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When I see the assembly code of a C app, like this:

emacs hello.c clang -S -O hello.c -o hello.s cat hello.s

Function names are prefixed with an underscore (e.g. callq _printf). Why is this done and what advantages does it have?

Example:

hello.c

#include <stdio.h> #include <stdlib.h> #include <string.h> int main() { char *myString = malloc(strlen("Hello, World!") + 1); memcpy(myString, "Hello, World!", strlen("Hello, World!") + 1); printf("%s", myString); return 0; }

hello.s

_main: ; Here Leh_func_begin0: pushq %rbp Ltmp0: movq %rsp, %rbp Ltmp1: movl $14, %edi callq _malloc ; Here movabsq $6278066737626506568, %rcx movq %rcx, (%rax) movw $33, 12(%rax) movl $1684828783, 8(%rax) leaq L_.str1(%rip), %rdi movq %rax, %rsi xorb %al, %al callq _printf ; Here xorl %eax, %eax popq %rbp ret Leh_func_end0:
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    I'd be interested in knowing which OS this is. I thought this silly practice had been long abandoned. Maybe Mac? Commented May 6, 2011 at 10:57
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    OSX and perhaps some other BSD derivatives are the only unix-like OS's I know of that might still do this. Linux definitely abandoned it about 12-15 years ago. Commented May 6, 2011 at 13:13
  • @R..: do you mean a.out did that? Commented Oct 23, 2011 at 11:54
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    possible duplicate of Why do C compilers prepend underscores to external names? Commented Jan 31, 2013 at 20:10
  • It's been 13 years since the question was answered, but macOS still does this. Commented Sep 25, 2024 at 16:08

3 Answers 3

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From Linkers and Loaders:

At the time that UNIX was rewritten in C in about 1974, its authors already had extensive assember language libraries, and it was easier to mangle the names of new C and C-compatible code than to go back and fix all the existing code. Now, 20 years later, the assembler code has all been rewritten five times, and UNIX C compilers, particularly ones that create COFF and ELF object files, no longer prepend the underscore.

Prepending an underscore in the assembly results of C compilation is just a name-mangling convention that arose as a workaround. It stuck around for (as far as I know) no particular reason, and has now made its way into Clang.

Outside of assembly, the C standard library often has implementation-defined functions prefixed with an underscore to convey notions of magicalness and don't touch this to the ordinary programmers that stumble across them.

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7 Comments

As for leading underscores in C source code: That's a name spacing issue, cf. section 7.1.3 of the C standard. To put it more bluntly: If your C code defines an identifier starting with two underscores or with an underscore followed by a capital letter, it is broken. If it defines identifiers with file scope or larger that start with an underscore, it is broken. Those are reserved for the compiler and standard library implementation.
Not broken, if you're writing C code for, say, your standard library implementation, though. In other words, you really need to know what youre' doing, and if you can't explain why what you're doing is OK, you're doing it wrong. But just in case you are writing the library, the compiler does not prevent you from breaking those rules, just in case anyone was wondering why.
@Lars: Unfortunately a lot of bloated egos in semi-system-level but not standard library code, like legacy X libs, sound libs, graphics libs, etc. think they're entitled to use underscores as if they were part of the standard library... And then some people blindly import code from various implementations of the standard library without understanding it, and keep the underscores... These usages are all definitely broken.
I am a fresher and not that knowledgeable about C. Can someone please explain what is a broken identifier? I am seeing these underscores everywhere and cannot understand why are they used.
@ChristopherCreutzig to which version of the standard are you refering? ANSI C88 doesn't have a 7.1.3. flash-gordon.me.uk/ansi.c.txt nevermind. I found the C99 ISO standard 9899
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A lot of compilers used to translate C to assembly language, and then run an assembler on that to generate an object file. It's a lot easier than generating binary code directly. (AFAIK GCC still does this. But it also has its own assembler.) During this translation, function names become labels in the assembly source. If you have a function called (for example) ret, though, some assemblers can get confused and think it's an instruction rather than a label. (YASM does, for example, mostly because labels can appear pretty much anywhere and don't require colons. You have to prepend a $ if you want a label called ret.)

Prepending a character (like, say, an underscore) to the C-generated labels was a whole lot easier than writing one's own C-friendly assembler or worrying about labels clashing with assembly instructions/directives.

These days, assemblers and compilers have evolved a bit, and most people work at the C level or higher anyway. So the original need to mangle names in C is largely gone.

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At first glance the operating system is a Unix/Unix-like running on a PC. According to me, there is nothing much surprising to find _printf in the generated assembly language. C printf is a function which performs an I/O. So it is the responsibility of the kernel + driver to perform the requested I/O.

The machine instructions path taken on any Unix/Unix-like OS is the following:

printf (C code)-> _printf (libc) -> trap -> kernel + driver work -> return from trap -> return from _printf (libc) -> printf completion and return -> next machine instruction in C code

In the case of this assembly code extract, it looks like the C printf is inlined by the compilateur which caused the _printf entry point to be visible in the assembly code.

To make sure the C printf does not get decorated with a prefix (an underscore in this case), best if searching in all C headers for a _printf with a command like:

find /usr/include -name *.h -exec grep _printf {} \; -print

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