What is the Illegal Instruction 4 error and why does -mmacosx-version-min 10 x fix it

Question

I get Illegal Instruction  4 errors with binaries compiled with GCC 4 7 2 under Mac OS X 10 8 2   Mountain Lion    when those binaries are run under Mac OS X 10 7 x   Lion   and earlier versions  The binaries work properly under Mac OS X 10 8 x   I added -mmacosx-version-min 10 5 to my compile flags and this seems to help resolve the issue for 10 5 x  10 6 x and 10 7 x clients  whatever that issue is    Which gets to my question s     What is the Illegal Instruction  4 error  Why does -mmacosx-version-min 10 x fix this specific error on 10 x and greater clients    I d like to apply this fix to my makefiles  but would like to know what it is doing before I pull the trigger   Will I have larger binaries  Do I still have 64-bit binaries  Are there gotchas with this approach I should know about  Unintended side-effects  Etc

User · Answer

I got this error when attempting to build with Xcode 10. It appears to be a bug in the Swift compiler. Building with Whole Module Optimization on, resolves the issue: https://forums.swift.org/t/illegal-instruction-4-when-trying-to-compile-project/16118

This is not an ideal solution, I will continue to use Xcode 9.4.1 until this issue is resolved.

User · Answer

I m consciously writing this answer to an old question with this in mind  because the other answers didn t help me   I got the Illegal Instruction  4 while running the binary on the same system I had compiled it on  so -mmacosx-version-min didn t help   I was using gcc in Code Blocks 16 on Mac OS X 10 11   However  turning off all of Code Blocks  compiler flags for optimization worked  So look at all the flags Code Blocks set  right-click on the Project -   Build Properties   and turn off all the flags you are sure you don t need  especially -s and the -Oflags for optimization  That did it for me

User · Answer

In my case  I got this while overloading   ostream  amp  operator  lt  lt   ostream  amp out  const MyClass  amp obj    and forgot to return out  In other systems this just generates a warning  but on macos it also generated an error  although it seems to print correctly    The error was resolved by adding the correct return value  In my case  adding the -mmacosx-version-min flag had no effect

User · Answer

I recently got this error  I had compiled the binary with -O3  Google told me that this means  illegal opcode   which seemed fishy to me  I then turned off all optimizations and reran  Now the error transformed to a segfault  Hence by setting -g and running valgrind I tracked the source down and fixed it  Reenabling all optimizations showed no further appearances of illegal instruction 4   Apparently  optimizing wrong code can yield weird results

User · Answer

The  illegal instruction  message is simply telling you that your binaries contain instructions the version of the OS that you are attempting to run them under does not understand  I can t give you the precise meaning of 4 but I expect that is internal to Apple   Otherwise take a look at these     they are a little old  but probably tell you what you need to know  How does 64 bit code work on OS-X 10 5  what does macosx-version-min imply

User · Answer

From the Apple Developer Forum  account required        The compiler and linker are capable of using features and performing optimizations that do not work on older OS versions  -mmacosx-version-min tells the tools what OS versions you need to work with  so the tools can disable optimizations that won t run on those OS versions  If you need to run on older OS versions then you must use this flag        The downside to -mmacosx-version-min is that the app s performance may be worse on newer OS versions then it could have been if it did not need to be backwards-compatible  In most cases the differences are small

User · Answer

I found my issue was an improper  if  leaf   NULL        where it should have been  if  leaf    NULL        Check those compiler warnings

[macos] What is the "Illegal Instruction: 4" error and why does "-mmacosx-version-min=10.x" fix it?

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