The Linux Programming Interface

940 Chapter 46

case 'x':

flags |= IPC_EXCL;

break;

default:

usageError(argv[0], "Bad option\n");

}

}

if (numKeyFlags != 1)

usageError(argv[0], "Exactly one of the options -f, -k, "

"or -p must be supplied\n");

perms = (optind == argc)? (S_IRUSR | S_IWUSR) :

getInt(argv[optind], GN_BASE_8, "octal-perms");

msqid = msgget(key, flags | perms);

if (msqid == -1)

errExit("msgget");

printf("%d\n", msqid);

exit(EXIT_SUCCESS);

}

––––––––––––––––––––––––––––––––––––––––––––––––––––––svmsg/svmsg_create.c

46.2 Exchanging Messages

The msgsnd() and msgrcv() system calls perform I/O on message queues. The first

argument to both system calls (msqid) is a message queue identifier. The second

argument, msgp, is a pointer to a programmer-defined structure used to hold the

message being sent or received. This structure has the following general form:

struct mymsg {

long mtype; /* Message type */

char mtext[]; /* Message body */

}

This definition is really just shorthand for saying that the first part of a message

contains the message type, specified as a long integer, while the remainder of the

message is a programmer-defined structure of arbitrary length and content; it need

not be an array of characters. Thus, the mgsp argument is typed as void * to allow it

to be a pointer to any type of structure.

A zero-length mtext field is permitted, and is sometimes useful if the informa-

tion to be conveyed can be encoded solely in the message type or if the existence of

a message is in itself sufficient information for the receiving process.

46.2.1 Sending Messages

The msgsnd() system call writes a message to a message queue.