Fork() and Posix Queues. Send and Receive strings - c

Question

what I want is this:
1 main process that create 4 children process where:
-> The main process receive messages from the children through the queue and print the message recieved.
-> The children send messages (a string with priority+message) through the queue and finish.
All in a while (1), so, when you CTRL+C, the children finish first (the signal is in the children code) and then, the parent finish.
For the moment, I am having problem with mq_send() and mq_recieve().
Well, this is my code:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <signal.h>
#include <sys/wait.h>
#include <mqueue.h>
void sigint_handler()
{
/*do something*/
printf("killing process %d\n",getpid());
exit(0);
}
int main ()
{
mqd_t mqd;
struct mq_attr atributos;
// atributos.mq_maxmsg = 10;
//
// atributos.mq_msgsize = 50;
printf ("This is the parent. PID=%d\n",getpid ());
int num_children = 4;
int i;
int pid;
int status;
char buffer [50];
while (1){
for (i=0; i<num_children ;i++){
if ((pid=fork()==0)){
signal(SIGINT, sigint_handler);
int prio = rand () % 3;
printf ("%d\n",prio);
char * msg= "Hi dude";
char * priority=NULL;
if (prio == 0){
priority = "NORMAL";
}
else {
priority = "URGENT";
}
char* toSend=NULL;
toSend = malloc(strlen(msg)+1+strlen(priority));
strcpy (toSend,priority);
strcat (toSend,msg);
printf ("%s\n",toSend);
if ((mqd=mq_open("/queue.txt", O_CREAT|O_WRONLY, 0777, &atributos))==-1){
printf ("Error mq_open\n");
exit(-1);
}
if (mq_send(mqd, msg , strlen(toSend), prio) == -1) {
printf ("Error mq_send\n");
exit (-1);
}
mq_close(mqd);
printf ("This is children %d\n",getpid());
sleep(1);
exit(0);
}
}
if ((mqd=mq_open("/queue.txt", O_CREAT|O_WRONLY, 0777, &atributos))==-1){
printf ("Error mq_open\n");
exit(-1);
}
//Rest Parent code
if (mq_receive(mqd, buffer, strlen(buffer),0)==-1){
printf ("Error mq_recieve\n");
exit(-1);
}
printf("Received: %s\n",buffer);
sleep (1);
waitpid(pid,&status,0);
printf ("This is the parent again %d, children should have finished\n",getpid());
mq_close(mqd);
}
}
I don't know why both mq_send() and mq_receive() returns -1, what am I doing wrong¿?
And you you see something wrong in my code in order to do what I intend apart from the error I am talking about, let me know.
Thank you in advance, I appreciate any help.

Answer 1

user58697 touched upon the biggest problems.
(1) Your queue opens were failing with EINVAL because you wee passing uninitialized attributes because you commented out assignments.
(2) You were opening both queues for write-only. The parent queue needed to be opened in read mode.
(3) Execute permissions don't mean anything to a queue so 777 permissions while not invalid are unnecessary.
(4) Your sends/receives were failing because of invalid lengths. In many if not most cases it is just easier and safer to allocate your buffers to the length attribute of the queue. In this case you know the length before hand but in programs that don't you can get the value via mq_getattr.
(5) You weren't calling srand to seed the RNG before calling rand.
(6) You had a memory leak where you allocate space (unnecessarily) for the message but never freed it.
(7) What you were trying to do with passing priorities is redundant. POSIX MQs have priorities already built in. You can just use those.
I took out some of the fluff (mainly the loops & signals) to concentrate more on the queue aspects of your program.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <time.h>
#include <signal.h>
#include <sys/wait.h>
#include <mqueue.h>
int main()
{
srand(time(NULL));
mqd_t mqd;
struct mq_attr atributos = {.mq_maxmsg = 10, .mq_msgsize = 50};
int i;
int pid;
int status;
int num_children = 4;
char buffer[atributos.mq_msgsize];
for (i = 0; i < num_children; i++)
{
if ((pid = fork() == 0))
{
int prio = rand () % 3;
char* msg = "Hi dude";
strncpy (buffer, msg, sizeof(buffer));
if ((mqd = mq_open("/queue.txt", O_CREAT | O_WRONLY, 0666, &atributos)) == -1)
{
perror("child mq_open");
exit(1);
}
if (mq_send(mqd, buffer, sizeof(buffer), prio) == -1)
{
perror("mq_send");
exit(1);
}
mq_close(mqd);
exit(0);
}
}
// parent
if ((mqd = mq_open("/queue.txt", O_CREAT | O_RDONLY, 0666, &atributos)) == -1)
{
perror("parent mq_open");
exit(1);
}
int priority;
for (int i = 0; i < num_children; ++i)
{
if (mq_receive(mqd, buffer, sizeof(buffer), &priority) == -1)
{
perror("mq_recieve");
exit(1);
}
printf("Received (%s): %s\n", (priority == 0) ? "NORMAL" : "URGENT", buffer);
pid_t childpid;
if ((childpid = waitpid(-1, &status, 0)) > 0)
{
if (WIFEXITED(status))
printf("PID %d exited normally. Exit status: %d\n",
childpid, WEXITSTATUS(status));
else
if (WIFSTOPPED(status))
printf("PID %d was stopped by %d\n",
childpid, WSTOPSIG(status));
else
if (WIFSIGNALED(status))
printf("PID %d exited due to signal %d\n.",
childpid,
WTERMSIG(status));
}
}
mq_close(mqd);
}

Answer 2

First and foremost, when a system call fails, print errno (and strerror(errno)).
Now, obvious mistakes:
as was mentioned, you need a read access to be able to mq_receive()
what is strlen(buffer)?
you are passing attributes without initializing them.
To summarize, print errno and see what is wrong.