readlinkat() is not available in older systems

[Chun Tian (binghe)]

Greetings,

Currently Mac OS X 10.6 is the only MacOS environment that I can
successfully build Axiom (may2017) (with MacTeX 2016, so that all
potential LaTeX-related building issues were gone). This is done with
GCL 2.6.12.

But in recent GCL 2.6.13pre, a Unix API readlinkat() is called in
o/unixfsys.c, which seems to export this function to Lisp level:

DEFUN_NEW("READLINKAT",object,fSreadlinkat,SI,2,2,NONE,OI,OO,OO,OO,(fixnum
d,object s),"") {
  ssize_t l,z1;

  check_type_string(&s);
  /* l=s->st.st_hasfillp ? s->st.st_fillp : s->st.st_dim; */
  z1=length(s);
  massert(z1<sizeof(FN1));
  memcpy(FN1,s->st.st_self,z1);
  FN1[z1]=0;
#ifndef __MINGW32__
  massert((l=readlinkat(d ? dirfd((DIR *)d) :
AT_FDCWD,FN1,FN2,sizeof(FN2)))>=0 && l<sizeof(FN2));
#else
  l=0;
#endif
  FN2[l]=0;
  RETURN1(make_simple_string(FN2));

}

According to Linux man page [1], readlinkat() is only available when
_POSIX_C_SOURCE >= 200809L, which unfortunately is not the case for Mac
OS X 10.6 (although released in 2009).  By reading this man page, it
sees that, whenever the first argument is the special value AT_FDCWD,
the entire call is equivalent to the more available readlink()
(_XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L || _BSD_SOURCE) with
the rest of the arguments. Thus I suppose the following minimal patch
should behave correctly for most of the cases:

--- gcl-2.6.13pre.orig/o/unixfsys.c     2022-11-09 01:43:51.000000000 +0100
+++ gcl-2.6.13pre/o/unixfsys.c  2022-11-12 18:38:11.000000000 +0100
@@ -268,8 +268,12 @@
   memcpy(FN1,s->st.st_self,z1);
   FN1[z1]=0;
 #ifndef __MINGW32__
+#if _POSIX_C_SOURCE >= 200809L
   massert((l=readlinkat(d ? dirfd((DIR *)d) :
AT_FDCWD,FN1,FN2,sizeof(FN2)))>=0 && l<sizeof(FN2));
 #else
+  massert((l=readlink(FN1,FN2,sizeof(FN2)))>=0 && l<sizeof(FN2));
+#endif
+#else
   l=0;
 #endif
   FN2[l]=0;

(If the first argument were not AT_FDCWD, which means the current
directory, then for how to emulate the behavior of readlinkat() with
readlink(), I have no idea at all...)

So this was how I successfully built the latest GCL 2.6.13pre on Mac OS
X 10.6, to debug the remaining Axiom building issues, before moving to
more recent macOS releases.

Regards,

Chun Tian

[1] https://man7.org/linux/man-pages/man2/readlink.2.html

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