[SOLVED] Raspberry PI4 Segmentation Fault

Issue

I have a short program that is causing Segmentation Fault on RPi4 after run several times (e.g.: 10 times in a loop). I am using Raspbian GNU/Linux 10 (buster) and default gcc compiler (sudo apt install build-essential)

gcc --version
gcc (Raspbian 8.3.0-6+rpi1) 8.3.0

Do you think this is a gcc compiler problem? Maybe I am missing some special settings for RPi4. I am using this to build:

gcc threads.c -o threads -l pthread

The output is sometimes (not always) something like this:

...
in thread_dummy, loop: 003
Segmentation fault

The code is here:

#include <stdio.h>  /* for puts() */
#include <unistd.h> /* for sleep() */
#include <stdlib.h> /* for EXIT_SUCCESS */
#include <pthread.h>

#define PTR_SIZE (0xFFFFFF)
#define PTR_CNT (10)

void* thread_dummy(void* param)
{
    void* ptr = malloc(PTR_SIZE);

    //fprintf(stderr, "thread num: %03i, stack: %08X, heap: %08X - %08X\n", (int)param, (unsigned int)&param, (unsigned int)ptr, (unsigned int)((unsigned char*)ptr + PTR_SIZE));
    fprintf(stderr, "in thread_dummy, loop: %03i\n", (int)param);

    sleep(1);

    free(ptr);

    pthread_detach(pthread_self());
    return NULL;
}

int main(void)
{
    void* ptrs[PTR_CNT];
    pthread_t threads[PTR_CNT];
    for(int i=0; i<PTR_CNT; ++i)
    {
        ptrs[i] = malloc(PTR_SIZE);
        //fprintf(stderr, "main   num: %03i, stack: %08X, heap: %08X - %08X\n", i, (unsigned int)&ptrs, (unsigned int)ptrs[i], (unsigned int)((unsigned char*)ptrs[i] + PTR_SIZE));
        fprintf(stderr, "in main, loop: %03i\n", i);
    }

    fprintf(stderr, "-----------------------------------------------------------\n");

    for(int i=0; i<PTR_CNT; ++i)
        pthread_create(&threads[i], 0, thread_dummy, (void*)i);

    for(int i=0; i<PTR_CNT; ++i)
        pthread_join(threads[i], NULL);

    for(int i=0; i<PTR_CNT; ++i)
        free(ptrs[i]);

    return EXIT_SUCCESS;
}

UPDATE: I also tested it with new gcc, but the problem remains...

gcc -v

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/arm-linux-gnueabihf/11.1.0/lto-wrapper
Target: arm-linux-gnueabihf
Configured with: ../configure --enable-languages=c,c++,fortran --with-cpu=cortex-a72 --with-fpu=neon-fp-armv8 --with-float=hard --build=arm-linux-gnueabihf --host=arm-linux-gnueabihf --target=arm-linux-gnueabihf
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.1.0 (GCC)

Solution

pthread_create is like malloc, and pthread_detach or pthread_join is like free. You are basically doing something like "double free" - you detach a thread and join it at the same time. Either detach or join the thread.

You could remove pthread_join from main. But you should logically remove pthread_detach(...) from inside the thread, which is actually useless because the thread terminates right after anyway.

Answered By - KamilCuk