My C application creates 64-128 UDP sockets.

It creates sockets using this code:

int sock = socket(AF_INET, SOCK_DGRAM, 0);
assert(sock != -1, strerror(errno));

const u_int yes = 1;
int result = setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));
printf("sock=%i result=%i errno=%i\n", sock, result, errno);
if(result != 0)
{
    FATAL(strerror(errno));
}

However, at the moment it's only creating 2 sockets because setsockopt() returns -1 on the third request:

sock=1023 result=-1 errno=0

(stderror(errno) just says success)

I'm puzzled because when I run ss it doesn't look like many sockets are in use:

ss -s
Total: 238
TCP:   85 (estab 16, closed 40, orphaned 0, timewait 37)

Transport Total     IP        IPv6
RAW       2         1         1        
UDP       24        17        7        
TCP       45        30        15       
INET      71        48        23       
FRAG      0         0         0 

My understanding is you may have 1023 sockets. So the above implies I should be able to create 64-128?

How/what is the problem here?

CodePudding user response：

There is no limit on number of open sockets, but there is more general limit on number of opened file descriptors. The fact that it fails on fd=1023 suggests that this limit was truly hit, since on a typical Linux:

1. file descriptors are assigned consecutive numbers starting with 0
2. default limit (ulimit -n) is 1024 opened file descriptors

You can check the number of opened file descriptors with ls -l /proc/<pid>/fd | wc -l. I suspect that you have many opened (regular) files.