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The C and C++ Include Header Files

cat -n /usr/include/linux/msg.h

     1	/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
     2	#ifndef _LINUX_MSG_H
     3	#define _LINUX_MSG_H
     4	
     5	#include <linux/ipc.h>
     6	
     7	/* ipcs ctl commands */
     8	#define MSG_STAT 11
     9	#define MSG_INFO 12
    10	#define MSG_STAT_ANY 13
    11	
    12	/* msgrcv options */
    13	#define MSG_NOERROR     010000  /* no error if message is too big */
    14	#define MSG_EXCEPT      020000  /* recv any msg except of specified type.*/
    15	#define MSG_COPY        040000  /* copy (not remove) all queue messages */
    16	
    17	/* Obsolete, used only for backwards compatibility and libc5 compiles */
    18	struct msqid_ds {
    19		struct ipc_perm msg_perm;
    20		struct msg *msg_first;		/* first message on queue,unused  */
    21		struct msg *msg_last;		/* last message in queue,unused */
    22		__kernel_old_time_t msg_stime;	/* last msgsnd time */
    23		__kernel_old_time_t msg_rtime;	/* last msgrcv time */
    24		__kernel_old_time_t msg_ctime;	/* last change time */
    25		unsigned long  msg_lcbytes;	/* Reuse junk fields for 32 bit */
    26		unsigned long  msg_lqbytes;	/* ditto */
    27		unsigned short msg_cbytes;	/* current number of bytes on queue */
    28		unsigned short msg_qnum;	/* number of messages in queue */
    29		unsigned short msg_qbytes;	/* max number of bytes on queue */
    30		__kernel_ipc_pid_t msg_lspid;	/* pid of last msgsnd */
    31		__kernel_ipc_pid_t msg_lrpid;	/* last receive pid */
    32	};
    33	
    34	/* Include the definition of msqid64_ds */
    35	#include <asm/msgbuf.h>
    36	
    37	/* message buffer for msgsnd and msgrcv calls */
    38	struct msgbuf {
    39		__kernel_long_t mtype;          /* type of message */
    40		char mtext[1];                  /* message text */
    41	};
    42	
    43	/* buffer for msgctl calls IPC_INFO, MSG_INFO */
    44	struct msginfo {
    45		int msgpool;
    46		int msgmap; 
    47		int msgmax; 
    48		int msgmnb; 
    49		int msgmni; 
    50		int msgssz; 
    51		int msgtql; 
    52		unsigned short  msgseg; 
    53	};
    54	
    55	/*
    56	 * MSGMNI, MSGMAX and MSGMNB are default values which can be
    57	 * modified by sysctl.
    58	 *
    59	 * MSGMNI is the upper limit for the number of messages queues per
    60	 * namespace.
    61	 * It has been chosen to be as large possible without facilitating
    62	 * scenarios where userspace causes overflows when adjusting the limits via
    63	 * operations of the form retrieve current limit; add X; update limit".
    64	 *
    65	 * MSGMNB is the default size of a new message queue. Non-root tasks can
    66	 * decrease the size with msgctl(IPC_SET), root tasks
    67	 * (actually: CAP_SYS_RESOURCE) can both increase and decrease the queue
    68	 * size. The optimal value is application dependent.
    69	 * 16384 is used because it was always used (since 0.99.10)
    70	 *
    71	 * MAXMAX is the maximum size of an individual message, it's a global
    72	 * (per-namespace) limit that applies for all message queues.
    73	 * It's set to 1/2 of MSGMNB, to ensure that at least two messages fit into
    74	 * the queue. This is also an arbitrary choice (since 2.6.0).
    75	 */
    76	
    77	#define MSGMNI 32000   /* <= IPCMNI */     /* max # of msg queue identifiers */
    78	#define MSGMAX  8192   /* <= INT_MAX */   /* max size of message (bytes) */
    79	#define MSGMNB 16384   /* <= INT_MAX */   /* default max size of a message queue */
    80	
    81	/* unused */
    82	#define MSGPOOL (MSGMNI * MSGMNB / 1024) /* size in kbytes of message pool */
    83	#define MSGTQL  MSGMNB            /* number of system message headers */
    84	#define MSGMAP  MSGMNB            /* number of entries in message map */
    85	#define MSGSSZ  16                /* message segment size */
    86	#define __MSGSEG ((MSGPOOL * 1024) / MSGSSZ) /* max no. of segments */
    87	#define MSGSEG (__MSGSEG <= 0xffff ? __MSGSEG : 0xffff)
    88	
    89	
    90	#endif /* _LINUX_MSG_H */