Close all sockets linux
The close() function deallocates the file descriptor indicated by fildes . To deallocate means to make the file descriptor available for return by subsequent calls to open (2) or other functions that allocate file descriptors. All outstanding record locks owned by the process on the file associated with the file descriptor will be removed (that is, unlocked).
If close() is interrupted by a signal that is to be caught, it will return -1 with errno set to EINTR and the state of fildes is unspecified. If an I/O error occurred while reading from or writing to the file system during close() , it returns -1, sets errno to EIO , and the state of fildes is unspecified.
When all file descriptors associated with a pipe or FIFO special file are closed, any data remaining in the pipe or FIFO will be discarded.
When all file descriptors associated with an open file description have been closed the open file description will be freed.
If the link count of the file is 0, when all file descriptors associated with the file are closed, the space occupied by the file will be freed and the file will no longer be accessible.
If a STREAMS-based (see Intro (2)) fildes is closed and the calling process was previously registered to receive a SIGPOLL signal (see signal (3C)) for events associated with that STREAM (see I_SETSIG in streamio (7I)), the calling process will be unregistered for events associated with the STREAM. The last close() for a STREAM causes the STREAM associated with fildes to be dismantled. If O_NONBLOCK and O_NDELAY are not set and there have been no signals posted for the STREAM, and if there is data on the module’s write queue, close() waits up to 15 seconds (for each module and driver) for any output to drain before dismantling the STREAM. The time delay can be changed via an I_SETCLTIME ioctl (2) request (see streamio (7I)). If the O_NONBLOCK or O_NDELAY flag is set, or if there are any pending signals, close() does not wait for output to drain, and dismantles the STREAM immediately.
If fildes is associated with one end of a pipe, the last close() causes a hangup to occur on the other end of the pipe. In addition, if the other end of the pipe has been named by fattach (3C), then the last close() forces the named end to be detached by fdetach (3C). If the named end has no open file descriptors associated with it and gets detached, the STREAM associated with that end is also dismantled.
If fildes refers to the master side of a pseudo-terminal, a SIGHUP signal is sent to the session leader, if any, for which the slave side of the pseudo-terminal is the controlling terminal. It is unspecified whether closing the master side of the pseudo-terminal flushes all queued input and output.
If fildes refers to the slave side of a STREAMS-based pseudo-terminal, a zero-length message may be sent to the master.
When there is an outstanding cancelable asynchronous I/O operation against fildes when close() is called, that I/O operation is canceled. An I/O operation that is not canceled completes as if the close() operation had not yet occurred. All operations that are not canceled will complete as if the close() blocked until the operations completed.
If a shared memory object or a memory mapped file remains referenced at the last close (that is, a process has it mapped), then the entire contents of the memory object will persist until the memory object becomes unreferenced. If this is the last close of a shared memory object or a memory mapped file and the close results in the memory object becoming unreferenced, and the memory object has been unlinked, then the memory object will be removed.
If fildes refers to a socket, close() causes the socket to be destroyed. If the socket is connection-mode, and the SO_LINGER option is set for the socket with non-zero linger time, and the socket has untransmitted data, then close() will block for up to the current linger interval until all data is transmitted.
RETURN VALUES
Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.
ERRORS
The close() function will fail if:
EBADF The fildes argument is not a valid file descriptor.
EINTR The close() function was interrupted by a signal.
ENOLINK The fildes argument is on a remote machine and the link to that machine is no longer active.
ENOSPC There was no free space remaining on the device containing the file.
The close() function may fail if:
EIO An I/O error occurred while reading from or writing to the file system.
EXAMPLES
Example 1 Reassign a file descriptor.
The following example closes the file descriptor associated with standard output for the current process, re-assigns standard output to a new file descriptor, and closes the original file descriptor to clean up. This example assumes that the file descriptor 0, which is the descriptor for standard input, is not closed.
#include unistd.h> . int pfd; . close(1); dup(pfd); close(pfd); .
Incidentally, this is exactly what could be achieved using:
Example 2 Close a file descriptor.
In the following example, close() is used to close a file descriptor after an unsuccessful attempt is made to associate that file descriptor with a stream.
#include stdio.h> #include unistd.h> #include stdlib.h> #define LOCKFILE "/etc/ptmp" . int pfd; FILE *fpfd; . if ((fpfd = fdopen (pfd, "w")) == NULL) < close(pfd); unlink(LOCKFILE); exit(1); > .
USAGE
An application that used the stdio function fopen (3C) to open a file should use the corresponding fclose (3C) function rather than close() .
ATTRIBUTES
See attributes (5) for descriptions of the following attributes:
|
How to close sockets in C (GCC on Linux)
Can you please explain. This question might seem a little too dumb. Here’s the Client code:
#include #include #include #include #include int main() < // Create a socket int network_socket; network_socket = socket(AF_INET, SOCK_STREAM, 0); // Address for the socket struct sockaddr_in server_address; server_address.sin_family = AF_INET; server_address.sin_port = htons(9002); server_address.sin_addr.s_addr = INADDR_ANY; int connection = connect(network_socket, (struct sockaddr *) &server_address, sizeof(server_address)); printf("%d\n", connection); char response[256]; recv(network_socket, &response, sizeof(response), 0); printf("%s\n", response); return 0; >Right before the cut screen, you can see Eduanix actually correcting the close line: youtu.be/LtXEMwSG5-8?t=1406.
2 Answers 2
There’s no variable called sock. The socket is called something else.
Yes, it’s network_socket in your code. So you want close(network_socket); .
I can’t find the close function. The compiler is saying that the function doesn’t exist
You need #include , which is where close() is declared. (If you look at man close you’ll see this at the top.)
Thanks! This answered all my questions. Still curious about what went wrong since the guy in the vid didn’t include unistd.h. But that doesn’t really matter because its working
@RushiSrinivas.K Obviously, another header must have been implicitly including unistd.h , which is why the tutorial’s code didn’t need to. Don’t rely on that. If you need to use something from a particular header, then just include that header yourself. If it has already been included, that is OK (provided the header is using proper include guards, which 99% of the time will be the case)
How to completely destroy a socket connection in C
I have made a chat client in linux using socket, and i wish to destroy the connection completely. Following is the relevant portions of the code:
int sock, connected, bytes_recieved , true = 1, pid; char send_data [1024] , recv_data[1024]; struct sockaddr_in server_addr,client_addr; int sin_size; label: if ((sock = socket(AF_INET, SOCK_STREAM, 0)) == -1) < perror("Socket"); exit(1); >if (setsockopt(sock,SOL_SOCKET,SO_REUSEADDR,&true,sizeof(int)) == -1) < perror("Setsockopt"); exit(1); >server_addr.sin_family = AF_INET; server_addr.sin_port = htons(3128); server_addr.sin_addr.s_addr = INADDR_ANY; bzero(&(server_addr.sin_zero),8); if (bind(sock, (struct sockaddr *)&server_addr, sizeof(struct sockaddr))== -1) < perror("Unable to bind"); exit(1); >if (listen(sock, 5) == -1) < perror("Listen"); exit(1); >printf("\nTCPServer Waiting for client on port 3128"); fflush(stdout); connected = accept(sock, (struct sockaddr *)&client_addr,&sin_size); //necessary code close(sock); goto label; but the close(sock) doesnot seem to close the destroy the connection completely, because after going to ‘label’ the code is exiting showing the error message
Unable to bind: Address already in use That is the connection is not happening again. What can the problem be? Thanks in advance. EDIT: What I actually want is, when I run the script from the beginning after destroying the connection, it should run as a fresh program. How can I do it?
Close open sockets from crashed program
I am opening thousand of sockets and sometimes the program crashes leaving me with a lot less available sockets. Is there a way to clean those hanging sockets?
1 Answer 1
Sockets are finally closed by the Unix kernel; a crashed program is no different from a normal program exit w/o a close()/shutdown() call.
Your problem may have to do with the TIME_WAIT state of the TCP/IP state machine and should be solved with the SO_REUSEADDR option. One way to confirm this is wait for about 5 minutes before starting again after a crash. If you find sufficient sockets are now available you should study the TIME_WAIT logic and work around it. If the wait trick does not solve your problem there might be a different issue in your program which needs to be identified then.
Two quick extracts from there for reference,
TIME_WAIT is often also known as the 2MSL wait state. This is because the socket that transitions to TIME_WAIT stays there for a period that is 2 x Maximum Segment Lifetime in duration. The MSL is the maximum amount of time that any segment, for all intents and purposes a datagram that forms part of the TCP protocol, can remain valid on the network before being discarded. This time limit is ultimately bounded by the TTL field in the IP datagram that is used to transmit the TCP segment. Different implementations select different values for MSL and common values are 30 seconds, 1 minute or 2 minutes. RFC 793 specifies MSL as 2 minutes and Windows systems default to this value but can be tuned using the TcpTimedWaitDelay registry setting.
(PS: hence the 4+1 minute wait for my test suggested above)
Changing the 2MSL delay is usually a machine wide configuration change. You can instead attempt to work around TIME_WAIT at the socket level with the SO_REUSEADDR socket option. This allows a socket to be created whilst an existing socket with the same address and port already exists. The new socket essentially hijacks the old socket. You can use SO_REUSEADDR to allow sockets to be created whilst a socket with the same port is already in TIME_WAIT but this can also cause problems such as denial of service attacks or data theft.
The article describes one more way. But that comes with other caveats.
There’s another way to terminate a TCP connection and that’s by aborting the connection and sending an RST rather than a FIN. This is usually achieved by setting the SO_LINGER socket option to 0. This causes pending data to be discarded and the connection to be aborted with an RST rather than for the pending data to be transmitted and the connection closed cleanly with a FIN. It’s important to realise that when a connection is aborted any data that might be in flow between the peers is discarded and the RST is delivered straight away; usually as an error which represents the fact that the «connection has been reset by the peer». The remote peer knows that the connection was aborted and neither peer enters TIME_WAIT.
Before using these schemes it is a good idea to understand the TCP machine behavior so you do not inadvertently introduce other situations which will need debug later. So at least read that article completely 🙂