CODEWITHSUNDEEP
linuxnetwork-programming
daurnimator
daurnimator

Reputation: 4311

Why is this read blocking when O_NONBLOCK is set?

Strace excerpt (please ignore sending xml to ssh):

dup(12)                                 = 13
getsockname(13, {sa_family=AF_INET, sin_port=htons(46811), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
getsockopt(13, SOL_SOCKET, SO_TYPE, [1], [4]) = 0
fstat(13, {st_mode=S_IFSOCK|0777, st_size=0, ...}) = 0
getsockname(13, {sa_family=AF_INET, sin_port=htons(46811), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
getsockopt(13, SOL_SOCKET, SO_TYPE, [1], [4]) = 0
fcntl(13, F_SETFD, FD_CLOEXEC)          = 0
fcntl(13, F_GETFL)                      = 0x802 (flags O_RDWR|O_NONBLOCK)
fcntl(13, F_SETFL, O_RDWR|O_NONBLOCK)   = 0
setsockopt(13, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
setsockopt(13, SOL_SOCKET, SO_REUSEPORT, [0], 4) = 0
setsockopt(13, SOL_TCP, TCP_NODELAY, [0], 4) = 0
setsockopt(13, SOL_TCP, TCP_CORK, [0], 4) = 0
fcntl(12, F_GETFL)                      = 0x802 (flags O_RDWR|O_NONBLOCK)
fcntl(12, F_SETFL, O_RDWR)              = 0
close(12)                               = 0
getpeername(13, {sa_family=AF_INET, sin_port=htons(22), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
sendto(13, "<stream:stream xmlns:stream='htt"..., 133, MSG_NOSIGNAL, NULL, 0) = 133
read(13, "SSH-2.0-OpenSSH_6.6.1\r\n", 4096) = 23
read(13,  <unfinished ...>

(At the end I kill -9 it.)

The read() at the end was blocking; even though O_NONBLOCK was set via fcntl().

How is this possible?

Version info:

$ uname -r
3.16.4-1-ARCH

Upvotes: 2

Views: 503

Answers (1)

Habbie
Habbie

Reputation: 2230

Because FD 13 is a dup() of FD 12, this line removes your O_NONBLOCK:

fcntl(12, F_SETFL, O_RDWR)              = 0

Upvotes: 4

Related Questions