POSIX sem_wait（）SIGABRT

Question

I am working on a school project where we have to make a multithreaded web server. I am having a problem where when I call sem_wait on my semaphore (which should be initialized to 0 but already seems to be sem_post() ed to 1). I get a SIGABRT.

I am attaching my code below, and I put a comment on the line that is causing my problem. I've spent a few hours with the debugger with little luck.

#include <iostream>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <netinet/in.h>
#include <netdb.h>
#include <string>
#include <string.h>
#include <iostream>
#include <fcntl.h>
#include <errno.h>
#include <pthread.h>
#include <vector>
#include <semaphore.h>
#include <stdio.h>
#include <cstdlib>
#include <strings.h>

#define PORTNUM 7000
#define NUM_OF_THREADS 5
#define oops(msg) { perror(msg); exit(1);}
#define FCFS 0
#define SJF 1;

void bindAndListen();
void acceptConnection(int socket_file_descriptor);
void* dispatchJobs(void*);
void* replyToClient(void* pos);

//holds ids of worker threads
pthread_t threads[NUM_OF_THREADS];

//mutex variable for sleep_signal_cond
pthread_mutex_t sleep_signal_mutex[NUM_OF_THREADS];
//holds the condition variables to signal when the thread should be unblocked
pthread_cond_t sleep_signal_cond[NUM_OF_THREADS];

//mutex for accessing sleeping_thread_list
pthread_mutex_t sleeping_threads_mutex = PTHREAD_MUTEX_INITIALIZER;
//list of which threads are sleeping so they can be signaled and given a job
std::vector<bool> *sleeping_threads_list = new std::vector<bool>();

//number of threads ready for jobs
sem_t available_threads;
sem_t waiting_jobs;


//holds requests waiting to be given to one of the threads for execution
//request implemented as int[3] with int[0]== socket_descriptor int[1]== file_size int[2]== file_descriptor of requested file
//if file_size == 0 then HEAD request
std::vector<std::vector<int> >* jobs = new std::vector<std::vector<int> >();

pthread_mutex_t jobs_mutex = PTHREAD_MUTEX_INITIALIZER;


int main (int argc, char * const argv[]) {
    //holds id for thread responsible for removing jobs from ready queue and assigning them to worker thread
    pthread_t dispatcher_thread;

    //initializes semaphores
    if(sem_init(&available_threads, 0, NUM_OF_THREADS) != 0){
        oops("Error Initializing Semaphore");
    }

    if(sem_init(&waiting_jobs, 0, 0) !=0){
        oops("Error Initializing Semaphore");
    }

    //initializes condition variables and guarding mutexes
    for(int i=0; i<NUM_OF_THREADS; i++){
        pthread_cond_init(&sleep_signal_cond[i], NULL);
        pthread_mutex_init(&sleep_signal_mutex[i], NULL);
    }

    if(pthread_create(&dispatcher_thread, NULL, dispatchJobs, (void*)NULL) !=0){
        oops("Error Creating Distributer Thread");
    }

    for (int i=0; i<NUM_OF_THREADS; i++) {
        pthread_mutex_lock(&sleeping_threads_mutex);
        printf("before");
        sleeping_threads_list->push_back(true);
        printf("after");
        pthread_mutex_unlock(&sleeping_threads_mutex);
    }

    printf("here");
    for (int i=0; i<NUM_OF_THREADS; i++) {
        //creates threads and stores ID in threads
        if(pthread_create(&threads[i], NULL, replyToClient, (void*)i) !=0){
            oops("Error Creating Thread");
        }
    }

    /*
    if(sem_init(&available_threads, 0, NUM_OF_THREADS) !=0){
        oops("Error Initializing Semaphore");
    }

    if(sem_init(&waiting_jobs, 0, 0) !=0){                 //this is the semaphore thats used in the sem_wait
        oops("Error Initializing Semaphore");
    }*/

    bindAndListen();
}


//binds to socket and listens for connections
//being done by main thead
void bindAndListen(){
    struct sockaddr_in saddr;
    struct hostent *hp;
    char hostname[256];
    int sock_id, sock_fd;

    gethostname(hostname, 256);
    hp = gethostbyname(hostname);
    bzero(&saddr, sizeof(saddr));

    //errno = 0;

    bcopy(hp->h_addr, &saddr.sin_addr, hp->h_length);

    saddr.sin_family = AF_INET;
    saddr.sin_port = htons(PORTNUM);
    saddr.sin_addr.s_addr = INADDR_ANY;

    sock_id = socket(AF_INET, SOCK_STREAM, 0);

    if(sock_id == -1){
        oops("socket");
        printf("socket");
    }

    if(bind(sock_id, (const sockaddr*)&saddr, sizeof(saddr)) ==0){

        if(listen(sock_id, 5) ==-1){
            oops("listen");
        }

        //each time a new connection is accepted, get file info and push to ready queue
        while(1){
            int addrlen = sizeof(saddr);
            sock_fd = accept(sock_id, (sockaddr*)&saddr, (socklen_t*)&addrlen);
            if (sock_fd > 0) {
                acceptConnection(sock_fd);
            }else {
                oops("Error Accepting Connection");
            }
        }
    }else{
        oops("there was an error binding to socket");
    }
}// end of bindAndListen()


//accepts connection and gets file info of requested file
//being done by main thread
void acceptConnection(int sock_fd){
    printf("**Server: A new client connected!");

    //only using loop so on error we can break out on error
    while(true){
        //used to hold input from client
        char* inputBuff = new char[BUFSIZ];
        int slen = read(sock_fd, inputBuff, BUFSIZ);

        //will sit on space between HEAD/GET and path
        int pos1 = 0;
        //will sit on space between path and HTTP version
        int pos2 = 0;

        //need duplicate ptr so we can manipulate one in the loop
        char* buffPtr = inputBuff;
        //parses client input  breaks up query by spaces
        for(int i=0; i<slen; i++){
            if(*buffPtr == ' '){
                if (pos1 == 0) {
                    pos1 = i;
                }else {
                    pos2 = i;
                    break;
                }
            }
            buffPtr++;
        }

        if((pos1 - pos2) >=0){
            std::string str = "Invalid Query";
            write(sock_fd, str.c_str(), strlen(str.c_str()));
            break;
        }

        printf("slen length %d\n", slen);

        std::string* method = new std::string(inputBuff, pos1);

        printf("method length %lu\n",method->length());

        //increment the ptr for buff to the starting pos of the path
        inputBuff += ++pos1;

        printf("pos2 - pos1 %d\n", (pos2 - pos1));

        printf("pos1 = %d  pos2 = %d\n", pos1, pos2);

        std::string* path = new std::string(inputBuff, (pos2 - pos1));

        printf("path length %lu\n", path->length());

        printf("part1 %s\n", method->c_str());

        printf("part2 %s\n", path->c_str());

        //opens file requested by client
        int fd = open(path->c_str(), O_RDONLY);
        if(fd < 0){
            std::string* error = new std::string("Error Opening File");
            *error += *path + std::string(strerror(errno), strlen(strerror(errno)));
            write(sock_fd, error->c_str(), strlen(error->c_str()));
            break;
        }

        int file_size;
        if(method->compare("GET") == 0){
            //gets file info and puts the resulting struct in file_info
            struct stat file_info;
            if(fstat(fd, &file_info) !=0){
                oops("Error getting file info");
            }
            file_size = file_info.st_size;
        }else if(method->compare("HEAD")){
            file_size = 0;
        }else{
            write(sock_fd, "Invalid Query", strlen("Invalid Query"));
            break;
        }

        //job to be pushed to ready queue
        std::vector<int> job;
        job.push_back(sock_fd);
        job.push_back(file_size);
        job.push_back(fd);

        //check mutex guarding the ready queue
        pthread_mutex_lock(&jobs_mutex);
        //push job to back of ready queue
        jobs->push_back(job);
        //unlock mutex guarding the ready queue
        pthread_mutex_unlock(&jobs_mutex);

        //increment number of jobs in ready queue
        sem_post(&waiting_jobs);

    } //end of while(true)
      // we only end up here if there was an error
    fflush(stdout);
    close(sock_fd);
}// end of acceptConnection()


//routine run by dispather thread
void *dispatchJobs(void*){
    while(true){
        //wait for a thread to be available to execute a job
        sem_wait(&available_threads);
        //wait for a job to be waiting in the ready queue
        sem_wait(&waiting_jobs);                    //this is the line thats crashing
        //aquire lock to check which threads are waiting
        pthread_mutex_lock(&sleeping_threads_mutex);
        //go through list of threads to see which is waiting
        for(int i=0; i<sleeping_threads_list->size(); i++){
            if(sleeping_threads_list->at(i)){
                //unlocks lock for access to list of waiting threads
                pthread_mutex_unlock(&sleeping_threads_mutex);
                //allows us access to the list of condition variables to signal the thread to resume execution
                pthread_mutex_lock(&sleep_signal_mutex[i]);
                pthread_cond_signal(&sleep_signal_cond[i]);
                pthread_mutex_unlock(&sleep_signal_mutex[i]);
            }
        }

    }//end of while(true)
}//end of dispatchJobs()


//sends file or metadata to client
//run by worker thread
//pos is position of condition variable that it waits to be signaled in the sleep_signal_cond[] array
void* replyToClient(void* pos){
    int position = (long)pos;
    while(true){
        //waits for dispather thread to signal it
        pthread_mutex_lock(&sleep_signal_mutex[position]);
        pthread_cond_wait(&sleep_signal_cond[position], &sleep_signal_mutex[position]);
        pthread_mutex_unlock(&sleep_signal_mutex[position]);


        //lock mutex to get job to be executed
        pthread_mutex_lock(&jobs_mutex);
        std::vector<int> job = jobs->front();
        //removes job from front of vector
        jobs->erase(jobs->begin());
        //releases mutex
        pthread_mutex_unlock(&jobs_mutex);

        //socket file descriptor, used for writing to socket
        int sock_fd =job[0];
        int file_size = job[1];
        //file descriptor for requested job
        int fd = job[2];

        //holds output to be written to socket
        char* outputBuffer = new char[BUFSIZ];

        //GET request, send file
        if(file_size !=0){
            int readResult = 0;
            while ((readResult = read(fd, outputBuffer, BUFSIZ)) > 0) {
                if(write(sock_fd, outputBuffer, readResult) != readResult){
                    printf("We may have a write error");
                }
            }
            if(readResult < 0){
                oops("Error Reading File");
            }
            if(readResult == 0){
                printf("finished sending file");
            }
        }else{    // HEAD request

        }
        //increment number of available threads
        sem_post(&available_threads);
    }
}// end of replyToClient()

Answer 1

Check again the whole logic of the code - it is possible to reach here:

pthread_mutex_lock(&jobs_mutex);
std::vector<int> job = jobs->front();
//removes job from front of vector
jobs->erase(jobs->begin());
//releases mutex
pthread_mutex_unlock(&jobs_mutex);

with jobs->size () == 0 , in which case front() and erase() invoke undefined behavior, which may well result in the effects you observe.

Check whether your program still crashes after the following change:

//lock mutex to get job to be executed
pthread_mutex_lock(&jobs_mutex);
if (jobs->size () == 0)
  {
    pthread_mutex_unlock (&jobs_mutex);
    continue;
  }
std::vector<int> job = jobs->front();
//removes job from front of vector
jobs->erase(jobs->begin());
//releases mutex
pthread_mutex_unlock(&jobs_mutex);

Answer 2

I haven't used POSIX semaphores, but I believe this is what is happening. I'm only familiar with Linux kernel semaphores, and you don't mention your system. The init function's 3rd parameter probably sets the count variable. You set it to 0 (= busy but no other processes waiting). The wait function probably invokes down(), which begins by decreasing the count variable by 1: to -1, which means the semaphore you mean to use is locked now. There is nothing in your program to ever unlock it I believe (from browsing your code - it's pretty long), so you are in trouble. Try setting it to 1 in init. This might be all that is needed.