diff --git a/Foundation/FileHandle.swift b/Foundation/FileHandle.swift
index 04e2d690b6..0f86c59789 100644
--- a/Foundation/FileHandle.swift
+++ b/Foundation/FileHandle.swift
@@ -142,13 +142,12 @@ open class FileHandle : NSObject, NSSecureCoding {
     open func seek(toFileOffset offset: UInt64) {
         lseek(_fd, off_t(offset), SEEK_SET)
     }
-    
+
     open func truncateFile(atOffset offset: UInt64) {
-        if lseek(_fd, off_t(offset), SEEK_SET) == 0 {
-            ftruncate(_fd, off_t(offset))
-        }
+        if lseek(_fd, off_t(offset), SEEK_SET) < 0 { fatalError("lseek() failed.") }
+        if ftruncate(_fd, off_t(offset)) < 0 { fatalError("ftruncate() failed.") }
     }
-    
+
     open func synchronizeFile() {
         fsync(_fd)
     }
diff --git a/TestFoundation/TestFileHandle.swift b/TestFoundation/TestFileHandle.swift
index 1b389ff0f7..ca728bdbe2 100644
--- a/TestFoundation/TestFileHandle.swift
+++ b/TestFoundation/TestFileHandle.swift
@@ -21,6 +21,7 @@ class TestFileHandle : XCTestCase {
             ("test_constants", test_constants),
             ("test_pipe", test_pipe),
             ("test_nullDevice", test_nullDevice),
+            ("test_truncateFile", test_truncateFile)
         ]
     }
 
@@ -64,4 +65,39 @@ class TestFileHandle : XCTestCase {
         fh.seek(toFileOffset: 0)
         XCTAssertEqual(fh.readDataToEndOfFile().count, 0)
     }
+
+    func test_truncateFile() {
+        mkstemp(template: "test_truncateFile.XXXXXX") { (fh) in
+            fh.truncateFile(atOffset: 50)
+            XCTAssertEqual(fh.offsetInFile, 50)
+
+            fh.truncateFile(atOffset: 0)
+            XCTAssertEqual(fh.offsetInFile, 0)
+
+            fh.truncateFile(atOffset: 100)
+            XCTAssertEqual(fh.offsetInFile, 100)
+
+            fh.write(Data(bytes: [1, 2]))
+            XCTAssertEqual(fh.offsetInFile, 102)
+
+            fh.seek(toFileOffset: 4)
+            XCTAssertEqual(fh.offsetInFile, 4)
+
+            (0..<20).forEach { fh.write(Data(bytes: [$0])) }
+            XCTAssertEqual(fh.offsetInFile, 24)
+
+            fh.seekToEndOfFile()
+            XCTAssertEqual(fh.offsetInFile, 102)
+
+            fh.truncateFile(atOffset: 10)
+            XCTAssertEqual(fh.offsetInFile, 10)
+
+            fh.seek(toFileOffset: 0)
+            XCTAssertEqual(fh.offsetInFile, 0)
+
+            let data = fh.readDataToEndOfFile()
+            XCTAssertEqual(data.count, 10)
+            XCTAssertEqual(data, Data(bytes: [0, 0, 0, 0, 0, 1, 2, 3, 4, 5]))
+        }
+    }
 }
diff --git a/TestFoundation/TestProcess.swift b/TestFoundation/TestProcess.swift
index 6af3e37df3..58ffafa896 100644
--- a/TestFoundation/TestProcess.swift
+++ b/TestFoundation/TestProcess.swift
@@ -297,20 +297,6 @@ class TestProcess : XCTestCase {
 #endif
 }
 
-private func mkstemp(template: String, body: (FileHandle) throws -> Void) rethrows {
-    let url = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent("TestProcess.XXXXXX")
-    
-    try url.withUnsafeFileSystemRepresentation {
-        switch mkstemp(UnsafeMutablePointer(mutating: $0!)) {
-        case -1: XCTFail("Could not create temporary file")
-        case let fd:
-            defer { url.withUnsafeFileSystemRepresentation { _ = unlink($0!) } }
-            try body(FileHandle(fileDescriptor: fd, closeOnDealloc: true))
-        }
-    }
-    
-}
-
 private enum Error: Swift.Error {
     case TerminationStatus(Int32)
     case UnicodeDecodingError(Data)
diff --git a/TestFoundation/TestUtils.swift b/TestFoundation/TestUtils.swift
index 860e1291d9..c9ab02b917 100644
--- a/TestFoundation/TestUtils.swift
+++ b/TestFoundation/TestUtils.swift
@@ -50,3 +50,16 @@ func ensureFiles(_ fileNames: [String]) -> Bool {
     }
     return result
 }
+
+func mkstemp(template: String, body: (FileHandle) throws -> Void) rethrows {
+    let url = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(template)
+
+    try url.withUnsafeFileSystemRepresentation {
+        switch mkstemp(UnsafeMutablePointer(mutating: $0!)) {
+        case -1: XCTFail("Could not create temporary file")
+        case let fd:
+            defer { url.withUnsafeFileSystemRepresentation { _ = unlink($0!) } }
+            try body(FileHandle(fileDescriptor: fd, closeOnDealloc: true))
+        }
+    }
+}