Auto merge of #3393 - Brijeshkrishna:main, r=JohnTitor

feat: Added ifconf struct

Adding ifconf struct

As per defined in C

```c
struct ifconf
  {
    int	ifc_len;			/* Size of buffer.  */
    union
      {
	__caddr_t ifcu_buf;
	struct ifreq *ifcu_req;
      } ifc_ifcu;
  };
```

libc-test/build.rs

@@ -4086,6 +4086,8 @@ fn test_linux(target: &str) {
         (struct_ == "sockaddr_vm" && field == "svm_zero") ||
         // the `ifr_ifru` field is an anonymous union
         (struct_ == "ifreq" && field == "ifr_ifru") ||
+        // the `ifc_ifcu` field is an anonymous union
+        (struct_ == "ifconf" && field == "ifc_ifcu") ||
         // glibc uses a single array `uregs` instead of individual fields.
         (struct_ == "user_regs" && arm)
     });

libc-test/semver/linux.txt

@@ -3248,6 +3248,7 @@ if_freenameindex
 if_nameindex
 ifaddrs
 ifreq
+ifconf
 in6_ifreq
 in6_pktinfo
 in6_rtmsg

src/unix/linux_like/linux/mod.rs

@@ -792,6 +792,23 @@ s_no_extra_traits! {
         pub ifr_ifru: ::sockaddr,
     }
 
+    #[cfg(libc_union)]
+    pub union __c_anonymous_ifc_ifcu {
+        pub ifcu_buf: *mut ::c_char,
+        pub ifcu_req: *mut ::ifreq,
+    }
+
+    /*  Structure used in SIOCGIFCONF request.  Used to retrieve interface
+    configuration for machine (useful for programs which must know all
+    networks accessible).  */
+    pub struct ifconf {
+        pub ifc_len: ::c_int,       /* Size of buffer.  */
+        #[cfg(libc_union)]
+        pub ifc_ifcu: __c_anonymous_ifc_ifcu,
+        #[cfg(not(libc_union))]
+        pub ifc_ifcu: *mut ::ifreq,
+    }
+
     pub struct hwtstamp_config {
         pub flags: ::c_int,
         pub tx_type: ::c_int,
@@ -1229,6 +1246,23 @@ cfg_if! {
             }
         }
 
+        #[cfg(libc_union)]
+        impl ::fmt::Debug for __c_anonymous_ifc_ifcu {
+            fn fmt(&self, f: &mut ::fmt::Formatter) -> ::fmt::Result {
+                f.debug_struct("ifr_ifru")
+                    .field("ifcu_buf", unsafe { &self.ifcu_buf })
+                    .field("ifcu_req", unsafe { &self.ifcu_req })
+                    .finish()
+            }
+        }
+        impl ::fmt::Debug for ifconf {
+            fn fmt(&self, f: &mut ::fmt::Formatter) -> ::fmt::Result {
+                f.debug_struct("ifconf")
+                    .field("ifc_len", &self.ifc_len)
+                    .field("ifc_ifcu", &self.ifc_ifcu)
+                    .finish()
+            }
+        }
         impl ::fmt::Debug for hwtstamp_config {
             fn fmt(&self, f: &mut ::fmt::Formatter) -> ::fmt::Result {
                 f.debug_struct("hwtstamp_config")