The purpose of this project is to code a small data exchange program using UNIX signals.
2 - Analysing the subject
You must create a communication program in the form of a client and a server.
- The server must be started first. After its launch, it has to print its PID.
- The client takes two parameters: 1) The server PID 2) The string to send.
- The client must send the string passed as a parameter to the server. Once the string has been received, the server must print it.
- The server has to display the string pretty quickly. Quickly means that if you think it takes too long, then it is probably too long.
- Your server should be able to receive strings from several clients in a row without needing to restart.
- The communication between your client and your server has to be done only using UNIX signals.
- You can only use these two signals: SIGUSR1 and SIGUSR2.
To be noted that as per the subject, it cannot be displayed char-by-char on the server-side, it must be displayed the whole message received instead:
Once the string has been received, the server must print it.
Therefore, it must be known in advance the length of the message so the server can allocate memory in the heap as required only once (best approach). Or alternatively, don't send the lenght and keep modifying the allocation in memory - joining a char to already received string - till receiving the last char (not so good approach since will have to repeat allocation for every new char received).
The following functions from libft could be used:
ft_strlen(), to know the length of the message to be sent from client to serverft_calloc(), in order to save the chars being received till the string is completed received on server side, so it can be then displayed
- The server has to display the string pretty quickly. Quickly means that if you think it takes too long, then it is probably too long.
Researching about the theme, two options arise:
-
using a delay function like
sleep()orusleep()avoiding the communication of signals back and forward between server/client, which would impose a delay per char, since we would be sending in blind way (no feedback loop) chars from client to server. Depending on the delay set signals could queue. -
the other option would be to implement a feedback loop so whenever client sends a char to server, it waits till server sends back a ACK signal informing client that the next bit can be sent. This later solution should be a lot quicker since there would be no delay function. This solution is aligned with the bonus:
The server acknowledges every message received by sending back a signal to the client.
As noted on the subject, there should not be sent signals blindly, making the two-way communication the best implementation.
Linux system does NOT queue signals when you already have pending signals of this type! Bonus time?
The subject states that libft library can be used!
You can definitely use your libft.
Reading the subject it can be understood some functions included in the libft will be required to fulfill the requirements, as shown below:
The server must be started first. After its launch, it has to print its PID.
- The client takes two parameters:
- The server PID.
- The string to send.
ft_atoi(), to convert the PID argument received from command line to integer type for further processing of signal processing
Below is made a brief analysis of other functions that can be used.
In order to complete the mandatory part, you are allowed to use the following functions:
- write
- ft_printf and any equivalent YOU coded
- signal
sig_t signal(int sig, sig_t func);This signal() facility is a simplified interface to the more general sigaction(2) facility.
(...)
some signals instead cause the process receiving them to be stopped, or are simply discarded
if the process has not requested otherwise.
Except for the SIGKILL and SIGSTOP signals, the signal() function allows for a signal to be
caught, to be ignored, or to generate an interrupt.
These signals are defined in the file <signal.h>:
Name Default Action Description
1 SIGHUP terminate process terminal line hangup
2 SIGINT terminate process interrupt program
(...)
30 SIGUSR1 terminate process User defined signal 1
31 SIGUSR2 terminate process User defined signal 2
The sig argument specifies which signal was received.
The func procedure allows a user to choose the action upon receipt of a signal.
The handled signal is unblocked when the function returns and the process continues
from where it left off when the signal occurred.
For some system calls, if a signal is caught while the call is executing and the call is
prematurely terminated, the call is automatically restarted.
Any handler installed with signal(3) will have the SA_RESTART flag set,
meaning that any restartable system call will not return on receipt of a signal.
The affected system calls include read(2), write(2), sendto(2), recvfrom(2), sendmsg(2),
and recvmsg(2) on a communications channel or a low speed device and during a ioctl(2) or wait(2).
However, calls that have already committed are not restarted, but instead return a partial success
(for example, a short read count).
These semantics could be changed with siginterrupt(3).
See sigaction(2) for a list of functions that are considered safe for use in signal handlers.
The previous action is returned on a successful call.
Otherwise, SIG_ERR is returned and the global variable errno is set to indicate the error.
macOS 12.4 June 7, 2004 macOS 12.4
- sigemptyset
- sigaddset
int sigemptyset(sigset_t *set);
int sigaddset(sigset_t *set, int signo);These functions manipulate signal sets, stored in a sigset_t.
sigemptyset() must be called for every object of type sigset_t before any other use of the object.
The sigemptyset() function initializes a signal set to be empty.
The sigaddset() function adds the specified signal signo to the signal set.
These functions are provided as macros in the include file <signal.h>.
Actual functions are available if their names are undefined (with #undef name).
Functions return 0.
macOS 12.4 June 4, 1993 macOS 12.4
- sigaction
#define sa_handler __sigaction_u.__sa_handler
#define sa_sigaction __sigaction_u.__sa_sigaction
int sigaction(int sig, const struct sigaction *restrict act, struct sigaction *restrict oact);The system defines a set of signals that may be delivered to a process.
Signal delivery resembles the occurrence of a hardware interrupt:
the signal is normally blocked from further occurrence, the current process
context is saved, and a new one is built.
A process may specify a handler to which a signal is delivered, or specify that
a signal is to be ignored.
A process may also specify that a default action is to be taken by the
system when a signal occurs.
A signal may also be blocked, in which case its delivery is postponed until it is unblocked.
The action to be taken on delivery is determined at the time of delivery.
Normally, signal handlers execute on the current stack of the process.
This may be changed, on a per-handler basis, so that signals are taken on a special signal stack.
The sigaction() system call assigns an action for a signal specified by sig.
If act is non-zero, it specifies an action (SIG_DFL, SIG_IGN, or a handler routine) and mask
to be used when delivering the specified signal.
If oact is non-zero, the previous handling information for the signal is returned to the user.
Options may be specified by setting sa_flags. The meaning of the various bits is as follows:
(...)
SA_NODEFER If this bit is set, further occurrences of the
delivered signal are not masked during the execution
of the handler.
SA_RESETHAND If this bit is set, the handler is reset back to
SIG_DFL at the moment the signal is delivered.
SA_RESTART See paragraph below.
SA_SIGINFO If this bit is set, the handler function is assumed
to be pointed to by the sa_sigaction member of struct
sigaction and should match the prototype shown above
or as below in EXAMPLES. This bit should not be set
when assigning SIG_DFL or SIG_IGN.
If a signal is caught during the system calls listed below, the call may be
forced to terminate with the error EINTR, the call may return with a data
transfer shorter than requested, or the call may be restarted.
Restart of pending calls is requested by setting the SA_RESTART bit in sa_flags.
The affected system calls include (...), write(2) and (...) on a communications channel or a
slow device (such as a terminal, but not a regular file) and during a wait(2) or (...).
However, calls that have already committed are not restarted, but instead return a partial success
(for example, a short read count).
NOTE
The sa_mask field specified in act is not allowed to block SIGKILL or
SIGSTOP. Any attempt to do so will be silently ignored.
The following functions are either reentrant or not interruptible by
signals and are async-signal safe. Therefore applications may invoke them,
without restriction, from signal-catching functions:
Base Interfaces:
(...), getpid(), (...), kill(), (...), pause(), (...), sigaction(), sigaddset(), (...),
sigemptyset(), (...), signal(), (...), sleep(), (...), wait(), (...), write().
Realtime Interfaces:
aio_error(), sigpause(), aio_return(), aio_suspend(), sem_post(), sigset().
ANSI C Interfaces:
strcpy(), strcat(), strncpy(), strncat(), and perhaps some others.
Extension Interfaces:
strlcpy(), strlcat().
All functions not in the above lists are considered to be unsafe with
respect to signals. That is to say, the behaviour of such functions when
called from a signal handler is undefined. In general though, signal
handlers should do little more than set a flag; most other actions are not
safe.
Also, it is good practice to make a copy of the global variable errno and
restore it before returning from the signal handler. This protects against
the side effect of errno being set by functions called from inside the
signal handler.
- kill
Using kill() to check the PID server input
DESCRIPTION
The kill() function sends the signal specified by sig to pid, a process or
a group of processes. Typically, Sig will be one of the signals specified
in sigaction(2). A value of 0, however, will cause error checking to be
performed (with no signal being sent). This can be used to check the
validity of pid.This is a nice finding to check the validity of the PID in a robust way, rather than just checking if all the chars are digits.
else if (kill(ft_atoi(argv[1]), 0) < 0)
{
ft_putstr_fd("\e[31m## error - PID is invalid ##\n\e[0m", STDOUT_FILENO);
return (EXIT_FAILURE);
} If pid is greater than zero:
Sig is sent to the process whose ID is equal to pid.
If pid is zero:
Sig is sent to all processes whose group ID is equal to the process
group ID of the sender, and for which the process has permission;
this is a variant of killpg(2).
If pid is -1:
If the user has super-user privileges, the signal is sent to all
processes excluding system processes and the process sending the
signal. If the user is not the super user, the signal is sent to
all processes with the same uid as the user, excluding the process
sending the signal. No error is returned if any process could be
signaled.
- getpid
pid_t getpid(void);DESCRIPTION
getpid() returns the process ID of the calling process. The ID is
guaranteed to be unique and is useful for constructing temporary file
names.
- malloc
- free
- pause
int pause(void);DESCRIPTION
The pause() function causes the calling thread to pause until a signal is
received from either the kill(2) function or an interval timer. (See
setitimer(2).) Upon termination of a signal handler started during a
pause(), the pause() call will return.
- sleep
- usleep
int usleep(useconds_t microseconds);DESCRIPTION
The usleep() function suspends execution of the calling thread until either
microseconds have elapsed or a signal is delivered to the
thread and its action is to invoke a signal-catching function or to
terminate the process. System activity or limitations may lengthen the
sleep by an indeterminate amount.
This function is implemented using nanosleep(2) by pausing for microseconds
microseconds or until a signal occurs. Consequently, in this
implementation, sleeping has no effect on the state of process timers, and
there is no special handling for SIGALRM. Also, this implementation does
not put a limit on the value of microseconds (other than that limited by
the size of the useconds_t type); some other platforms require it to be
less than one million.
- exit
DESCRIPTION
The exit() function terminates a process.
Before termination, exit() performs the following functions in the order
listed:
1. Call the functions registered with the atexit(3) function, in
the reverse order of their registration.
2. Flush all open output streams.
3. Close all open streams.
4. Unlink all files created with the tmpfile(3) function.
Function make the low-order eight bits of the status argument available to
a parent process which has called a wait(2)-family function.
The C Standard (ISO/IEC 9899:1999 (“ISO C99”)) defines the values 0,
EXIT_SUCCESS, and EXIT_FAILURE as possible values of status.
- Below is a demonstration of the usage of the
serverandclientprograms- Erro handling, e.g. incorrect number of arguments and invalid PID server
- Sending empty messages with
""and''- transmission of empty message is received by server (length 0) and displayed - Sending Large message sent +1000 chars using Lorem Ipsum site
- Receiving a message from a 2nd client after 1st client (in a row)
- Client showing transmission log