Why would one need to use `MSG_WAITALL` FLAG instead of `0` FLAG? Why to use it with UDP?

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At some point when coding sockets one will face the receive-family of functions (recv, recvfrom, recvmsg).

This function accepts a FLAG argument, in which I see that the MSG_WAITALL is used in many examples on the web, such as this example on UDP.

Here is a definition of the MSG_WAITALL flag

MSG_WAITALL (since Linux 2.2)

This flag requests that the operation block until the full request is satisfied. However, the call may still return less data than requested if a signal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned. This flag has no effect for datagram sockets.

Hence, my two questions:

  1. Why would one need to use MSG_WAITALL FLAG instead of 0 FLAG? (Could someone explain a scenario of a problem for which the use of this would be the solution?)
  2. Why to use it with UDP?
sockets
1 Answers

As the quoted man page mentions, MSG_WAITALL has no effect on UDP sockets, so there's no reason to use it there. Examples that do use it are probably confused and/or the result of several generations of cargo-cult/copy-and-paste programming. :)

For TCP, OTOH, the default behavior of recv() is to block until at least one byte of data can be copied into the user's buffer from the sockets incoming-data-buffer. The TCP stack will try to provide as many bytes of data as it can, of course, but in a case where the socket's incoming-data-buffer contains fewer bytes of data than the user has passed in to recv(), the TCP stack will copy as many bytes as it can, and return the byte-count indicating how many bytes it actually provided.

However, some people find would prefer to have their recv() call keep blocking until all of the bytes in their passed-in array have been filled in, regardless of how long that might take. For those people, the MSG_WAITALL flag provides a simple way to obtain that behavior. (The flag is not strictly necessary, since the programmer could always emulate that behavior by writing a while() loop that calls recv() multiple times as necessary, until all the bytes in the buffer have been populated... but it's provided as a convenience nonetheless)

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