Creating a C Library: Difference between revisions

Then you can simply build your C library source files using:

 

x86_64-myos-gcc -c strfoo.c -o strfoo.o

x86_64-myos-as x86_64/crt0.s -o x86_64/crt0.o

x86_64-myos-ar rcs libc.a strfoo.o x86_64/crt0.o

 

Note that unlike the kernel, you don't need to add extra special options that disable standard include directories and libraries. After all, this is the C library meant for user-space. You don't need to add -I options to the compiler if you already have installed your libc and kernel headers into your sysroot. Otherwise, simply add the appropriate -I options to the above commands. It may be useful to use the same libc for both user-space and the kernel, in that case don't make the mistake of thinking that user-space-libc and kernel-libcs are the same thing. First of all many user-space things don't make sense (or even work) in the kernel. Secondly, a kernel-usable libc must be built differently than the user-space libc because it is <em>freestanding</em>. A kernel libc must have all the special options that a kernel binary is passed. For instance, don't forget to add -mno-red-zone on x86_64 to both your kernel and libc, or interrupts may corrupt the stack.

:''See also: [[Calling Global Constructors]]''

 

 

=== crt0.o ===

C library headers conventionally use include guards to prevent multiple declarations. It may be wise to use the same format as GNU's libc as some programs (and GCC fixincludes) may erroneously rely on these macros to detect whether a given header has been included. GNU libc's headers usually follow this simple scheme:

 

#ifndef _STDIO_H

#define _STDIO_H 1

 

#endif

 

== Repeated Declarations ==

'''For GCC versions < 9''', telling GCC that your headers understand C++ is done by having the following in your OS-specific gcc/gcc/config/myos.h:

 

/* Don't assume anything about the header files. */

#undef NO_IMPLICIT_EXTERN_C

#define NO_IMPLICIT_EXTERN_C 1

 

'''For GCC versions >= 9''', [https://patchwork.ozlabs.org/patch/934478/ this patch] changed the behaviour; they have made <tt>NO_IMPLICIT_EXTERN_C</tt> a so-called "poisoned identifier", so compiling your OS-specific toolchain will fail. If you read the linked patch above, you will realise that they decided to invert the behaviour, such that system headers are assumed to understand C++ by default. So, in theory, you do not need those two lines above at all, and it should Just Work (tm).

Adding support for C++ in your C header files (such as stdio.h) is very simple. Previously GCC automatically added extern "C" around all headers if compiling C++, but now we'll need to do it ourselves. The key feature is that we don't need to do this on C++-only headers, meaning that C++ headers will now actually work, instead of GCC assuming they have C-linkage. For instance, you can change your stdio.h to be of this form:

 

#ifndef _STDIO_H

#define _STDIO_H 1

 

#endif

 

[[Category:C]]

[[Category:Tutorials]]