Hi,
As it comes from the manual the 'notty' option will enable pppd to read
and write from stdin/stdout. In order to do it, pppd allocates a
master-slave terminal pair, forks and starts a new 'charshunting'
process, which proxies data between pseudo terminal and stdin/stdout.
At high transfer rates the pppd-2.3.11 dies with "Error writing standard
output" error.
After looking thru the pppd/main.c code (charshunt() function) I figured
that there is a small problem (or call it a bug, if you want).
The following block is part of the proxying loop.
2096 if (select(nfds, &ready, &writey, NULL, NULL) < 0) {
2097 if (errno != EINTR)
2098 fatal("select");
2099 continue;
2100 }
2101 if (FD_ISSET(ifd, &ready)) {
2102 ibufp = inpacket_buf;
2103 nibuf = read(ifd, ibufp, sizeof(inpacket_buf));
2104 if (nibuf < 0 && errno == EIO)
2105 nibuf = 0;
2106 if (nibuf < 0) {
2107 if (!(errno == EINTR || errno == EAGAIN)) {
2108 error("Error reading standard input: %m");
2109 break;
2110 }
2111 nibuf = 0;
2112 } else if (nibuf == 0) {
2113 /* end of file from stdin */
2114 stdin_readable = 0;
2115 /* do a 0-length write, hopefully this will
generate
2116 an EOF (hangup) on the slave side. */
2117 write(pty_master, inpacket_buf, 0);
2118 if (recordf)
2119 if (!record_write(recordf, 4, NULL, 0,
&lasttime))
2120 recordf = NULL;
2121 } else {
***2122 FD_SET(pty_master, &writey);
2123 if (recordf)
2124 if (!record_write(recordf, 2, ibufp, nibuf,
&lasttime))
2125 recordf = NULL;
2126 }
2127 }
Note that when this code successfully reads some data from stdin, it
sets a flag in writey, which will fool the following code, and make it
believe that the pty_master is ready for writing:
2169 if (FD_ISSET(pty_master, &writey)) {
2170 n = write(pty_master, ibufp, nibuf);
2171 if (n < 0) {
2172 if (errno != EIO) {
2173 error("Error writing pseudo-tty master:
%m");
2174 break;
2175 }
2176 stdin_readable = 0;
2177 nibuf = 0;
2178 } else {
2179 ibufp += n;
2180 nibuf -= n;
2181 }
2182 }
The write() function in the above block will return EAGAIN, which will
make the charshunting process exit. Note that the error is returned
because we're attempting to write in pty_master, which is not ready for
writing. The attempt is done because we modified the 'writey' flags in
the first block.
I have no idea why it's necessary to modify flags in the first block, so
I patched this code on my system by just ignoring EAGAIN in the second
block.
- Andrei.
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