[libc] Add limits.h

[alfredfo]

Summary

This header contains implementation specific constants.

The compiler already provides its own limits.h with numerical limits
conforming to freestanding ISO C. But it is missing extensions like
POSIX, and does for example not include <linux/limits.h> which is
expected on a Linux system, therefore, an LLVM libc implementation of
limits.h is needed for hosted (STDC_HOSTED) environments.

[michaelrj]

The limits.h header is provided by the compiler, so us providing it will just cause problems

[alfredfo]

In D156961#4558158, @michaelrj wrote:

The limits.h header is provided by the compiler, so us providing it will just cause problems

Hmm, are you sure? I thought Clang headers and libc headers were both supposed to be used. Also, Clang headers only have standard C things, no POSIX/GNU extensions... etc.
I know SSIZE_MAX isn't there, also PATH_MAX is not available so it does not include <linux/limits.h> like musl and glibc.

[alfredfo]

Yeah prob should be provided by both :)

/* System headers include a number of constants from POSIX in <limits.h>.
   Include it if we're hosted. */
#if __STDC_HOSTED__ && __has_include_next(<limits.h>)
#include_next <limits.h>
#endif

https://github.com/llvm/llvm-project/blob/eac78fdf68f58e113b2cf18a14baccb8f5ebcf50/clang/lib/Headers/limits.h#L18

[michaelrj]

Ah! In that case, this looks good.

[michaelrj]

libc/include/llvm-libc-macros/limits-macros.h

9

as an added thing, can we make it so that we only define these if they aren't already defined?

[sivachandra]

libc/include/llvm-libc-macros/limits-macros.h

14

Considering LONG_MAX is already defined in the freestanding limits.h, may be all you need is:

#define SSIZE_MAX __LONG_MAX__

[alfredfo]

remove PATH_MAX definition

[alfredfo]

libc/include/llvm-libc-macros/limits-macros.h

9

Sure, will fix tomorrow

14

Yes, done

[alfredfo]

only define SSIZE_MAX if not already defined

[alfredfo]

update msg

[alfredfo]

arc broken moment

[alfredfo]

update message

libc/include/llvm-libc-macros/limits-macros.h

 #ifndef __LLVM_LIBC_MACROS_LIMITS_MACROS_H
 #define __LLVM_LIBC_MACROS_LIMITS_MACROS_H
 
 #ifdef __linux__
 #include <linux/limits.h>
 #endif
 
-#if __LP64__
-#define __LONG_MAX 0x7fffffffffffffffL
-#else
-#define __LONG_MAX 0x7fffffffL
-#endif // __LP64__
-
-#define LONG_MAX __LONG_MAX
+#ifndef SSIZE_MAX
+#define SSIZE_MAX __LONG_MAX__
+#endif
 
 #endif // __LLVM_LIBC_MACROS_LIMITS_MACROS_H

libc/spec/posix.td

[
           [ArgSpec<IntType>, ArgSpec<ConstCharPtr>, ArgSpec<IntType>, ArgSpec<VarArgType>]
       >,
     ]
   >;
 
   HeaderSpec Limits = HeaderSpec<
     "limits.h",
     [
+        Macro<"SSIZE_MAX">,
     ],
     [], // Types
     [], // Enumerations
     [] // Functions
   >;
 
   HeaderSpec SysMMan = HeaderSpec<
       "sys/mman.h",