os.fork fails on FreeBSD with tcc

[lcheylus]

Describe the bug

During debug of issue #24683, I found an issue with os.fork on FreeBSD with tcc compiler => child process crashes after fork.

No issue with os.fork on FreeBSD with clang/gcc compiler.

Reproduction Steps

• Build V on FreeBSD/amd64 from Git sources
• Code to test os.fork:

import os

fn main() {
        pid := os.fork()

        if pid > 0 {
                /*parent process*/
                println('Parent [Process id: ${os.getpid()}]')
        } else if pid == 0 {
                 /*child process*/
                println('Child [Process id: ${os.getpid()}]')
                println('Child - parent process id: ${os.getppid()}')
        } else {
                /*fork creation faile*/
                println('fork creation failed!!!')
        }
}

• Run code to test os.fork => no child process

$  ./v run /tmp/test_fork.v
Parent [Process id: 98428]

Expected Behavior

os.fork works as expected.

Test with clang on FreeBSD:

$ ./v -cc clang run /tmp/test_fork.v
Parent [Process id: 97029]
Child [Process id: 97082]
Child - parent process id: 97029

Current Behavior

See above

Possible Solution

No response

Additional Information/Context

With the same code to test os.fork, no issue on OpenBSD/amd64 and Linux/amd64.

V version

V 0.4.10 f250636

Environment details (OS name and version, etc.)

V full version	V 0.4.10 0c495d0.f250636
OS	freebsd, 14.3-RELEASE, FreeBSD 14.3-RELEASE releng/14.3-n271432-8c9ce319fef7 GENERIC
Processor	4 cpus, 64bit, little endian
Memory	0.33GB/1.96GB
V executable	/home/fox/dev/vlang.git/v
V last modified time	2025-06-10 16:19:06
V home dir	OK, value: /home/fox/dev/vlang.git
VMODULES	OK, value: /home/fox/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/fox/dev/vlang.git
Git version	git version 2.49.0
V git status	weekly.2025.24-4-gf2506368-dirty
.git/config present	true
cc version	FreeBSD clang version 19.1.7 (https://github.com/llvm/llvm-project.git llvmorg-19.1.7-0-gcd708029e0b2)
gcc version	gcc (FreeBSD Ports Collection) 13.3.0
clang version	FreeBSD clang version 19.1.7 (https://github.com/llvm/llvm-project.git llvmorg-19.1.7-0-gcd708029e0b2)
tcc version	tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 FreeBSD)
tcc git status	thirdparty-freebsd-amd64 2f0f0f13
emcc version	N/A
glibc version	N/A

Note

You can use the 👍 reaction to increase the issue's priority for developers.

Please note that only the 👍 reaction to the issue itself counts as a vote.
Other reactions and those to comments will not be taken into account.

[huly-for-github]

Connected to Huly®: V_0.6-23064

[lcheylus]

No issue with C fork when building a C example with tcc on FreeBSD/amd64.

• C code to test fork:

#include <stdio.h>
#include <unistd.h>

int main()
{
    int id;

    id = fork();
    if (id > 0) {
        /*parent process*/
        printf("Parent [Process id: %d]\n", getpid());
    }
    else if (id == 0) {
        /*child process*/
        printf("Child [Process id: %d]\n", getpid());
        printf("Child - parent process id: %d\n", getppid());
    }
    else {
        /*fork creation faile*/
        printf("fork creation failed!!!\n");
    }

    return 0;
}

• Build C code with tcc on FreeBSD:

$ ~/dev/vlang.git/thirdparty/tcc/tcc.exe -L/home/fox/dev/vlang.git/thirdparty/tcc/lib/tcc/ -o test_fork-c test_fork.c

• Run C example to test fork:

$ ./test_fork-c
Parent [Process id: 36564]
Child [Process id: 36655]
Child - parent process id: 36564

[lcheylus]

Debug V code with lldb

• Compile V code to test os.fork with tcc (see code above): ./v -cc tcc /tmp/test_fork.v
• Run test_fork with lldb:

$ lldb /tmp/test_fork
(lldb) target create "/tmp/test_fork"
Current executable set to '/tmp/test_fork' (x86_64).
(lldb) settings set target.process.follow-fork-mode child
(lldb) run
Process 68280 launched: '/tmp/test_fork' (x86_64)
Parent [Process id: 68280]
Process 68416 stopped
* thread #2, name = 'test_fork', stop reason = signal SIGSEGV: address not mapped to object (fault address: 0xa6a30f)
    frame #0: 0x00000000006276a4 test_fork`fork_child_proc + 68
test_fork`fork_child_proc:
->  0x6276a4 <+68>: addq   0x442c64(%rip), %r14
    0x6276ab <+75>: xorl   %r15d, %r15d
    0x6276ae <+78>: jmp    0x6276c6       ; <+102>
    0x6276b0 <+80>: xorl   %edi, %edi

=> child process crashes with SIGSEGV after fork.

[lcheylus]

No issue when running my V code to test os.fork using dynamic_boehm.

$ ./v -cc tcc -d dynamic_boehm run /tmp/test_fork.v
Parent [Process id: 87674]
Child [Process id: 87716]
Child - parent process id: 87674

[lcheylus]

@felipensp Please could you add labels for FreeBSD, tcc and "Boehm Garbage Collection (GC)"?

[lcheylus]

I have a solution to fix this issue using a local build of GC library for FreeBSD/amd64 (same build script than the one used in https://github.com/vlang/tccbin/tree/thirdparty-linux-amd64/lib => static library libgc.a for Linux/amd64).

• Static library libgc.a built for FreeBSD/amd64 from bdwgc Git sources:

$ ls -l thirdparty/tcc/lib/libgc.a
-rw-r--r--  1 fox fox 239278 Jun 13 18:18 thirdparty/tcc/lib/libgc.a
$ file thirdparty/tcc/lib/libgc.a
thirdparty/tcc/lib/libgc.a: current ar archive

• Modification of vlib/builtin/builtin_d_gcboehm.c.v to use libgc.a when building on FreeBSD with tcc compiler.

My test code to test os.fork works as expected :)

$ ./v -cc tcc -showcc run /tmp/test_fork.v
> C compiler cmd: '/home/fox/dev/vlang.git/thirdparty/tcc/tcc.exe' '@/tmp/v_1000/test_fork.01JXPR5VNWYS60EZ3090CDDQSN.tmp.c.rsp'
> C compiler response file "/tmp/v_1000/test_fork.01JXPR5VNWYS60EZ3090CDDQSN.tmp.c.rsp":
  -D__RUNETYPE_INTERNAL -o "/tmp/test_fork" -D GC_THREADS=1 -D GC_BUILTIN_ATOMIC=1 -D BUS_PAGE_FAULT=T_PAGEFLT -I "/usr/local/include" "/tmp/v_1000/test_fork.01JXPR5VNWYS60EZ3090CDDQSN.tmp.c" -std=c99 -D_DEFAULT_SOURCE -bt25 "/home/fox/dev/vlang.git/thirdparty/tcc/lib/libgc.a" -lgc-threaded -lpthread -lexecinfo -lelf  
Parent [Process id: 61057]
Child [Process id: 61185]
Child - parent process id: 61057

[lcheylus]

Plan to fix this issue:

• build a dedicated version of static library libgc.a for FreeBSD/amd64 and commit it in vlang/tccbin repository, branch thirdparty-freebsd-amd64.
• add thirdparty/build_scripts/thirdparty-freebsd-amd64_bdwgc.sh script in vlang repository
• modify vlib/builtin/builtin_d_gcboehm.c.v to use local static library libgc.a on FreeBSD with tcc compiler.

[spytheman]

What is the modification to vlib/builtin/builtin_d_gcboehm.c.v ?