individual char extraction in c while using messages and msgsnd - c

Question

This is bugging me for days.
The problem is my not so good understanding of pointers and addresses in c so i hope someone will be able to help me out.
I need to pass some strings as input parameters and create as much producer processes + one consumer process.
Producers should take the string apart and send each letter as message to queue. At the end it should send NULL("").
The consumer should wait for messages and print them out.
The whole code and output is below. By looking at the output i'd say that the problem is somewhere in the producer. To be more precise it is in the first line of te for loop but i can not get it right.
manager.c - This is the main program that operates processes
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/msg.h>
int main( int argc, char *argv[], char *envp[] ) {
printf("Starting %d processes \n", argc);
putenv("MSG_KEY=12345");
for (int i = 1; i < argc; i++) {
printf("argv[%d] = %s \n", i, argv[i]);
pid_t producer = fork();
if (producer == 0) {
printf("producer pid - %d\n", getpid());
execl("./producer", "producer", argv[i], NULL);
}
}
pid_t consumer = fork();
if (consumer == 0) {
printf("consumer pid - %d\n", getpid());
execl("./consumer", "consumer", NULL);
exit(0);
} else {
printf("manager pid - %d\n", getpid());
wait(NULL);
}
int status;
while(waitpid(consumer, &status, 0) == -1);
printf("DONE consumer\n");
printf("DONE manager\n");
return 0;
}
producer.c
/*
** writes to message queue
*/
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <stddef.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
struct my_msgbuf {
long mtype;
char mtext[1];
};
int main( int argc, char *argv[], char *envp[] ) {
struct my_msgbuf buf;
int msqid;
key_t key = atoi(getenv("MSG_KEY"));
if ((msqid = msgget(key, 0600 | IPC_CREAT)) == -1) {
perror("msgget");
exit(1);
}
buf.mtype = getpid();
// I believe the error is in this for loop or to be more precise in the first line of the for loop.
// takes the first argument and sends characters in separate messages
for (int i = 0; i < strlen(argv[1]); ++i) {
char c = argv[1][i];
strcpy(buf.mtext, &c);
printf ("Sending -%s-\n", buf.mtext);
if (msgsnd(msqid, (struct msgbuf *)&buf, strlen(buf.mtext)+1, 0) == -1)
perror("msgsnd");
}
// send NULL at the end
memcpy(buf.mtext, "", strlen("")+1);
if (msgsnd(msqid, (struct msgbuf *)&buf, strlen("")+1, 0) == -1)
perror("msgsnd");
return 0;
}
consumer.c
/*
** reads from message queue
*/
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
struct my_msgbuf {
long mtype;
char mtext[1];
};
int main( int argc, char *argv[], char *envp[] ) {
struct my_msgbuf buf;
int msqid;
key_t key = atoi(getenv("MSG_KEY"));
if ((msqid = msgget(key, 0600 | IPC_CREAT)) == -1) {
perror("msgget");
exit(1);
}
int flag = 0;
int wait_counter = 0;
while (wait_counter < 10) {
msgrcv(msqid, (struct msgbuf *)&buf, sizeof(buf)-sizeof(long), 0, flag);
if (errno == ENOMSG){
wait_counter++;
printf ("Sleaping for one second...zzzZZZzzz...%d\n", wait_counter);
usleep(1000 * 1000);
} else {
printf("Received:\n\ttype: -%ld- \n\tchar: -%s- \n", buf.mtype, buf.mtext);
int compare = strcmp(buf.mtext, "");
if(compare == 0){
printf("NULL received\n");
flag = IPC_NOWAIT;
} else {
flag = 0;
}
wait_counter = 0;
}
errno = 0;
}
if (msgctl(msqid, IPC_RMID, NULL) == -1) {
perror("msgctl");
exit(1);
} else {
printf("Message queue removed\n");
}
return 0;
}
Output - i have to give you the screenshot here because c/p deletes the problem and everything looks ok
Any help will be much appreciated! Thank you!
Error when used as suggested in the #sergeya answer below
*buf.mtext = c;

Answer 1

Your problem (one of them, at least) is here:
char c = argv[1][i];
strcpy(buf.mtext, &c);
strcpy() will try to copy as many characters as there are until nul-terminator '\0' is encountered, starting from c. You need to copy one character exactly, so you just need
*buf.mtext = c;

Answer 2

As i said, the problem was in the producer inside the for loop. I will put the change here. Hope it helps anyone with the similar problem.
#SergeyA gave me excellent clue where the problem is so i switched from "strcpy" to "memcpy" and i have copied just the first character and not the nul-terminator.
Also i have changed the "strlen" to "sizeof" and removed the +1.
Producer.c
...
for (int i = 0; i < strlen(argv[1]); ++i) {
char c = argv[1][i];
memcpy(buf.mtext, &c, sizeof(&c)+1);
printf ("Sending -%c-\n", buf.mtext);
if (msgsnd(msqid, (struct msgbuf *)&buf, sizeof(buf.mtext), 0) == -1)
perror("msgsnd");
}
...