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Multiple external function declarations causes compiler segfault #529

snocl opened this Issue Oct 9, 2017 · 2 comments


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commented Oct 9, 2017

Trying to reference an extern function declared with two different types causes the compiler to segfault, even when the types should be “compatible”.

This is a problem if multiple parts of a code base (say, two different statically linked libraries) try to use the same external function, but ends up using different argument types, e.g., through different instances of extern struct.



// main.zig

const A = extern struct {
    field: c_int,

extern "c" fn f(?*A) void;

pub fn main() void {
    _ = @import("other.zig").f;
    _ = f;
// other.zig

pub const A = extern struct {
    field: c_int,

pub extern "c" fn f(?*A) void;

Note that main.A and other.A are distinct types; by replacing both definitions with const A = c_int; the example compiles.

Build command:

$ zig build-exe main.zig
$ zig build-obj main.zig

Back trace:

* thread #1, queue = '', stop reason = EXC_BAD_ACCESS (code=1, address=0x6661656c2d86)
  * frame #0: 0x00000001021b89c1 zig`llvm::AttributeList::addAttributes(llvm::LLVMContext&, unsigned int, llvm::AttrBuilder const&) const + 91
    frame #1: 0x00000001021b8c01 zig`llvm::AttributeList::addAttribute(llvm::LLVMContext&, unsigned int, llvm::Attribute) const + 93
    frame #2: 0x000000010220e177 zig`llvm::Function::addAttribute(unsigned int, llvm::Attribute) + 41
    frame #3: 0x000000010002da9a zig`addLLVMAttr(val=0x000000010722bed8, attr_index=4294967295, attr_name="nounwind") at codegen.cpp:282
    frame #4: 0x000000010002ef74 zig`addLLVMFnAttr(fn_val=0x000000010722bed8, attr_name="nounwind") at codegen.cpp:303
    frame #5: 0x000000010002d8f5 zig`fn_llvm_value(g=0x0000000105017000, fn_table_entry=0x000000010669fa90) at codegen.cpp:462
    frame #6: 0x000000010002628f zig`do_code_gen(g=0x0000000105017000) at codegen.cpp:4222
    frame #7: 0x0000000100025771 zig`codegen_build(g=0x0000000105017000) at codegen.cpp:5562
    frame #8: 0x000000010008f723 zig`main(argc=3, argv=0x00007fff5fbff7c0) at main.cpp:788
    frame #9: 0x00007fffac045235 libdyld.dylib`start + 1

It looks like the problem occurs because the extern functions instead of being referenced through their local declarations, get referenced through a global table in:

The invalid cast on line 365 then seems to be what causes the crash later.

@andrewrk andrewrk added the bug label Oct 9, 2017

@andrewrk andrewrk added this to the 0.1.0 milestone Oct 9, 2017


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commented Oct 9, 2017

Strange, that bitcast is supposed to make exactly this situation work correctly. I'll look into this.

@andrewrk andrewrk modified the milestones: 0.1.0, 0.2.0 Oct 16, 2017

andrewrk added a commit that referenced this issue Nov 10, 2017


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commented Nov 10, 2017

I think this issue is fixed but I'm going to leave it open until I add a unit test for it.

@andrewrk andrewrk modified the milestones: 0.2.0, 0.3.0 Mar 15, 2018

@andrewrk andrewrk modified the milestones: 0.3.0, 0.4.0 Sep 3, 2018

@andrewrk andrewrk closed this in 557983e Feb 27, 2019

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