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I am working on performing iptables update through a custom c program using libiptc. The requirement is to invoke iptc APIs from a separate thread every 2 seconds.

I have written a simple C program to try out invoking of iptc APIs from a separate thread. The c program is pasted below. 

/*** thread_iptc.c ***/

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

void* thread_func(void* unused)
{
        struct iptc_handle *handle = NULL;
        char *table = "filter";

        while (1)
        {
                printf("\nthread_func(): While loop.\n");
                handle = iptc_init(table);
                if (handle) {
                        printf("thread_func(): handle is not NULL.\n");
                        iptc_free(handle);
                }
                else
                        printf("thread_func(): handle is NULL.\n");

                sleep(2);
        }

        return NULL;
}


int main()
{
        struct iptc_handle *handle = NULL;
        char *table = "filter";
        pthread_t thread_id;

        handle = iptc_init(table);
        if (handle) {
                printf("main(): handle is not NULL.\n");
                iptc_free(handle);
        }
        else
                printf("main(): handle is NULL.\n");


        pthread_create(&thread_id, NULL, &thread_func, NULL);
        pthread_join(thread_id, NULL);

        return 0;
}

The problem, I am facing is that call to both iptc_init() and iptc_free() works well when called from main function. However, call to iptc_free() fails with "Segmentation Fault" when called from thread_func().

Program Output:

# ./test 
main(): handle is not NULL.

thread_func(): While loop.
thread_func(): handle is not NULL.
Segmentation fault

Compilation:

# gcc -o test thread_iptc.c -lpthread -lext4 -lext6 -lip4tc -lip6tc -liptc -lxtables -ldl

GDB Backtrace

#0  0x00007ffff79be303 in iptc_free () from /lib64/libip4tc.so.0
#1  0x00000000004007f3 in thread_func ()
#2  0x00007ffff7bc77e1 in start_thread () from /lib64/libpthread.so.0
#3  0x00007ffff6efb8ed in clone () from /lib64/libc.so.6

Am I missing something during compilation or while invoking a new thread?

SKD
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Kamal K Garg
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    What happens if you don't call `iptc_init` in main? Or call it with a different table name? Does it make the thread call work, or work crash on the second time (or just crash the same way)? – ugoren Jan 17 '12 at 07:43
  • Tried both ("Removing iptc_init call in main", "using different table name"). Result is same. Crashing the same way. – Kamal K Garg Jan 17 '12 at 08:56
  • Perhaps the iptc library has some per-thread element. Have you ever tried to call iptc_init inside the second thread instead of in the main one? – Giuseppe Guerrini Jan 17 '12 at 11:00
  • Another aproach to get insight, would be to use a debug version of the iptc library (or build one if none is available). – alk Jan 17 '12 at 11:08
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    I added #include and #include to your code and compiled without warnings. On my x86_64 debian system it works perfectly. I compiled with those switches: gcc -Wall -pthread -liptc -o thread_iptc thread_iptc.c – dAm2K Mar 17 '12 at 12:52

0 Answers0