I am trying to debug a problem with linux that sometimes my TCP server blocks the socket forever, although the socket is already gone. I wrote test code to understand the behavior of the close (), shutdown (), select (), and recv () methods.
I have 2 threads - 1 block block in the socket using select () and shutdown 2 calls (SHUT_RDWR) in the same socket. When shutdown () is called from thread 2, thread 1 wakes up, select () returns 1, but errno reads 0. Also, after select () returns 1, recv () is called and returns 0, again errno reads 0.
If I changed the implementation of thread 2 to use close () instead of shutdown (), select () never wakes up.
If I changed the implementation of thread 2 to call shutdown (), followed by close (), select () returns 1 (errno remains 0), and then recv () returns -1 and errno reads EBADF.
My questions:
When the close () function is called, should you choose () not to wake up? I looked at the close () page, but couldn't figure out what was going to happen.
When a shutdown (SHUT_RDWR) is called, should () return normally, like what I see? Or should it return 1 and set errno to EBADF? If I look at man () man select (), it says errno is set to EBADF if one or more sets of file descriptors specify a file descriptor that is not a valid open file descriptor. It looks like shutdown () only sends TCP FIN to the remote side, but doesn’t actually close the fd socket?
If I call shutdown () followed by close (), it seems to help. How is this different from calling close () yourself?
Note. I understand that blocking a socket in one thread and closing the same socket from another thread is not a good design, but I am dealing with an outdated code base, so I cannot change this design. I can only configure it to call shutdown (), and then close (), as it seems to work, but I want to make sure that I understand the point correctly.
Thanks in advance.
Here is the test code:
Theme 1:
#include "sys/socket.h" #include "net/if.h" #include "linux/sockios.h" #include "netinet/in.h" #include "fcntl.h" #include "signal.h" #include "errno.h" #include "strings.h" #include "pthread.h" #include "errno.h" void* f1(void* p) { int f1_ret, bytes, ret; struct sockaddr_in f1so1_addr; fd_set read_f1, error_fds; char f1buf[20]; struct sockaddr clientInfo; socklen_t clientAddrLen = sizeof(struct sockaddr); f1so1 = socket(AF_INET, SOCK_STREAM, 0); printf("f1: open fd - f1fd1 = %d, f1so1 = %d\n", f1fd1, f1so1); f1so1_addr.sin_family = AF_INET; f1so1_addr.sin_addr.s_addr = htonl(INADDR_ANY); //ret = inet_aton("10.20.30.100", &(f1so1_addr.sin_addr)); f1so1_addr.sin_port = htons(7777); ret = bind(f1so1, (const struct sockaddr*) &f1so1_addr, sizeof(f1so1_addr)); printf("f1: bind returned %x, errno = %x\n", ret, errno); listen(f1so1, 5); printf("f1- listening on f1so1, blocking on accept\n"); f1so2 = accept(f1so1, &clientInfo, &clientAddrLen); printf("f1- accept returned %x, errno = %x\n", f1so2, errno); if(errno) { printf("f1: accept failed, return...\n"); return; } FD_ZERO(&read_f1); FD_SET(f1so2, &read_f1); FD_ZERO(&error_fds); FD_SET(f1so2, &error_fds); printf("f1: start loop\n"); while (1) { printf("f1: call select _read and error__ - cBLOCKING\n"); errno = 0; FD_ZERO(&read_f1); FD_SET(f1so2, &read_f1); FD_ZERO(&error_fds); FD_SET(f1so2, &error_fds); f1_ret = select(f1so2+1, &read_f1, 0, &error_fds, 0); printf("f1: select returned = %x, errno = %x\n", f1_ret, errno); if (errno) { printf("f1: select failed...\n"); } else if (f1_ret) { if (FD_ISSET(f1so2,&error_fds)) { printf("f1: error on socket %x\n",f1so2); } if (FD_ISSET(f1so2,&read_f1)) { sleep(1); bytes = recv(f1so2, f1buf, 20, 0); printf("f1: errno after recv = %x\n", errno); if (errno) { printf("!!!recv failed!!!\n"); return; } printf("f1: read from socket %x, bytes = %x, data = %s\n",f1so2, bytes, f1buf); } } } printf("f1: exiting\n"); }
Theme 2:
#include "stdio.h" #include "pthread.h" #include "linux/socket.h" #include "sys/socket.h" #include "net/if.h" #include "linux/sockios.h" #include "netinet/in.h" #include "fcntl.h" #include "signal.h" #include "errno.h" #include "strings.h" extern int f1so2; void f2() { int f2_ret, i, choice; struct sockaddr_in f2so2_addr; fd_set read_f2; char f2buf[20]; struct sigaction f2_act; while (1) { printf("f2: 1: close socket, 2: shutdown socket, 2: exit f2\n"); scanf("%d", &choice); if (choice == 1) { f2_ret = close(f1so2); printf("f2: close on socket %x returned %x, errno = %x\n", f1so2, f2_ret, errno); } else if (choice == 2) { f2_ret = shutdown(f1so2, SHUT_RDWR); printf("f2: shutdown on socket %x returned %x, errno = %x\n", f1so2, f2_ret, errno); } else if (choice == 3) { continue; } } printf("f2: exiting\n"); return; }