glibc: add backwards compatibility for some symbols

- `fcntl` was renamed to `fcntl64` in glibc 2.28 (see ziglang#9485)
- `res_{,n}{search,query,querydomain}` became "their own" symbols since
  glibc 2.34: they were prefixed with `__` before.

This PR makes it possible to use `fcntl` with glibc 2.27 or older and
the `res_*` functions with glibc 2.33 or older.

These patches will become redundant with universal-headers and can be
dropped. But we have to do with what we have now.

Closes ziglang#9485

Related: 39083c3

lib/libc/glibc/io/fcntl.h

@@ -167,6 +167,10 @@ typedef __pid_t pid_t;
 					   effective IDs, not real IDs.  */
 #endif
 
+
+/* fcntl was a simple symbol until glibc 2.27 inclusive. glibc 2.28 onwards
+ * re-defines it to fcntl64 (via #define) if _FILE_OFFSET_BITS == 64. */
+#if (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 28) || __GLIBC__ > 2
 /* Do the file control operation described by CMD on FD.
    The remaining arguments are interpreted depending on CMD.
 
@@ -197,6 +201,9 @@ extern int __fcntl_time64 (int __fd, int __request, ...) __THROW;
 #  define fcntl __fcntl_time64
 # endif
 #endif
+#else /* glibc 2.27 or lower */
+extern int fcntl (int __fd, int __cmd, ...);
+#endif
 
 /* Open FILE and return a new file descriptor for it, or -1 on error.
    OFLAG determines the type of access used.  If O_CREAT or O_TMPFILE is set

lib/libc/include/generic-glibc/fcntl.h

@@ -167,6 +167,11 @@ typedef __pid_t pid_t;
 					   effective IDs, not real IDs.  */
 #endif
 
+
+/* fcntl was a simple symbol until glibc 2.27 inclusive.
+ * glibc 2.28 onwards converted it to a macro when compiled with
+ * USE_LARGEFILE64. */
+#if (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 28) || __GLIBC__ > 2
 /* Do the file control operation described by CMD on FD.
    The remaining arguments are interpreted depending on CMD.
 
@@ -197,6 +202,9 @@ extern int __fcntl_time64 (int __fd, int __request, ...) __THROW;
 #  define fcntl __fcntl_time64
 # endif
 #endif
+#else /* glibc 2.27 or lower */
+extern int fcntl (int __fd, int __cmd, ...);
+#endif
 
 /* Open FILE and return a new file descriptor for it, or -1 on error.
    OFLAG determines the type of access used.  If O_CREAT or O_TMPFILE is set

lib/libc/include/generic-glibc/resolv.h

@@ -169,6 +169,28 @@ __END_DECLS
 #define res_init		__res_init
 #define res_isourserver		__res_isourserver
 
+/* In glibc 2.33 and earlier res_search, res_nsearch, res_query, res_nquery,
+ * res_querydomain, res_nquerydomain were #define'd to __res_search,
+ * __res_nsearch, etc. glibc 2.34 onwards removes the macros and exposes the
+ * symbols directly. New glibc exposes compat symbols with underscores for
+ * backwards compatibility. Applications linked to glibc 2.34+ are expected
+ * to use the non-underscored symbols.
+ *
+ * It will be enough to bring the macros back when compiling against the older
+ * glibc versions.
+ *
+ * See glibc commit ea9878ec271c791880fcbbe519d70c42f8113750.
+ */
+#if (__GLIBC__ == 2 && __GLIBC_MINOR__ < 34)
+#define res_search        __res_search
+#define res_nsearch       __res_nsearch
+#define res_query         __res_query
+#define res_nquery        __res_nquery
+#define res_querydomain   __res_querydomain
+#define res_nquerydomain  __res_nquerydomain
+#endif
+/* end glibc compat hacks */
+
 #ifdef _LIBC
 # define __RESOLV_DEPRECATED
 # define __RESOLV_DEPRECATED_MSG(msg)

test/link.zig

@@ -20,6 +20,10 @@ pub const cases = [_]Case{
         .build_root = "test/link/interdependent_static_c_libs",
         .import = @import("link/interdependent_static_c_libs/build.zig"),
     },
+    .{
+        .build_root = "test/link/glibc_compat",
+        .import = @import("link/glibc_compat/build.zig"),
+    },
 
     // WASM Cases
     .{

test/link/glibc_compat/build.zig

@@ -0,0 +1,25 @@
+const std = @import("std");
+
+pub fn build(b: *std.Build) void {
+    const test_step = b.step("test", "Test");
+    b.default_step = test_step;
+
+    const targets: []const []const u8 = &.{
+        "aarch64-linux-gnu.2.27",
+        "aarch64-linux-gnu.2.28",
+        "aarch64-linux-gnu.2.33",
+        "aarch64-linux-gnu.2.34",
+    };
+
+    for (targets) |target| {
+        const exe = b.addExecutable(.{
+            .name = target,
+            .root_source_file = .{ .path = "main.c" },
+            .target = std.zig.CrossTarget.parse(
+                .{ .arch_os_abi = target },
+            ) catch unreachable,
+        });
+        exe.linkLibC();
+        test_step.dependOn(&exe.step);
+    }
+}

test/link/glibc_compat/main.c

@@ -0,0 +1,37 @@
+#define _FILE_OFFSET_BITS 64
+#include <unistd.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <resolv.h>
+
+int main() {
+    /* in glibc 2.28+ and _FILE_OFFSET_BITS=64 fcntl is #define'd to fcntl64
+     * Thus headers say `fcntl64` exists, but libc.so.6 (the old one)
+     * disagrees, resulting in a linking error unless headers are made
+     * backwards-compatible.
+     *
+     * Glibc 2.28+:
+     *   FUNC    GLOBAL DEFAULT  UND fcntl64@GLIBC_2.28 (3):
+     *
+     * Glibc 2.27 or older:
+     *   FUNC    GLOBAL DEFAULT  UND fcntl@GLIBC_2.2.5
+     */
+    printf("address to fcntl: %p\n", fcntl);
+
+    /* The following functions became symbols of their own right with glibc
+     * 2.34+. Before 2.34 resolv.h would #define res_search __res_search; and
+     * __res_search is a valid symbol since the beginning of time.
+     *
+     * On glibc 2.34+ these symbols are linked this way:
+     *   FUNC    GLOBAL DEFAULT  UND res_search@GLIBC_2.34 (2)
+     *
+     * Pre-glibc 2.34:
+     *   FUNC    GLOBAL DEFAULT  UND __res_search@GLIBC_2.2.5 (4)
+     */
+    printf("address to res_search: %p\n", res_search);
+    printf("address to res_nsearch: %p\n", res_nsearch);
+    printf("address to res_query: %p\n", res_query);
+    printf("address to res_nquery: %p\n", res_nquery);
+    printf("address to res_querydomain: %p\n", res_querydomain);
+    printf("address to res_nquerydomain: %p\n", res_nquerydomain);
+}