#include <sys/msg.h> int msgget(key_t key, int msgflg);
The msgget() argument returns the message queue identifier associated with key.
A message queue identifier and associated message queue and data structure (see Intro(2)) are created for key if one of the following are true:
key is IPC_PRIVATE.
key does not already have a message queue identifier associated with it, and (msgflg&IPC_CREAT) is true.
On creation, the data structure associated with the new message queue identifier is initialized as follows:
msg_perm.cuid, msg_perm.uid, msg_perm.cgid, and msg_perm.gid are set to the effective user ID and effective group ID, respectively, of the calling process.
The low-order 9 bits of msg_perm.mode are set to the low-order 9 bits of msgflg.
msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime are set to 0.
msg_ctime is set to the current time.
msg_qbytes is set to the system limit. See NOTES.
Upon successful completion, a non-negative integer representing a message queue identifier is returned. Otherwise, -1 is returned and errno is set to indicate the error.
The msgget() function will fail if:
A message queue identifier exists for key, but operation permission (see Intro(2)) as specified by the low-order 9 bits of msgflg would not be granted.
A message queue identifier exists for key but (msgflg&IPC_CREAT) and (msgflg&IPC_EXCL) are both true.
A message queue identifier does not exist for key and (msgflg&IPC_CREAT) is false.
A message queue identifier is to be created but the system-imposed limit on the maximum number of allowed message queue identifiers system wide would be exceeded. See NOTES.
See attributes(5) for descriptions of the following attributes:
The system-defined limit used to initialize msg_qbytes is the minimum enforced value of the calling process's process.max-msg-qbytes resource control.
The system-imposed limit on the number of message queue identifiers is maintained on a per-project basis using the project.max-msg-ids resource control.