C++ – How to prevent SIGPIPE or prevent the server from ending

cclient-serversignalssockets

A quite standard C++ TCP server program using pthreads, bind, listen and accept. I have the scenario that the server ends (read: crashes) when I kill a connected client.

The reason for the crash is that the write() call on the file fails, thus the program receives a SIGPIPE. And I guess, this makes the server exit.

I thought, "of course, unhandled signal means exit", so let's use signal():

signal(SIGPIPE, SIG_IGN);

because, taken from man 2 write:

EPIPE fd is connected to a pipe or socket whose reading end is closed. When this happens the writing process will also receive a SIGPIPE signal. (Thus, the write return value is seen only if the program catches, blocks or ignores this signal.)

Alas, no. Neither in the server thread nor the client threads does this seem to help.

So, how do I prevent the write() call from raising that signal, or (to be pragmatic) how do I stop the server from exiting.

My diagnostics are:

server thread started, binding, listening, accepting.
let a client connect (via telnet for example)
send a pkill telnet to crash the client

unwanted behavior: server exits, in gdb with

... in write () at ../sysdeps/unix/syscall-template.S:82
82      T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)

and the backtrace:

#0  ... in write () at ../sysdeps/unix/syscall-template.S:82
#1  ... in ClientHandler::mesg(std::string) ()
#2  ... in ClientHandler::handle() ()
#3  ... in start_thread (arg=<value optimized out>) at pthread_create.c:300
#4  ... in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#5  ... in ?? ()

Best Answer

Late to the party, but just wanted to add to this for future reference: If you are debugging your code in gdb, don't forget that it overrides your signal handlers.

So if you have set a signal handler such as: signal(SIGPIPE, SIG_IGN) and it doesn't seem to be working, try running the code outside the debugger.

Or set handle SIGPIPE nostop (in gdb prompt) to prevent gdb stopping on the signal.

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How to determine the size of the array in C

Executive summary:

int a[17];
size_t n = sizeof(a)/sizeof(a[0]);

Full answer:

To determine the size of your array in bytes, you can use the sizeof operator:

int a[17];
size_t n = sizeof(a);

On my computer, ints are 4 bytes long, so n is 68.

To determine the number of elements in the array, we can divide the total size of the array by the size of the array element. You could do this with the type, like this:

int a[17];
size_t n = sizeof(a) / sizeof(int);

and get the proper answer (68 / 4 = 17), but if the type of a changed you would have a nasty bug if you forgot to change the sizeof(int) as well.

So the preferred divisor is sizeof(a[0]) or the equivalent sizeof(*a), the size of the first element of the array.

int a[17];
size_t n = sizeof(a) / sizeof(a[0]);

Another advantage is that you can now easily parameterize the array name in a macro and get:

#define NELEMS(x)  (sizeof(x) / sizeof((x)[0]))

int a[17];
size_t n = NELEMS(a);

How to initialize all members of an array to the same value

Unless that value is 0 (in which case you can omit some part of the initializer and the corresponding elements will be initialized to 0), there's no easy way.

Don't overlook the obvious solution, though:

int myArray[10] = { 5, 5, 5, 5, 5, 5, 5, 5, 5, 5 };

Elements with missing values will be initialized to 0:

int myArray[10] = { 1, 2 }; // initialize to 1,2,0,0,0...

So this will initialize all elements to 0:

int myArray[10] = { 0 }; // all elements 0

In C++, an empty initialization list will also initialize every element to 0. This is not allowed with C:

int myArray[10] = {}; // all elements 0 in C++

Remember that objects with static storage duration will initialize to 0 if no initializer is specified:

static int myArray[10]; // all elements 0

And that "0" doesn't necessarily mean "all-bits-zero", so using the above is better and more portable than memset(). (Floating point values will be initialized to +0, pointers to null value, etc.)

Best Answer

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How to determine the size of the array in C

How to initialize all members of an array to the same value

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