Include error-reason when failining in CryptoVec unix

cryptovec/src/platform/unix.rs

@@ -1,11 +1,13 @@
+use std::ffi::CStr;
+
 use libc::c_void;
 
 /// Unlock memory on drop for Unix-based systems.
 pub fn munlock(ptr: *const u8, len: usize) {
     unsafe {
         #[allow(clippy::panic)]
         if libc::munlock(ptr as *const c_void, len) != 0 {
-            panic!("Failed to unlock memory");
+            panic_libc_error("Failed to unlock memory");
         }
     }
 }
@@ -14,7 +16,7 @@ pub fn mlock(ptr: *const u8, len: usize) {
     unsafe {
         #[allow(clippy::panic)]
         if libc::mlock(ptr as *const c_void, len) != 0 {
-            panic!("Failed to lock memory");
+            panic_libc_error("Failed to lock memory");
         }
     }
 }
@@ -24,3 +26,20 @@ pub fn memset(ptr: *mut u8, value: i32, size: usize) {
         libc::memset(ptr as *mut c_void, value, size);
     }
 }
+
+unsafe fn panic_libc_error(msg: &str) {
+    let errno = *libc::__errno_location();
+    const ERRMAXLEN: usize = 255;
+    let mut errdesc = [0u8; ERRMAXLEN];
+    let errdesc = if libc::strerror_r(errno, errdesc.as_mut_ptr() as _, ERRMAXLEN) == 0 {
+        CStr::from_bytes_until_nul(&errdesc)
+            .ok()
+            .and_then(|msg| msg.to_str().ok())
+            .unwrap_or("Invalid error description")
+    } else {
+        "Invalid error"
+    };
+    // Note: if you get 'Cannot allocate memory (0xc)' here,
+    // check if your RLIMIT_MEMLOCK (`ulimit -l`) is configured low! 
+    panic!("{}: {} (0x{:x})", msg, errdesc, errno);
+}