Identifier Removed (EIDRM) error when sending message with a IPC queue

c, ipc, message-queue

Solution

The problem lies in the improper handling of the precedence and order of evaluation in C, whenever you evaluate the msgget return value against values lower than 0:

if(msgqid = msgget(CLAVE,PERMISOS|IPC_CREAT)<0){
    fprintf(stderr,"Problema al crear la cola de mensages IPC\n");
    exit(0);
}

The precedence of the assignment operator = falls below the comparison one <, which implies that you were assigning the result of `msgget() < 0` to msgqid and not checking if `(msgqid = msgget()) < 0`.

You should then surround the assignment with parentheses:

if((msgqid = msgget(CLAVE, PERMISOS | IPC_CREAT)) < 0){
    fprintf(stderr,"Problema al crear la cola de mensages IPC\n");
    exit(0);
}

Problem

I'm using an IPC queue to do process synchronization. I always get the EIDRM eror when sending an receiving messages from an IPC queue, but i can see that the queue is there with ipcs. I've been for 2 hours searching but i can't see the error. the following code is a stripped down version that gives me the same error. ``` #define CLAVE 53543961 #define TAM_BUFFER 1024 #define PERMISOS 0777 #define DEBUG int Cola_Mensages; int msgqid; typedef struct { long mtype; char mtext[TAM_BUFFER]; }msgbuf; int main (int argc, char *argv[]){ msgbuf msg_ipc; int num_cli,i, i_aux; if(argc == 2){ num_cli = atoi(argv[1]); }else{ num_cli = 1; } //Creating the queue if(msgqid = msgget(CLAVE,PERMISOS|IPC_CREAT)<0){ fprintf(stderr,"Problema al crear la cola de mensages IPC\n"); exit(0); } if(msgqid < 0){ fprintf(stderr,"Problema al crear la cola de mensages IPC\n"); exit(0); } for(i = 0;i<num_cli;i++){ //here i get the error i_aux=msgrcv(msgqid,&msg_ipc,TAM_BUFFER,1,0); if(i_aux == -1) fprintf(stderr,"Error enviando msg ipc %s \n",strerror(errno)); } msg_ipc.mtype = 2; strcpy(msg_ipc.mtext,"COMIENZO"); printf("Enviando msg\n"); for(i = 0;i<num_cli;i++){ printf("Enviado msg %d\n",i); //here i also get the same error if (msgsnd(msgqid,(char *) &msg_ipc,strlen(msg_ipc.mtext),0)!=0) { fprintf(stderr,"Ocurrio el error %s en msgsnd\n",strerror(errno)); exit(4); } } if (msgctl(msgqid,IPC_RMID,(struct msqid_ds *) NULL)<0) { fprintf(stderr,"Error al borrar la cola de mensajes de clave %d\n",CLAVE); exit(4); } return 0; ``` }

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