sys/kern/sysv_msg.c

/*
 * Implementation of SVID messages
 *
 * Author:  Daniel Boulet
 *
 * Copyright 1993 Daniel Boulet and RTMX Inc.
 *
 * This system call was implemented by Daniel Boulet under contract from RTMX.
 *
 * Redistribution and use in source forms, with and without modification,
 * are permitted provided that this entire comment appears intact.
 *
 * Redistribution in binary form may occur without any restrictions.
 * Obviously, it would be nice if you gave credit where credit is due
 * but requiring it would be too onerous.
 *
 * This software is provided ``AS IS'' without any warranties of any kind.
 */

#ifdef SYSVMSG

#include "param.h"
#include "systm.h"
#include "kernel.h"
#include "proc.h"
#include "msg.h"
#include "malloc.h"

static int      msgctl(), msgget(), msgsnd(), msgrcv();

int     (*msgcalls[])() = { msgctl, msgget, msgsnd, msgrcv };

int nfree_msgmaps;              /* # of free map entries */
short free_msgmaps;             /* head of linked list of free map entries */
struct msg *free_msghdrs;       /* list of free msg headers */

void
msginit()
{
    register int i;
    vm_offset_t whocares1, whocares2;

    /*
     * msginfo.msgssz should be a power of two for efficiency reasons.
     * It is also pretty silly if msginfo.msgssz is less than 8
     * or greater than about 256 so ...
     */

    i = 8;
    while ( i < 1024 && i != msginfo.msgssz ) {
        i <<= 1;
    }
    if ( i != msginfo.msgssz ) {
        printf("msginfo.msgssz=%d (0x%x)\n",msginfo.msgssz,msginfo.msgssz);
        panic("msginfo.msgssz not a small power of 2");
    }

    if ( msginfo.msgseg > 32767 ) {
        printf("msginfo.msgseg=%d\n",msginfo.msgseg);
        panic("msginfo.msgseg > 32767");
    }

    if ( msgmaps == NULL ) {
        panic("msgmaps is NULL");
    }
    for ( i = 0; i < msginfo.msgseg; i += 1 ) {
        if ( i > 0 ) {
            msgmaps[i-1].next = i;
        }
        msgmaps[i].next = -1;           /* implies entry is available */
    }
    free_msgmaps = 0;
    nfree_msgmaps = msginfo.msgseg;

    if ( msghdrs == NULL ) {
        panic("msghdrs is NULL");
    }
    for ( i = 0; i < msginfo.msgtql; i += 1 ) {
        msghdrs[i].msg_type = 0;
        if ( i > 0 ) {
            msghdrs[i-1].msg_next = &msghdrs[i];
        }
        msghdrs[i].msg_next = NULL;
    }
    free_msghdrs = &msghdrs[0];

    if ( msqids == NULL ) {
        panic("msqids is NULL");
    }
    for ( i = 0; i < msginfo.msgmni; i += 1 ) {
        msqids[i].msg_qbytes = 0;       /* implies entry is available */
        msqids[i].msg_perm.seq = 0;     /* reset to a known value */
    }

}

TEXT_SET(pseudo_set, msginit);

/*
 * Entry point for all MSG calls
 */

struct msgsys_args {
        u_int   which;
};

int
msgsys(p, uap, retval)
        struct caller *p;
        struct msgsys_args *uap;
        int *retval;
{
        if (uap->which >= sizeof(msgcalls)/sizeof(msgcalls[0]))
                return (EINVAL);
        return ((*msgcalls[uap->which])(p, &uap[1], retval));
}

static
void
msg_freehdr(msghdr)
struct msg *msghdr;
{
    while ( msghdr->msg_ts > 0 ) {
        short next;
        if ( msghdr->msg_spot < 0 || msghdr->msg_spot >= msginfo.msgseg ) {
            panic("msghdr->msg_spot out of range");
        }
        next = msgmaps[msghdr->msg_spot].next;
        msgmaps[msghdr->msg_spot].next = free_msgmaps;
        free_msgmaps = msghdr->msg_spot;
        nfree_msgmaps += 1;
        msghdr->msg_spot = next;
        if ( msghdr->msg_ts >= msginfo.msgssz ) {
            msghdr->msg_ts -= msginfo.msgssz;
        } else {
            msghdr->msg_ts = 0;
        }
    }
    if ( msghdr->msg_spot != -1 ) {
        panic("msghdr->msg_spot != -1");
    }
    msghdr->msg_next = free_msghdrs;
    free_msghdrs = msghdr;
}

struct msgctl_args {
        int     msqid;
        int     cmd;
        struct  msqid_ds *user_msqptr;
};

static int
msgctl(p, uap, retval)
        struct proc *p;
        register struct msgctl_args *uap;
        int *retval;
{
    int msqid = uap->msqid;
    int cmd = uap->cmd;
    struct msqid_ds *user_msqptr = uap->user_msqptr;
    struct ucred *cred = p->p_ucred;
    int i, rval, eval;
    struct msqid_ds msqbuf;
    register struct msqid_ds *msqptr;

#ifdef MSG_DEBUG
    printf("call to msgctl(%d,%d,0x%x)\n",msqid,cmd,user_msqptr);
#endif

    msqid = IPCID_TO_IX(msqid);

    if ( msqid < 0 || msqid >= msginfo.msgmni ) {
#ifdef MSG_DEBUG
        printf("msqid (%d) out of range (0<=msqid<%d)\n",msqid,msginfo.msgmni);
#endif
        return(EINVAL);
    }

    msqptr = &msqids[msqid];

    if ( msqptr->msg_qbytes == 0 ) {
#ifdef MSG_DEBUG
        printf("no such msqid\n");
#endif
        return(EINVAL);
    }
    if ( msqptr->msg_perm.seq != IPCID_TO_SEQ(uap->msqid) ) {
#ifdef MSG_DEBUG
        printf("wrong sequence number\n");
#endif
        return(EINVAL);
    }

    eval = 0;
    rval = 0;

    switch (cmd) {

    case IPC_RMID:
#ifdef MSG_DEBUG
        printf("IPC_RMID\n");
#endif
        {
            struct msg *msghdr;

            if ( cred->cr_uid != 0
            && msqptr->msg_perm.cuid != cred->cr_uid
            && msqptr->msg_perm.uid != cred->cr_uid ) {
                return(EPERM);
            }
            msghdr = msqptr->msg_first;

            /* Free the message headers */

            while ( msghdr != NULL ) {
                struct msg *msghdr_tmp;

                /* Free the segments of each message */

                msqptr->msg_cbytes -= msghdr->msg_ts;
                msqptr->msg_qnum -= 1;
                msghdr_tmp = msghdr;
                msghdr = msghdr->msg_next;
                msg_freehdr(msghdr_tmp);

            }

            if ( msqptr->msg_cbytes != 0 ) {
                panic("msg_cbytes is screwed up");
            }
            if ( msqptr->msg_qnum != 0 ) {
                panic("msg_qnum is screwed up");
            }

            msqptr->msg_qbytes = 0;     /* Mark it as free */

            /* Make sure that anybody who is waiting notices the deletion */

            wakeup( (caddr_t)msqptr );
        }

        break;

    case IPC_SET:
#ifdef MSG_DEBUG
        printf("IPC_SET\n");
#endif
        if ( cred->cr_uid != 0
        && msqptr->msg_perm.cuid != cred->cr_uid
        && msqptr->msg_perm.uid != cred->cr_uid ) {
            return(EPERM);
        }
        if ( (eval = copyin(user_msqptr, &msqbuf, sizeof(msqbuf))) != 0 ) {
            return(eval);
        }
        if ( msqbuf.msg_qbytes > msqptr->msg_qbytes
        && cred->cr_uid != 0 ) {
            return(EPERM);
        }
        if ( msqbuf.msg_qbytes > msginfo.msgmnb ) {
#ifdef MSG_DEBUG
            printf("can't increase msg_qbytes beyond %d (truncating)\n",msginfo.msgmnb);
#endif
            msqbuf.msg_qbytes = msginfo.msgmnb; /* silently restrict qbytes to system limit */
        }
        if ( msqbuf.msg_qbytes == 0 ) {
#ifdef MSG_DEBUG
            printf("can't reduce msg_qbytes to 0\n");
#endif
            return(EINVAL);             /* non-standard errno! */
        }
        msqptr->msg_perm.uid = msqbuf.msg_perm.uid;     /* change the owner */
        msqptr->msg_perm.gid = msqbuf.msg_perm.gid;     /* change the owner */
        msqptr->msg_perm.mode = (msqptr->msg_perm.mode & ~0777)
        | (msqbuf.msg_perm.mode & 0777);
        msqptr->msg_qbytes = msqbuf.msg_qbytes;
        msqptr->msg_ctime = time.tv_sec;
        break;

    case IPC_STAT:
#ifdef MSG_DEBUG
        printf("IPC_STAT\n");
#endif
        if ( (eval = ipcaccess(&msqptr->msg_perm, IPC_R, cred)) ) {
#ifdef MSG_DEBUG
            printf("requester doesn't have read access\n");
#endif
            return(eval);
        }
        rval = 0;
        eval = copyout((caddr_t)msqptr, user_msqptr, sizeof(struct msqid_ds));
        break;

    default:
#ifdef MSG_DEBUG
        printf("invalid command %d\n",cmd);
#endif
        return(EINVAL);
    }

    if ( eval == 0 ) {
        *retval = rval;
    }
    return(eval);
}

struct msgget_args {
        key_t   key;
        int     msgflg;
};

static int
msgget(p, uap, retval)
        struct proc *p;
        register struct msgget_args *uap;
        int *retval;
{
    int msqid, eval;
    int key = uap->key;
    int msgflg = uap->msgflg;
    struct ucred *cred = p->p_ucred;
    register struct msqid_ds *msqptr = NULL;

#ifdef MSG_DEBUG
    printf("msgget(0x%x,0%o)\n",key,msgflg);
#endif

    if ( key == IPC_PRIVATE ) {
#ifdef MSG_DEBUG
        printf("private key\n");
#endif
        msqid = msginfo.msgmni;
    } else {
        for ( msqid = 0; msqid < msginfo.msgmni; msqid += 1 ) {
            msqptr = &msqids[msqid];
            if ( msqptr->msg_qbytes != 0 && msqptr->msg_perm.key == key ) {
                break;
            }
        }
        if ( msqid < msginfo.msgmni ) {
#ifdef MSG_DEBUG
            printf("found public key\n");
#endif
            if ( (msgflg & IPC_CREAT) && (msgflg & IPC_EXCL) ) {
#ifdef MSG_DEBUG
                printf("not exclusive\n");
#endif
                return(EEXIST);
            }
            if ( (eval = ipcaccess(&msqptr->msg_perm, msgflg & 0700, cred)) ) {
#ifdef MSG_DEBUG
                printf("requester doesn't have 0%o access\n",msgflg & 0700);
#endif
                return(eval);
            }
        } else {
#ifdef MSG_DEBUG
            printf("didn't find public key\n");
#endif
        }
    }

    if ( msqid == msginfo.msgmni ) {
#ifdef MSG_DEBUG
        printf("need to allocate the msqid_ds\n");
#endif
        if ( key == IPC_PRIVATE || (msgflg & IPC_CREAT) ) {
            for ( msqid = 0; msqid < msginfo.msgmni; msqid += 1 ) {
                /*
                 * Look for an unallocated and unlocked msqid_ds.
                 * msqid_ds's can be locked by msgsnd or msgrcv while they
                 * are copying the message in/out.  We can't re-use the
                 * entry until they release it.
                 */

                msqptr = &msqids[msqid];
                if ( msqptr->msg_qbytes == 0
                && (msqptr->msg_perm.mode & MSG_LOCKED) == 0 ) {
                    break;
                }
            }
            if ( msqid == msginfo.msgmni ) {
#ifdef MSG_DEBUG
                printf("no more msqid_ds's available\n");
#endif
                return(ENOSPC);
            }
#ifdef MSG_DEBUG
            printf("msqid %d is available\n",msqid+1);
#endif
            msqptr->msg_perm.key = key;
            msqptr->msg_perm.cuid = cred->cr_uid;
            msqptr->msg_perm.uid = cred->cr_uid;
            msqptr->msg_perm.cgid = cred->cr_gid;
            msqptr->msg_perm.gid = cred->cr_gid;
            msqptr->msg_perm.mode = (msgflg & 0777);
            msqptr->msg_perm.seq += 1;          /* Make sure that the returned msqid is unique */
            msqptr->msg_first = NULL;
            msqptr->msg_last = NULL;
            msqptr->msg_cbytes = 0;
            msqptr->msg_qnum = 0;
            msqptr->msg_qbytes = msginfo.msgmnb;
            msqptr->msg_lspid = 0;
            msqptr->msg_lrpid = 0;
            msqptr->msg_stime = 0;
            msqptr->msg_rtime = 0;
            msqptr->msg_ctime = time.tv_sec;
        } else {
#ifdef MSG_DEBUG
            printf("didn't find it and wasn't asked to create it\n");
#endif
            return(ENOENT);
        }
    }

    *retval = IXSEQ_TO_IPCID(msqid,msqptr->msg_perm);   /* Construct the unique msqid */
    return(0);
}

struct msgsnd_args {
        int     msqid;
        void    *user_msgp;
        size_t  msgsz;
        int     msgflg;
};

int
msgsnd(p, uap, retval)
        struct proc *p;
        register struct msgsnd_args *uap;
        int *retval;
{
    int msqid = uap->msqid;
    void *user_msgp = uap->user_msgp;
    size_t msgsz = uap->msgsz;
    int msgflg = uap->msgflg;
    int segs_needed, eval;
    struct ucred *cred = p->p_ucred;
    register struct msqid_ds *msqptr;
    register struct msg *msghdr;
    short next;

#ifdef MSG_DEBUG
    printf("call to msgsnd(%d,0x%x,%d,%d)\n",msqid,user_msgp,msgsz,msgflg);
#endif

    msqid = IPCID_TO_IX(msqid);

    if ( msqid < 0 || msqid >= msginfo.msgmni ) {
#ifdef MSG_DEBUG
        printf("msqid (%d) out of range (0<=msqid<%d)\n",msqid,msginfo.msgmni);
#endif
        return(EINVAL);
    }

    msqptr = &msqids[msqid];
    if ( msqptr->msg_qbytes == 0 ) {
#ifdef MSG_DEBUG
        printf("no such message queue id\n");
#endif
        return(EINVAL);
    }
    if ( msqptr->msg_perm.seq != IPCID_TO_SEQ(uap->msqid) ) {
#ifdef MSG_DEBUG
        printf("wrong sequence number\n");
#endif
        return(EINVAL);
    }

    if ( (eval = ipcaccess(&msqptr->msg_perm, IPC_W, cred)) ) {
#ifdef MSG_DEBUG
        printf("requester doesn't have write access\n");
#endif
        return(eval);
    }

    segs_needed = (msgsz + msginfo.msgssz - 1) / msginfo.msgssz;
#ifdef MSG_DEBUG
    printf("msgsz=%d, msgssz=%d, segs_needed=%d\n",msgsz,msginfo.msgssz,segs_needed);
#endif
    while ( 1 ) {
        int need_more_resources = 0;

        /*
         * check msgsz
         * (inside this loop in case msg_qbytes changes while we sleep)
         */

        if ( msgsz > msqptr->msg_qbytes ) {
#ifdef MSG_DEBUG
            printf("msgsz > msqptr->msg_qbytes\n");
#endif
            return(EINVAL);
        }

        if ( msqptr->msg_perm.mode & MSG_LOCKED ) {
#ifdef MSG_DEBUG
            printf("msqid is locked\n");
#endif
            need_more_resources = 1;
        }
        if ( msgsz + msqptr->msg_cbytes > msqptr->msg_qbytes ) {
#ifdef MSG_DEBUG
            printf("msgsz + msg_cbytes > msg_qbytes\n");
#endif
            need_more_resources = 1;
        }
        if ( segs_needed > nfree_msgmaps ) {
#ifdef MSG_DEBUG
            printf("segs_needed > nfree_msgmaps\n");
#endif
            need_more_resources = 1;
        }
        if ( free_msghdrs == NULL ) {
#ifdef MSG_DEBUG
            printf("no more msghdrs\n");
#endif
            need_more_resources = 1;
        }

        if ( need_more_resources ) {

            int we_own_it;

            if ( (msgflg & IPC_NOWAIT) != 0 ) {
#ifdef MSG_DEBUG
                printf("need more resources but caller doesn't want to wait\n");
#endif
                return(EAGAIN);
            }

            if ( (msqptr->msg_perm.mode & MSG_LOCKED) != 0 ) {
#ifdef MSG_DEBUG
                printf("we don't own the msqid_ds\n");
#endif
                we_own_it = 0;
            } else {
                /* Force later arrivals to wait for our request */
#ifdef MSG_DEBUG
                printf("we own the msqid_ds\n");
#endif
                msqptr->msg_perm.mode |= MSG_LOCKED;
                we_own_it = 1;
            }
#ifdef MSG_DEBUG
            printf("goodnight\n");
#endif
            eval = tsleep( (caddr_t)msqptr, (PZERO - 4) | PCATCH, "msg wait", 0 );
#ifdef MSG_DEBUG
            printf("good morning, eval=%d\n",eval);
#endif
            if ( we_own_it ) {
                msqptr->msg_perm.mode &= ~MSG_LOCKED;
            }
            if ( eval != 0 ) {
#ifdef MSG_DEBUG
                printf("msgsnd:  interrupted system call\n");
#endif
                return( EINTR );
            }

            /*
             * Make sure that the msq queue still exists
             */

            if ( msqptr->msg_qbytes == 0 ) {
#ifdef MSG_DEBUG
                printf("msqid deleted\n");
#endif
                /* The SVID says to return EIDRM. */
#ifdef EIDRM
                return(EIDRM);
#else
                /* Unfortunately, BSD doesn't define that code (yet)! */
                return(EINVAL);
#endif
            }

        } else {
#ifdef MSG_DEBUG
            printf("got all the resources that we need\n");
#endif
            break;
        }

    }

    /*
     * We have the resources that we need.
     * Make sure!
     */

    if ( msqptr->msg_perm.mode & MSG_LOCKED ) {
        panic("msg_perm.mode & MSG_LOCKED");            /* bug somewhere */
    }
    if ( segs_needed > nfree_msgmaps ) {
        panic("segs_needed > nfree_msgmaps");           /* bug somewhere */
    }
    if ( msgsz + msqptr->msg_cbytes > msqptr->msg_qbytes ) {
        panic("msgsz + msg_cbytes > msg_qbytes");       /* bug somewhere */
    }
    if ( free_msghdrs == NULL ) {
        panic("no more msghdrs");                       /* bug somewhere */
    }

    /*
     * Re-lock the msqid_ds in case we page-fault when copying in the message
     */

    if ( (msqptr->msg_perm.mode & MSG_LOCKED) != 0 ) {
        panic("msqid_ds is already locked");
    }
    msqptr->msg_perm.mode |= MSG_LOCKED;

    /*
     * Allocate a message header
     */

    msghdr = free_msghdrs;
    free_msghdrs = msghdr->msg_next;
    msghdr->msg_spot = -1;
    msghdr->msg_ts = msgsz;

    /*
     * Allocate space for the message
     */

    while ( segs_needed > 0 ) {
        if ( nfree_msgmaps <= 0 ) {
            panic("not enough msgmaps");
        }
        if ( free_msgmaps == -1 ) {
            panic("nil free_msgmaps");
        }
        next = free_msgmaps;
        if ( next <= -1 ) {
            panic("next too low #1");
        }
        if ( next >= msginfo.msgseg ) {
            panic("next out of range #1");
        }
#ifdef MSG_DEBUG
        printf("allocating segment %d to message\n",next);
#endif
        free_msgmaps = msgmaps[next].next;
        nfree_msgmaps -= 1;
        msgmaps[next].next = msghdr->msg_spot;
        msghdr->msg_spot = next;
        segs_needed -= 1;
    }

    /*
     * Copy in the message type
     */

    if ( (eval = copyin(user_msgp,&msghdr->msg_type,sizeof(msghdr->msg_type))) != 0 ) {
#ifdef MSG_DEBUG
        printf("error %d copying the message type\n",eval);
#endif
        msg_freehdr(msghdr);
        msqptr->msg_perm.mode &= ~MSG_LOCKED;
        wakeup( (caddr_t)msqptr );              /* Somebody might care - we should check! */
        return(eval);
    }
    user_msgp += sizeof(msghdr->msg_type);

    /*
     * Validate the message type
     */

    if ( msghdr->msg_type < 1 ) {
        msg_freehdr(msghdr);
        msqptr->msg_perm.mode &= ~MSG_LOCKED;
        wakeup( (caddr_t)msqptr );              /* Somebody might care - we should check! */
#ifdef MSG_DEBUG
        printf("mtype (%d) < 1\n",msghdr->msg_type);
#endif
        return(EINVAL);
    }

    /*
     * Copy in the message body
     */

    next = msghdr->msg_spot;
    while ( msgsz > 0 ) {
        size_t tlen;
        if ( msgsz > msginfo.msgssz ) {
            tlen = msginfo.msgssz;
        } else {
            tlen = msgsz;
        }
        if ( next <= -1 ) {
            panic("next too low #2");
        }
        if ( next >= msginfo.msgseg ) {
            panic("next out of range #2");
        }
        if ( (eval = copyin(user_msgp, &msgpool[next * msginfo.msgssz], tlen)) != 0 ) {
#ifdef MSG_DEBUG
            printf("error %d copying in message segment\n",eval);
#endif
            msg_freehdr(msghdr);
            msqptr->msg_perm.mode &= ~MSG_LOCKED;
            wakeup( (caddr_t)msqptr );          /* Somebody might care - we should check! */
            return(eval);
        }
        msgsz -= tlen;
        user_msgp += tlen;
        next = msgmaps[next].next;
    }
    if ( next != -1 ) {
        panic("didn't use all the msg segments");
    }

    /*
     * We've got the message.  Unlock the msqid_ds.
     */

    msqptr->msg_perm.mode &= ~MSG_LOCKED;

    /*
     * Make sure that the msqid_ds is still allocated.
     */

    if ( msqptr->msg_qbytes == 0 ) {
        msg_freehdr(msghdr);
        wakeup( (caddr_t)msqptr );              /* Somebody might care - we should check! */
        /* The SVID says to return EIDRM. */
#ifdef EIDRM
        return(EIDRM);
#else
        /* Unfortunately, BSD doesn't define that code (yet)! */
        return(EINVAL);
#endif
    }

    /*
     * Put the message into the queue
     */

    if ( msqptr->msg_first == NULL ) {
        msqptr->msg_first = msghdr;
        msqptr->msg_last = msghdr;
    } else {
        msqptr->msg_last->msg_next = msghdr;
        msqptr->msg_last = msghdr;
    }
    msqptr->msg_last->msg_next = NULL;

    msqptr->msg_cbytes += msghdr->msg_ts;
    msqptr->msg_qnum += 1;
    msqptr->msg_lspid = p->p_pid;
    msqptr->msg_stime = time.tv_sec;

    wakeup( (caddr_t)msqptr );          /* Somebody might care - we should check! */
    *retval = 0;
    return(0);
}

struct msgrcv_args {
        int     msqid;
        void    *msgp;
        size_t  msgsz;
        long    msgtyp;
        int     msgflg;
};

static int
msgrcv(p, uap, retval)
        struct proc *p;
        register struct msgrcv_args *uap;
        int *retval;
{
    int msqid = uap->msqid;
    void *user_msgp = uap->msgp;
    size_t msgsz = uap->msgsz;
    long msgtyp = uap->msgtyp;
    int msgflg = uap->msgflg;
    size_t len;
    struct ucred *cred = p->p_ucred;
    register struct msqid_ds *msqptr;
    register struct msg *msghdr;
    int eval;
    short next;

#ifdef MSG_DEBUG
    printf("call to msgrcv(%d,0x%x,%d,%ld,%d)\n",msqid,user_msgp,msgsz,msgtyp,msgflg);
#endif

    msqid = IPCID_TO_IX(msqid);

    if ( msqid < 0 || msqid >= msginfo.msgmni ) {
#ifdef MSG_DEBUG
        printf("msqid (%d) out of range (0<=msqid<%d)\n",msqid,msginfo.msgmni);
#endif
        return(EINVAL);
    }

    msqptr = &msqids[msqid];
    if ( msqptr->msg_qbytes == 0 ) {
#ifdef MSG_DEBUG
        printf("no such message queue id\n");
#endif
        return(EINVAL);
    }
    if ( msqptr->msg_perm.seq != IPCID_TO_SEQ(uap->msqid) ) {
#ifdef MSG_DEBUG
        printf("wrong sequence number\n");
#endif
        return(EINVAL);
    }

    if ( (eval = ipcaccess(&msqptr->msg_perm, IPC_R, cred)) ) {
#ifdef MSG_DEBUG
        printf("requester doesn't have read access\n");
#endif
        return(eval);
    }

    msghdr = NULL;
    while ( msghdr == NULL ) {

        if ( msgtyp == 0 ) {

            msghdr = msqptr->msg_first;
            if ( msghdr != NULL ) {
                if ( msgsz < msghdr->msg_ts && (msgflg & MSG_NOERROR) == 0 ) {
#ifdef MSG_DEBUG
                    printf("first message on the queue is too big (want %d, got %d)\n",msgsz,msghdr->msg_ts);
#endif
                    return(E2BIG);
                }
                if ( msqptr->msg_first == msqptr->msg_last ) {
                    msqptr->msg_first = NULL;
                    msqptr->msg_last = NULL;
                } else {
                    msqptr->msg_first = msghdr->msg_next;
                    if ( msqptr->msg_first == NULL ) {
                        panic("msg_first/last screwed up #1");
                    }
                }
            }

        } else {
            struct msg *previous;
            struct msg **prev;

            previous = NULL;
            prev = &(msqptr->msg_first);
            while ( (msghdr = *prev) != NULL ) {

                /*
                 * Is this message's type an exact match or is this message's
                 * type less than or equal to the absolute value of a negative msgtyp?
                 * Note that the second half of this test can NEVER be true
                 * if msgtyp is positive since msg_type is always positive!
                 */

                if ( msgtyp == msghdr->msg_type || msghdr->msg_type <= -msgtyp ) {
#ifdef MSG_DEBUG
                    printf("found message type %d, requested %d\n",msghdr->msg_type,msgtyp);
#endif
                    if ( msgsz < msghdr->msg_ts && (msgflg & MSG_NOERROR) == 0 ) {
#ifdef MSG_DEBUG
                        printf("requested message on the queue is too big (want %d, got %d)\n",msgsz,msghdr->msg_ts);
#endif
                        return(E2BIG);
                    }
                    *prev = msghdr->msg_next;
                    if ( msghdr == msqptr->msg_last ) {
                        if ( previous == NULL ) {
                            if ( prev != &msqptr->msg_first ) {
                                panic("msg_first/last screwed up #2");
                            }
                            msqptr->msg_first = NULL;
                            msqptr->msg_last = NULL;
                        } else {
                            if ( prev == &msqptr->msg_first ) {
                                panic("msg_first/last screwed up #3");
                            }
                            msqptr->msg_last = previous;
                        }
                    }
                    break;
                }
                previous = msghdr;
                prev = &(msghdr->msg_next);
            }

        }

        /*
         * We've either extracted the msghdr for the appropriate message
         * or there isn't one.
         * If there is one then bail out of this loop.
         */

        if ( msghdr != NULL ) {
            break;
        }

        /*
         * Hmph!  No message found.  Does the user want to wait?
         */

        if ( (msgflg & IPC_NOWAIT) != 0 ) {
#ifdef MSG_DEBUG
            printf("no appropriate message found (msgtyp=%d)\n",msgtyp);
#endif
            /* The SVID says to return ENOMSG. */
#ifdef ENOMSG
            return(ENOMSG);
#else
            /* Unfortunately, BSD doesn't define that code (yet)! */
            return(EAGAIN);
#endif
        }

        /*
         * Wait for something to happen
         */

#ifdef MSG_DEBUG
        printf("msgrcv:  goodnight\n");
#endif
        eval = tsleep( (caddr_t)msqptr, (PZERO - 4) | PCATCH, "msg wait", 0 );
#ifdef MSG_DEBUG
        printf("msgrcv:  good morning (eval=%d)\n",eval);
#endif

        if ( eval != 0 ) {
#ifdef MSG_DEBUG
            printf("msgsnd:  interrupted system call\n");
#endif
            return( EINTR );
        }

        /*
         * Make sure that the msq queue still exists
         */

        if ( msqptr->msg_qbytes == 0
        || msqptr->msg_perm.seq != IPCID_TO_SEQ(uap->msqid) ) {
#ifdef MSG_DEBUG
            printf("msqid deleted\n");
#endif
            /* The SVID says to return EIDRM. */
#ifdef EIDRM
            return(EIDRM);
#else
            /* Unfortunately, BSD doesn't define that code (yet)! */
            return(EINVAL);
#endif
        }

    }

    /*
     * Return the message to the user.
     *
     * First, do the bookkeeping (before we risk being interrupted).
     */

    msqptr->msg_cbytes -= msghdr->msg_ts;
    msqptr->msg_qnum -= 1;
    msqptr->msg_lrpid = p->p_pid;
    msqptr->msg_rtime = time.tv_sec;

    /*
     * Make msgsz the actual amount that we'll be returning.
     * Note that this effectively truncates the message if it is too long
     * (since msgsz is never increased).
     */

#ifdef MSG_DEBUG
    printf("found a message, msgsz=%d, msg_ts=%d\n",msgsz,msghdr->msg_ts);
#endif
    if ( msgsz > msghdr->msg_ts ) {
        msgsz = msghdr->msg_ts;
    }

    /*
     * Return the type to the user.
     */

    eval = copyout((caddr_t)&(msghdr->msg_type), user_msgp, sizeof(msghdr->msg_type));
    if ( eval != 0 ) {
#ifdef MSG_DEBUG
        printf("error (%d) copying out message type\n",eval);
#endif
        msg_freehdr(msghdr);
        wakeup( (caddr_t)msqptr );              /* Somebody might care - we should check! */
        return(eval);
    }
    user_msgp += sizeof(msghdr->msg_type);

    /*
     * Return the segments to the user
     */

    next = msghdr->msg_spot;
    for ( len = 0; len < msgsz; len += msginfo.msgssz ) {
        size_t tlen;
        if ( msgsz > msginfo.msgssz ) {
            tlen = msginfo.msgssz;
        } else {
            tlen = msgsz;
        }
        if ( next <= -1 ) {
            panic("next too low #3");
        }
        if ( next >= msginfo.msgseg ) {
            panic("next out of range #3");
        }
        eval = copyout((caddr_t)&msgpool[next * msginfo.msgssz], user_msgp, tlen);
        if ( eval != 0 ) {
#ifdef MSG_DEBUG
            printf("error (%d) copying out message segment\n",eval);
#endif
            msg_freehdr(msghdr);
            wakeup( (caddr_t)msqptr );          /* Somebody might care - we should check! */
            return(eval);
        }
        user_msgp += tlen;
        next = msgmaps[next].next;
    }

    /*
     * Done, return the actual number of bytes copied out.
     */

    msg_freehdr(msghdr);
    wakeup( (caddr_t)msqptr );          /* Somebody might care - we should check! */
    *retval = msgsz;
    return(0);
}
#endif