Thread: TCP socket message size

Hi all,

I want to make a simple POP3 client in C and Linux and I reached a dead end:
the responses from the POP3 server differ in length (eg: RETR x).

Is there a way to query the length of the TCP message before actually reading it so I can assign a buffer large enough to hold the data?

What I want to do is something similar to:

int length= ??? //message length
char *buffer=malloc(length)
buffer=read(...)
//processing the message
free(buffer)

(I found the MSG_PEEK option but from what I have read so far is doesn't seem like a good option; is there another way?)

Thank you

fscanf on the socket file descriptor until EOF..

Originally Posted by ibanezul

Hi all,

I want to make a simple POP3 client in C and Linux and I reached a dead end:
the responses from the POP3 server differ in length (eg: RETR x).

Is there a way to query the length of the TCP message before actually reading it so I can assign a buffer large enough to hold the data?

No sane way, no. You just need to have some logic which allocates a buffer of some reasonable size, and reallocate it to a larger size if you find that it isn't big enough.

RFC 1939:

Each command may be up to 40 characters long.
Responses may be up to 512 characters long, including the terminating CRLF.

Responses to certain commands are multi-line. In these cases, which
are clearly indicated below, after sending the first line of the
response and a CRLF, any additional lines are sent, each terminated
by a CRLF pair. When all lines of the response have been sent, a
final line is sent, consisting of a termination octet (decimal code
046, ".") and a CRLF pair. If any line of the multi-line response
begins with the termination octet, the line is "byte-stuffed" by
pre-pending the termination octet to that line of the response.
Hence a multi-line response is terminated with the five octets
"CRLF.CRLF". When examining a multi-line response, the client checks
to see if the line begins with the termination octet. If so and if
octets other than CRLF follow, the first octet of the line (the
termination octet) is stripped away. If so and if CRLF immediately
follows the termination character, then the response from the POP
server is ended and the line containing ".CRLF" is not considered
part of the multi-line response.