for both input and output)
• But the receive buffer is memset() (no need)

You should refer to the API documentation and refer to the site where you are using it correctly.
(When I searched for it, there were a lot of NULL specification that I ignored, so I couldn't find any variable specification, but it should be somewhere.)