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Old 10-18-2004, 05:54 PM   #1
Registered: Dec 2003
Location: NC, US
Distribution: Novell Linux Eval (2.6.5)
Posts: 240

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programming for message queue and such ...

I am writing a practise for using message queue in linux, there are a few mistries i can't seem to figure out. code are posted after this
i have two programs, ./server and ./client, when I do ./client <cmd>, ./client supposes to send the <cmd> to the queue and server will retrieve it, exec it and send the results back to the queue and client gets it, display it and quit.
when I do one ./client <cmd> and then do ipcs, it shows me there is no messages
i open another terminal and do it again, it shows me there is one message
i open another terminal and do it again, it shows me there is 2 messages
and so on, why the first one didn't get added to the queue, that's my biggest question right now
queue struct
const long int  SERVER = 1L;    //msg type for server
typedef struct {
        long int msg_to;
        long int msg_fm;
        char buffer[BUFSIZ];
client code
#include "local.h"
#include <stdio.h>

int main (int argc, char *argv[]) {
	pid_t c_pid;
	int mid, n=0, i, pos =0;
	char *pch;
	bzero(msg.buffer, BUFSIZ);
	c_pid = getpid();
	if ((mid=msgget(2,IPC_CREAT)) == -1) {
		perror ("Server is not running");
		return 1;
printf("mid is %d\n", mid);
	while (1) {
		msg.msg_to = SERVER;
		msg.msg_fm = c_pid;
		memset(msg.buffer, 0x0, BUFSIZ);
		for(i=0; i<argc; i++) {
			strcat(msg.buffer, argv[i]);
			strcat(msg.buffer, " ");
printf("argv[%i] is %s and n is %d and str is %s and size %d\n", i, argv[i], n, msg.buffer, sizeof(msg.buffer));	
		n = sizeof(char) * strlen(msg.buffer);
		n += sizeof(msg.msg_fm);
printf("n is %d\n", n);

		if(msgsnd(mid, &msg, n, 0) == -1) {
			perror ("Client: msgsnd");
			return 3;
		if ( (n=msgrcv(mid, &msg, BUFSIZ, c_pid, 0)) != -1 ) {
			printf("Client MSQID: %d\n", c_pid);
			write(fileno(stdout), msg.buffer, n);
	printf("Client exit\n");
	return 0;
server code
#include "local.h"
#include <signal.h>
#include <fcntl.h>

int smid;
void server_cmd();
void sig_catch_u1(int);

int main (int argc, char *argv[]) {
	int n,t=0;
	FILE *fp;	
	signal(SIGUSR1, sig_catch_u1);

	if ((smid=msgget(2, 0)) == -1) {
		smid = msgget(2, IPC_CREAT | IPC_EXCL | 0660);		//create msg queue
	while (1) {
		memset(msg.buffer, 0x0, BUFSIZ);
		if ((n=msgrcv(smid, &msg, BUFSIZ, SERVER, 0)) == -1) {
			perror("Server: msgrcv");
			return 2;
		else if (n == 0) {
printf("server: get here and n is %d\n", n);
printf("server: msg.buffer is %s\n", msg.buffer);

		switch(fork()) {
			case -1:
				perror("Fork error");
			case 0:

printf("server: after fork\n");

		if ((fp = fopen("/tmp/server_tmp", "r")) == NULL) {	
			perror("Open file error");

		do {
			msg.buffer[t++] = getc(fp);
		}while(msg.buffer[t] != EOF || t>= BUFSIZ);

		msg.msg_to = msg.msg_fm;
		msg.msg_fm = SERVER;

		n = sizeof(char) * strlen(msg.buffer);
		n += sizeof(msg.msg_fm);
		if (msgsnd(smid, &msg, n, 0) == -1) {
			perror("Server: msgsnd");
			return 3;

void server_cmd() {
	char *s_cmd, *new_args[20];
	int s_fd, i=0;

	if ((s_fd=open("/tmp/server_tmp", O_WRONLY|O_CREAT|O_TRUNC, S_IRUSR|S_IWUSR))==-1) {
		perror ("Open error");
	if (dup2(s_fd, fileno(stdout)) == -1) {
		perror ("Dup2 erorr");
	s_cmd = msg.buffer;
	new_args[i] = strtok(s_cmd, " \t");
	while (*new_args[i] != '\0'){
	new_args[i] = NULL;
	execvp(new_args[1], &new_args[1]);
	perror("Execvp failure");

void sig_catch_u1 (int sig) 
	signal(sig, sig_catch_u1);
	msgctl(smid, IPC_RMID, (struct msqid_ds *) 0);		


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