How to tell gcc to stop using built-in functions?

Question

I am using my own modified glibc. I saw in the compiled code that compiler was not using many standard library functions from my glibc when I linked with it. Then I put -fno-builtin flag. Things got better and I could see that many functions which were not taken from glibc were now taken from there, such as malloc.

However, still for many functions, such as mmap, the compiler is using some built-in-code. Now how can I ask the compiler to please exclusively use the code from glibc rather than using its built-in-functions?

On my x86-64 function, if I do objdump of the compiled glibc, following is the generated mmap function. I can't find equivalent code in the glibc source.

0000000000000000 <__mmap>:
   0:   49 89 ca                mov    %rcx,%r10
   3:   b8 09 00 00 00          mov    $0x9,%eax
   8:   0f 05                   syscall 
   a:   48 3d 01 f0 ff ff       cmp    $0xfffffffffffff001,%rax
  10:   0f 83 00 00 00 00       jae    16 <__mmap+0x16>
  16:   c3                      retq

There is no built-in mmap() in gcc. You may want to use -O0 and disable inlining. — blaze, May 21 '12 at 14:32
does http://stackoverflow.com/questions/10684381/is-mmap-a-built-in-function help? — andrew cooke, May 21 '12 at 14:35
It calls into kernel using `syscall` instruction. It's not a built-in function. — Maxim Egorushkin, May 21 '12 at 15:16
Maxim: I know its a system call, but there must be a wrapper in user space somewhere. No? — MetallicPriest, May 21 '12 at 15:23
"there must be a wrapper" --- this *is* the wrapper. Load the syscall number to %eax, invoke the syscall instruction. Doesn't have to be anything more than that. — n. m. could be an AI, May 21 '12 at 15:34

Chris Dodd · Answer 1 · 2012-05-21T16:51:59.567

The wrapper you disassemble above comes from the INLINE_SYSCALL macro in sysdeps/unix/sysv/linux/x86_64/sysdep.h. This macro is the 'magic glue' used to turn a normal function call into a system call.

As part of the build process of glibc, for every defined system call foo that is not in a list of special exceptions for that architecture, it generates a function __foo that contains just a single INLINE_SYSCALL macro invocation. mmap is not in the exception list for x86_64 (in sysdeps/unix/sysv/linux/x86_64/syscalls.list), so it gets the generic treatment.

score 2 · Answer 2 · answered Dec 01 '21 at 01:25

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How about passing "-ffreestanding" to gcc as explained in this answer?

answered Dec 01 '21 at 01:25

ZeZNiQ

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I'm trying `-ffreestanding` and it still seems to use a lot of builtins. For instance, `__mulsi3` or other calls on really low-end systems. – Charles Lohr Jan 16 '23 at 21:09

score 0 · Answer 3 · answered May 21 '12 at 15:08

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This is what a call to mmap compiles to on my system:

movl    $1048837, 20(%esp)
movl    $1048836, 16(%esp)
movl    $1048835, 12(%esp)
movl    $1048834, 8(%esp)
movl    $1048833, 4(%esp)
movl    $1048832, (%esp)
call    mmap

I have used some constants like 0x100100 (1048832) as mmap arguments in order to be able to find corresponding instructions in the assembly easier.

This looks like pretty ordinary call to a pretty ordinary library function, and not a builtin.

The actual code for mmap is OS- and architecture-dependent. For instance, for i386 Linux it is in sysdeps/unix/sysv/linux/i386/mmap.S (yes it's written in assembly).

answered May 21 '12 at 15:08

n. m. could be an AI

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I am using x86-64 system. Can't find any correspoding code for that. Edited the question to show the generated mmap. – MetallicPriest May 21 '12 at 15:09
What you have displayed does look like the code for `mmap`. What your `sysdeps/unix/sysv/linux/i386/mmap.S` says? – n. m. could be an AI May 21 '12 at 15:28
No its not that code. And ofcourse i386 is not used for x86-64 code. – MetallicPriest May 21 '12 at 15:32
I suggest you recompile the `sysdeps/unix/sysv/linux` directory and see for yourself which source file is responsible for `mmap.o`. – n. m. could be an AI May 21 '12 at 16:17

How to tell gcc to stop using built-in functions?

3 Answers3