Adding leading underscores to assembly symbols with GCC on Win32?

by

One option, though dangerous, is to convince GCC to omit the ABI-required leading underscore.

  • -fleading-underscore

    This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.

    Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.

Another, safer option, is to explicitly tell GCC the name to use.

5.39 Controlling Names Used in Assembler Code

You can specify the name to be used in the assembler code for a C function or variable by writing the asm (or __asm__) keyword after the declarator as follows:

     int foo asm ("myfoo") = 2;

This specifies that the name to be used for the variable foo in the assembler code should be “myfoo' rather than the usual \``_foo‘.

On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore.

It does not make sense to use this feature with a non-static local variable since such variables do not have assembler names. If you are trying to put the variable in a particular register, see Explicit Reg Vars. GCC presently accepts such code with a warning, but will probably be changed to issue an error, rather than a warning, in the future.

You cannot use asm in this way in a function definition; but you can get the same effect by writing a declaration for the function before its definition and putting asm there, like this:

 extern func () asm ("FUNC");

 func (x, y)
      int x, y;
 /* ... */

It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GCC does not as yet have the ability to store static variables in registers. Perhaps that will be added.

In your case,

extern int bar(int x) asm("bar");

should tell GCC that “bar uses asm name “bar`’, even though it’s a ccall function”.

More Related Contents:

  1. Assembly code fsqrt and fmul instructions
  2. How can I indicate that the memory *pointed* to by an inline ASM argument may be used?
  3. What is the difference between ‘asm’, ‘__asm’ and ‘__asm__’?
  4. Using GCC to produce readable assembly?
  5. Can I use Intel syntax of x86 assembly with GCC?
  6. Why does GCC allocate more space than necessary on the stack, beyond what’s needed for alignment?
  7. Stack allocation, padding, and alignment
  8. When to use earlyclobber constraint in extended GCC inline assembly?
  9. How does GCC implement variable-length arrays?
  10. What does it mean to align the stack?
  11. Syscall implementation of exit()
  12. Looping over arrays with inline assembly
  13. Embedding binary blobs using gcc mingw
  14. CreateProcess: No such file or directory
  15. Why does GCC pad functions with NOPs?
  16. Why is the maximum size of an array “too large”?
  17. Reading a register value into a C variable [duplicate]
  18. Yet Another MinGW “gcc: error: CreateProcess: No such file or directory”
  19. Why does gcc use movl instead of push to pass function args?
  20. x86_64 : is stack frame pointer almost useless?
  21. execute binary machine code from C
  22. calling assembly function from c
  23. Why do compilers insist on using a callee-saved register here?
  24. Building a shared library using gcc on Linux and MinGW on Windows
  25. Why is the compiler throwing this warning: “missing initializer”? Isn’t the structure initialized?
  26. Why is gcc allowed to speculatively load from a struct?
  27. How to tell GCC to generate 16-bit code for real mode
  28. How to get the size of a C function from inside a C program or with inline assembly?
  29. What is the effect of second argument in _builtin_prefetch()?
  30. How do I compile the asm generated by GCC?

Leave a Comment