diff --git a/src/fd/ops.cpp b/src/fd/ops.cpp
index b0f2bef..d5991c6 100644
--- a/src/fd/ops.cpp
+++ b/src/fd/ops.cpp
@@ -15,15 +15,29 @@
 static void opExact(const char* name, Fun&& fun, Fd& fd, std::span<Byte> data,
                     Args&&... args)
 {
-    while (data.size() > 0)
+    std::size_t total = 0;
+    try
     {
-        auto ret = (fd.*fun)(data, std::forward<Args>(args)...);
-        if (ret.size() == 0)
+        while (total < data.size())
         {
-            throw exception::WouldBlock(
-                std::format("{} missing {}B", name, data.size()));
+            auto r = (fd.*fun)(data.subspan(total),
+                               std::forward<Args>(args)...);
+            if (r.size() == 0)
+            {
+                throw exception::WouldBlock(
+                    std::format("{} missing", name, data.size()));
+            }
+            total += r.size();
         }
-        data = data.subspan(ret.size());
+    }
+    catch (const std::system_error&)
+    {
+        if (total != 0)
+        {
+            throw exception::Incomplete(
+                std::format("{} is {}B/{}B", name, total, data.size()));
+        }
+        throw;
     }
 }
 
diff --git a/test/fd/ops.cpp b/test/fd/ops.cpp
index c0328fe..ef1d489 100644
--- a/test/fd/ops.cpp
+++ b/test/fd/ops.cpp
@@ -1,11 +1,19 @@
+#include "util.hpp"
+
+#include <stdplus/exception.hpp>
+#include <stdplus/fd/gmock.hpp>
 #include <stdplus/fd/ops.hpp>
+#include <stdplus/numeric/endian.hpp>
 
 #include <gtest/gtest.h>
 
-namespace stdplus
+namespace stdplus::fd
 {
-namespace fd
-{
+
+using testing::_;
+using testing::Ge;
+using testing::SizeIs;
+using std::literals::string_view_literals::operator""sv;
 
 TEST(Flags, Flags)
 {
@@ -14,5 +22,76 @@
     EXPECT_EQ(0, static_cast<int>(f));
 }
 
-} // namespace fd
-} // namespace stdplus
+TEST(ReadExact, Success)
+{
+    testing::StrictMock<FdMock> fd;
+    {
+        testing::InSequence seq;
+        EXPECT_CALL(fd, read(SizeIs(Ge(4)))).WillOnce(readSv("alph"));
+        EXPECT_CALL(fd, read(SizeIs(Ge(2)))).WillOnce(readSv("a "));
+        EXPECT_CALL(fd, read(SizeIs(Ge(3)))).WillOnce(readSv("one"));
+    }
+    char buf[9];
+    readExact(fd, buf);
+    EXPECT_EQ(std::string_view(buf, sizeof(buf)), "alpha one");
+}
+
+TEST(ReadExact, NotEnoughEof)
+{
+    testing::StrictMock<FdMock> fd;
+    {
+        testing::InSequence seq;
+        EXPECT_CALL(fd, read(_)).WillOnce(readSv("alph"));
+        EXPECT_CALL(fd, read(_)).WillOnce(readSv("a "));
+        EXPECT_CALL(fd, read(_))
+            .WillRepeatedly(testing::Throw(exception::Eof("test")));
+    }
+    char buf[9];
+    EXPECT_THROW(readExact(fd, buf), exception::Incomplete);
+    EXPECT_THROW(readExact(fd, buf), exception::Eof);
+}
+
+TEST(ReadExact, NotEnoughBlock)
+{
+    testing::StrictMock<FdMock> fd;
+    {
+        testing::InSequence seq;
+        EXPECT_CALL(fd, read(_)).WillOnce(readSv("alph"));
+        EXPECT_CALL(fd, read(_)).WillOnce(readSv("a "));
+        EXPECT_CALL(fd, read(_)).WillRepeatedly(readSv(""));
+    }
+    char buf[9];
+    EXPECT_THROW(readExact(fd, buf), exception::Incomplete);
+    EXPECT_THROW(readExact(fd, buf), exception::WouldBlock);
+}
+
+TEST(ReadExact, NotEnoughBlockInt)
+{
+    testing::StrictMock<FdMock> fd;
+    {
+        testing::InSequence seq;
+        EXPECT_CALL(fd, read(_)).WillOnce(readSv("\0\0\0"sv));
+        EXPECT_CALL(fd, read(_)).WillRepeatedly(readSv(""));
+    }
+    int32_t i;
+    EXPECT_THROW(readExact(fd, i), exception::Incomplete);
+    EXPECT_THROW(readExact(fd, i), exception::WouldBlock);
+}
+
+TEST(ReadExact, EnoughInt)
+{
+    testing::StrictMock<FdMock> fd;
+    {
+        testing::InSequence seq;
+        EXPECT_CALL(fd, read(_)).WillOnce(readSv("\0\0\0\0"sv));
+        EXPECT_CALL(fd, read(_)).WillOnce(readSv("\x01\x02"sv));
+        EXPECT_CALL(fd, read(_)).WillOnce(readSv("\x03\x04"sv));
+    }
+    int32_t i;
+    readExact(fd, i);
+    EXPECT_EQ(i, 0);
+    readExact(fd, i);
+    EXPECT_EQ(ntoh(i), 0x01020304);
+}
+
+} // namespace stdplus::fd
diff --git a/test/meson.build b/test/meson.build
index a0c37a1..b43cf9b 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -47,7 +47,7 @@
     'fd/line': [stdplus_fd_dep, stdplus_dep, gmock_dep, gtest_main_dep],
     'fd/mmap': [stdplus_fd_dep, gtest_main_dep],
     'fd/mock': [stdplus_fd_dep, gmock_dep, gtest_main_dep],
-    'fd/ops': [stdplus_fd_dep, gtest_main_dep],
+    'fd/ops': [stdplus_fd_dep, stdplus_dep, gmock_dep, gtest_main_dep],
   }
   if has_gtest
     gtests += {