Segmentation Fault - pthread mutex locking issue

Question

I am creating a simple kdb+ shared library which generates values on a seperate thread and executes a callback function when data is ready. The application is writing data to a file descriptor in the new thread and reading from this in the main event loop. The application appears to be segfaulting when trying to lock and unlock the mutex.

If I introduce a small sleep into the loop, the segfault seems to disappear. This would suggest that the pthread_mutex_lock call is not blocking the thread until a lock is obtained as I would have expected.

#include <k.h>
#include <pthread.h>
#include <time.h>

#define PIPE_CAPACITY 65536

static int fd;
static pthread_t thread;
static pthread_mutex_t lock;

K callback(int d)
{
    K data;

    // Aquire mutex lock and read from fd
    pthread_mutex_lock(&lock);
    read(d, &data, PIPE_CAPACITY);
    pthread_mutex_unlock(&lock);

    // kdb+ callback
    k(0, (char *)"callback", r1(data), (K)0);

    return (K)0;
}

void* loop()
{
    while (1) {
        struct timespec ts;
        struct tm *time;

        // Get seconds and nanoseconds since epoch
        clock_gettime(CLOCK_REALTIME, &ts);

        // Adjust for kdb+
        time = gmtime(&ts.tv_sec);
        time->tm_sec = 0;
        time->tm_min = 0;
        time->tm_hour = 0;
        ts.tv_sec -= mktime(time); // Subtract seconds between epoch and midnight

        // Create kdb+ timestamp
        K data = ktj(-KN, ts.tv_sec * 1000000000 + ts.tv_nsec);

        // Aquire mutex lock and write to fd
        pthread_mutex_lock(&lock);
        write(fd, &data, sizeof(K));
        pthread_mutex_unlock(&lock);
    }
}

K init()
{
    // Initialize mutex
    pthread_mutex_init(&lock, NULL);

    // Create file descriptor
    fd = eventfd(0, 0);

    // Register callback
    sd1(fd, callback);

    // Launch thread
    pthread_create(&thread, NULL, loop, NULL);
}

Not related, but you really should avoid one letter typenames such as `K`. — Jabberwocky, Jan 31 '17 at 09:42
Can you provide the "main" function? Does initialization take effect before thread creation? — Emisilve86, Jan 31 '17 at 10:07
@MichaelWalz This is down to the API I have to work with, not my personal choice - contained in [k.h](http://code.kx.com/wsvn/code/kx/kdb%2B/c/c/k.h) — Anthony Graham, Jan 31 '17 at 10:19
@Emisilve86 This is a shared library, there is no main function. The entry point will be `init`. — Anthony Graham, Jan 31 '17 at 10:21
@AnthonyGraham Fyi, `gmtime` is not thread-safe. You should use `gmtime_r` if offered on your platform (which is highly likely, since you're threading). Or `gmtime_s` if using a C11 compliant toolchain. — WhozCraig, Jan 31 '17 at 10:22
@AnthonyGraham i had this problem too and i was very very very confused about it...... finally i find your best question..... your very good....................... thankfulllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll — BattleTested_закалённый в бою, Apr 16 '18 at 14:11

Alexander Belopolsky · Accepted Answer · 2017-02-03T16:44:25.010

Recall that K is a pointer type defined in k.h as:

typedef struct k0{..}*K;

This means you are sending a pointer to an object created in the "loop" thread to the callback executed in the main thread. This does not work because kdb+ uses a separate memory pull for each thread. I would recommend passing a copy of the data instead.

Another problem is at the line

read(d, &data, PIPE_CAPACITY);

You are reading 65536 bytes, but pass the address of an 8-byte variable as the destination. The reason you don't get a segfault when you introduce the delay is that in this case the loop does not get a chance to write more than 8 bytes between the reads.

Finally, I am not sure you can use the file descriptor returned by eventfd as a read-write buffer. I would recommend using the good old pipe() call.

The following modification of your code works for me:

#include <k.h>
#include <pthread.h>
#include <time.h>
#include <unistd.h>

static int fd[2];
static pthread_t thread;
static pthread_mutex_t lock;

K callback(int d)
{
    K data = ktj(-KN, 0);

    // Aquire mutex lock and read from fd
    pthread_mutex_lock(&lock);
    read(d, (void *)&data->j, sizeof(data->j));
    pthread_mutex_unlock(&lock);

    // kdb+ callback
    k(0, (char *)"callback", data, (K)0);

    return (K)0;
}

void* loop()
{
    while (1) {
        struct timespec ts;
        struct tm *time;

        // Get seconds and nanoseconds since epoch
        clock_gettime(CLOCK_REALTIME, &ts);

        // Adjust for kdb+
        time = gmtime(&ts.tv_sec);
        time->tm_sec = 0;
        time->tm_min = 0;
        time->tm_hour = 0;
        ts.tv_sec -= mktime(time); // Subtract seconds between epoch and midnight

        // Create kdb+ timestamp
        J data = (J)ts.tv_sec * 1000000000 + ts.tv_nsec;

        // Aquire mutex lock and write to fd
        pthread_mutex_lock(&lock);
        write(fd[1], &data, sizeof(data));
        pthread_mutex_unlock(&lock);
    }
}

K1(init)
{
    // Initialize mutex
    pthread_mutex_init(&lock, NULL);

    // Create file descriptor
    pipe(fd);

    // Register callback
    sd1(fd[0], callback);

    // Launch thread
    pthread_create(&thread, NULL, loop, NULL);

    R ktj(0, 0);
}

To test, copy the code above in x.c, compile

$ gcc -Wall -shared -fPIC -I $(pwd) -DKXVER=3 x.c -o x.so

and run the following q code:

callback:0N!
init:`:./x 2:(`init;1)
init[]

`read(d, (void *)&data->j, sizeof(data->j));` yessssssssssssssssss. thats it you help me seriously with this statement thank you sooooooooooooooooooooooo much..... i had this `segmentation fault` with every mutex.lock and i had a read function from socket................... — BattleTested_закалённый в бою, Apr 16 '18 at 14:13

Segmentation Fault - pthread mutex locking issue

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