Wsrtjtyk

I am using uuntu 18.04.1LTS and studying IPC using C. I'm testing Unix i/o using LPC this time, and there's a problem when more than one client connects to the server at the same time.
(when only one client connected, there is no problem.)

sprintf(s1,"./%sA",t);

sprintf(s2, "./%sB", t);



if (MakeDirectory(s1, 0755) == -1) {

    return -1;

}



if (MakeDirectory(s2, 0755) == -1) {

    return -1;

}







for (i = 0; i < 5; i++)

{

    memset(dirName, 0, SIZE);

    sprintf(dirName, "%s/%d",s1,i);

    usleep(300000);

    if (MakeDirectory(dirName, 0755) == -1) {

        return -1;

    }

}

This code is client's main function. There is no problem at the top, but after running the repeat statement once (when i = 1), MakeDirectory() returns -1 with an error.
(t refers to the pid of the forked process converted into a string.)

int MakeDirectory(char* path, int mode) {

    memset(&pRequest, 0x00, LPC_REQUEST_SIZE);

    memset(&pResponse, 0x00, LPC_RESPONSE_SIZE);



    pRequest.pid = getpid();

    pRequest.service = LPC_MAKE_DIRECTORY;

    pRequest.numArg = 2;

    pRequest.lpcArgs[0].argSize = strlen(path);

    strcpy(pRequest.lpcArgs[0].argData, path);

    pRequest.lpcArgs[1].argSize = mode;



    msgsnd(rqmsqid, &pRequest, LPC_REQUEST_SIZE, 0);

    msgrcv(rpmsqid, &pResponse, LPC_RESPONSE_SIZE, getpid(), 0);



    int res = pResponse.responseSize;



    return res;

}

This is client's MakeDirectory, and

int MakeDirectory(LpcRequest* pRequest) {

    memset(&pResponse, 0x00, LPC_RESPONSE_SIZE);



    char *path = pRequest->lpcArgs[0].argData;

    int mode = pRequest->lpcArgs[1].argSize;



    int res = mkdir(path, mode);



    pResponse.errorno = 0;

    pResponse.pid = pRequest->pid;



    printf("%ldn", pResponse.pid);



    pResponse.responseSize = res;

    msgsnd(rpmsqid, &pResponse, LPC_RESPONSE_SIZE, 0);



    return res;

}

This is a function of the server that runs after checking the pRequest.service when the MakeDirectory function is enabled on the client.

Again, there's nothing wrong with having one client, and if there's more than one. I checked with printf(), but the server passes 0 and the client receives -1. I don't know why this happens.

There's too much missing from your code to know definitively what's happening. I'm placing my bet on either using unallocated memory, or not recognizing a syscall error.

I'm using LTS 16, and there's no definition on my system for LpcRequest or LPC_REQUEST_SIZE, etc. You don't show how they're defined, so we don't know for example if pRequest.lpcArgs[1] exists.

You're also not checking the return code for msgsnd and msgrcv, a sure recipe for endless hours of entertaining debugging.

I suggest you edit your question to include working code, and a shell script that produces the mysterious result. Then someone will be able, if willing, to debug it and explain where you went wrong.

My other suggestion in this area is pretty standard: W. Richard Stevens's books on TCP/IP, specifically Unix Network Programming. If you're studying this stuff, you'll absolutely be glad to have read it.