Msgrcv Error Invalid Argument
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Msgsnd Invalid Argument
data through message queue in C up vote 0 down vote favorite I am sending message using message queue mechanism in C linux. But there is some problem with the msgrcv function. It's showing error as invalid argument. Please check this. //msgrcv.c #include"msgbuf.h" int main() { int msqid; key_t key; message_buf *rbuf; rbuf=malloc(sizeof(*rbuf)); // rbuf->m=malloc(sizeof(M1)); key = ftok("/home/user",'a'); if ((msqid = msgget(key, IPC_CREAT)) ==(key)-1) { msgsnd msgrcv example perror("msgget"); exit(1); } /* Receive an answer of message type 1. */ if (msgrcv(msqid, &rbuf, sizeof(rbuf->m), 1, 0) < 0) { perror("msgrcv"); //invalid argument to msgrcv exit(1); } /* Print the answer. */ printf("Received message text= %s\n", rbuf->m.cp); return 0; } now msgbuf.h //msgbuf.h typedef struct msgclient { int msglen; int msgtype; char *cp; }M1; typedef struct msgbuf1 { long mtype; M1 m; } message_buf; I also would like to know how two way comm happen using message queue. Do I need to make two message queues to get communication done between two processes? sample code for the same is also welcomed. Thanks :) c linux ipc message-queue msgrcv share|improve this question edited Mar 25 '14 at 8:11 asked Mar 25 '14 at 6:51 user3436838 538 add a comment| 2 Answers 2 active oldest votes up vote 0 down vote accepted I guess this if ((msqid = msgget(key, 0666)) ==key-1) should be if ((msqid = msgget(key, 0666)) == -1) From msgrcv ERRORS The msgrcv() function will fail if: ... [EINVAL] msqid is not a valid message queue identifier. Furthermore, message_buf.m must not be a pointer but a member typedef struct msgbuf1 { l
here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About Us Learn more message queue in linux about Stack Overflow the company Business Learn more about hiring developers or posting ads with us Stack Overflow Questions Jobs Documentation Tags Users Badges Ask Question x Dismiss Join the Stack Overflow Community Stack Overflow is a community of 6.2 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up Error in msgrcv: Invalid Arguments up vote 0 down vote favorite I http://stackoverflow.com/questions/22627307/error-in-msgrcv-receiving-data-through-message-queue-in-c am writing a code in C on a unix system. I have created a message queue server. Each time I receive a new message I fork and the child process handles the new client. The server waits for new client. Here's the code. for (;;) { struct my_msgbuf buf; if (msgrcv (msqid, &(buf.mtype), sizeof (buf), 1, 0) == -1) perror ("msgrcv"); if((pid = fork())<0) perror("fork"); if(pid==0) { //child code } http://stackoverflow.com/questions/1586006/error-in-msgrcv-invalid-arguments } Now the code works for the first iteration but on the second iteration msgrcv gives the following error - msgrcv: Invalid Arguments instead of waiting for new messages. c unix ipc share|improve this question edited Feb 9 '13 at 8:16 Steven Penny 1 asked Oct 18 '09 at 20:54 user187707 add a comment| 1 Answer 1 active oldest votes up vote 0 down vote msgrcv does take a msgsz parameter, but it's not the size of a struct my_msgbuf, rather, it's the number of bytes in the mtext[] field of the structure. You aren't really supposed to use struct msgbuf directly, I think. It would be normal to define your own with appropriate space. The way you have it, your buf declaration allocates only 1 byte, and then the code tells the kernel that you have sizeof buf bytes, which you don't. As a result, the code is said to invoke Undefined Behavior or UB. This may or may not explain the EINVAL but one should always fix the known bugs and retest because UB taints further analysis... share|improve this answer answered Oct 18 '09 at 21:12 DigitalRoss 103k12178258 no didnt work... –user187707 Oct 18 '09 at 21:16 The code doe
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*msgp, size_t msgsz, long msgtyp, int msgflg); Description The msgsnd() and msgrcv() system calls are used, respectively, to send messages to, and receive messages from, a message queue. The calling process must have write permission on the message queue in order to send a message, and read permission to receive a message. The msgp argument is a pointer to caller-defined structure of the following general form: struct msgbuf { long mtype; /* message type, must be > 0 */ char mtext[1]; /* message data */ }; The mtext field is an array (or other structure) whose size is specified by msgsz, a nonnegative integer value. Messages of zero length (i.e., no mtext field) are permitted. The mtype field must have a strictly positive integer value. This value can be used by the receiving process for message selection (see the description of msgrcv() below). msgsnd() The msgsnd() system call appends a copy of the message pointed to by msgp to the message queue whose identifier is specified by msqid. If sufficient space is available in the queue, msgsnd() succeeds immediately. (The queue capacity is defined by the msg_qbytes field in the associated data structure for the message queue. During queue creation this field is initialized to MSGMNB bytes, but this limit can be modified using msgctl(2).) If insufficient space is available in the queue, then the default behavior of msgsnd() is to block until space becomes available. If IPC_NOWAIT is specified in msgflg, then the call instead fails with the error EAGAIN. A blocked msgsnd() call may also fail if: * the queue is removed, in which case the system call fails with errno set to EIDRM; or * a signal is caught, in which case the system call fails with errno set to EINTR;see signal(7). (msgsnd() is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTART flag when establishing a signal handler.) Upon successful completion the message queue data s