How to run the following message queue on Linux? - c

Question

The galactic imperium plans to send a star destroyer to attack the rebel‘s base. This star destroyer shall host 1024 imperial clone warriors.
At t=0 there is just one soldier availabe: the captain. From his first birthday on, a clone warrior is able to clone once a year. The imperator wants to get the star destroyer ready for action in short time.
The imperial command structure is very simple:
Every warrior sends commands to his clones
there is no communication to the superior
Write a Linux C-Programm with the following requirements:
Every clone warrior has to be represented by a separate process
Commands have to be transmitted via uniquely (!) named message queues
*There is an existing message queue /Imperator from the Imperator to the captain
After the cloning phase, every clone warrior has to wait for commands to receive and to transmit to his inferiors
Hints and requirements:
Consider, how many soldiers are available in which year: t=0 –just the captain, t=1 –captain and his first clone, etc.
Don‘t worry about error handling
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/stat.h>
#include <mqueue.h>
#include <errno.h>
// Exercise „clone warriors“
#define NUM 10
#define SIZE_MSGBUF 500
#define MODE (S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH |S_IWOTH)
mqd_t QueueArray[NUM]; // queues to my clones
void cleanQueueArray(void) { // support function for init: start with no queues
for (int i=0; i<NUM; i++) QueueArray[i] = 0;
}
int main(void) {
char cNameBossQueue[100] = "/Imperator"; // boss queue‘s default name
mqd_t BossQueue; // boss queue to receive commands of the father‘s process
struct mq_attr attr;
attr.mq_maxmsg = 10;
attr.mq_msgsize = SIZE_MSGBUF;
attr.mq_flags = 0;
int nPrio=0;
char cMsgbuf[SIZE_MSGBUF+1] = "";
cleanQueueArray(); // init: no queues to any clones at the beginning
// phase 1 / clone phase takes NUM years:
for (int i=0; i<NUM; i++) {
pid_t npid_child = fork();
if (npid_child > 0) { // Father. Create + store command channel to clone:
char cQueue[100];
sprintf(cQueue, "/Queue%d", npid_child);
QueueArray[i] = mq_open(cQueue, O_CREAT|O_WRONLY, MODE, &attr);
} else { // Child. Remember the name of the boss queue:
sprintf(cNameBossQueue, "/Queue%d", getpid());
cleanQueueArray(); // Child has no queues to clones currently
}
}
// Phase 2 / battle phase. Receive and transmit orders:
BossQueue = mq_open(cNameBossQueue, O_RDONLY, MODE, &attr);
mq_receive(BossQueue, cMsgbuf, SIZE_MSGBUF, &nPrio);
// Send orders to all of my clones:
for (int i=0; i<NUM; i++) {
if (QueueArray[i] > 0) {
mq_send (QueueArray[i], cMsgbuf, strlen(cMsgbuf), 0);
}
}
// Cleanup work...
return 0;
}
I tried running this using
gcc -o Wall clonew clonew.c -lrt"
./clonew
but I get no output

Answer 1

Your commandline (without the trailing ", which I consider a posting accident)
gcc -o Wall clonew clonew.c -lrt
means
compile clonew and clonew.c
into an executable named "Wall"
using the rt lib
I am not sure why the use of "clonew" (first in the compile command, later in the execution attempt) does not trigger any warning.
Clearly you intend to create and execute that file. But it is used as input file and should, even if it exists but is not a C syntax file, cause some complaint.
However, the reason why you do not get the expected output when trying to execute clonew is that your compile command cannot have resulted in the creation of an executable clonew.
Why the attempt to execute something, which either does not exist yet or is a C code file, does not result in any warnings, errors or any other output is a mystery.
If it already exists as an executable but silent program, it should also trigger a complaint.