diff --git a/lib/compiler/build_runner.zig b/lib/compiler/build_runner.zig
index 6e0e2d8ecaaa793c37b4fa23df7edba67d7e8534..36c73e96eb815d9f24b6cff0abba3873c9e231c0 100644
--- a/lib/compiler/build_runner.zig
+++ b/lib/compiler/build_runner.zig
@@ -824,7 +824,7 @@ fn runStepNames(
         }
         if (@bitSizeOf(usize) != 64) {
             // Current implementation depends on posix.mmap()'s second parameter, `length: usize`,
-            // being compatible with `std.fs.getEndPos() u64`'s return value. This is not the case
+            // being compatible with file system's u64 return value. This is not the case
             // on 32-bit platforms.
             // Affects or affected by issues #5185, #22523, and #22464.
             fatal("--fuzz not yet implemented on {d}-bit platforms", .{@bitSizeOf(usize)});
diff --git a/lib/std/Build/Fuzz.zig b/lib/std/Build/Fuzz.zig
index c95c9bd354b591639faa6bb85a75029f8ea4c565..8d5bc27f3a467b659792cba077c4d3ef94c76cca 100644
--- a/lib/std/Build/Fuzz.zig
+++ b/lib/std/Build/Fuzz.zig
@@ -413,7 +413,7 @@ fn prepareTables(fuzz: *Fuzz, run_step: *Step.Run, coverage_id: u64) error{ OutO
     };
     defer coverage_file.close(io);
 
-    const file_size = coverage_file.getEndPos() catch |err| {
+    const file_size = coverage_file.length(io) catch |err| {
         log.err("unable to check len of coverage file '{f}': {t}", .{ coverage_file_path, err });
         return error.AlreadyReported;
     };
diff --git a/lib/std/Io/test.zig b/lib/std/Io/test.zig
index 8b9d714ee857622b8a0c253cc2de54e26d544b74..e911031c7f697f894ac1736633acd9de4abd935e 100644
--- a/lib/std/Io/test.zig
+++ b/lib/std/Io/test.zig
@@ -47,7 +47,7 @@ test "write a file, read it, then delete it" {
         var file = try tmp.dir.openFile(io, tmp_file_name, .{});
         defer file.close(io);
 
-        const file_size = try file.getEndPos();
+        const file_size = try file.length(io);
         const expected_file_size: u64 = "begin".len + data.len + "end".len;
         try expectEqual(expected_file_size, file_size);
 
@@ -77,7 +77,7 @@ test "File seek ops" {
 
     // Seek to the end
     try file.seekFromEnd(0);
-    try expect((try file.getPos()) == try file.getEndPos());
+    try expect((try file.getPos()) == try file.length(io));
     // Negative delta
     try file.seekBy(-4096);
     try expect((try file.getPos()) == 4096);
@@ -100,17 +100,17 @@ test "setEndPos" {
     defer file.close(io);
 
     // Verify that the file size changes and the file offset is not moved
-    try expect((try file.getEndPos()) == 0);
+    try expect((try file.length(io)) == 0);
     try expect((try file.getPos()) == 0);
     try file.setEndPos(8192);
-    try expect((try file.getEndPos()) == 8192);
+    try expect((try file.length(io)) == 8192);
     try expect((try file.getPos()) == 0);
     try file.seekTo(100);
     try file.setEndPos(4096);
-    try expect((try file.getEndPos()) == 4096);
+    try expect((try file.length(io)) == 4096);
     try expect((try file.getPos()) == 100);
     try file.setEndPos(0);
-    try expect((try file.getEndPos()) == 0);
+    try expect((try file.length(io)) == 0);
     try expect((try file.getPos()) == 100);
 }
 
diff --git a/lib/std/debug.zig b/lib/std/debug.zig
index 7ede172d86f428f886a2b1f0aac54881fd2858f0..34f740bfee8635e07caa158ac66239308c4ac309 100644
--- a/lib/std/debug.zig
+++ b/lib/std/debug.zig
@@ -558,9 +558,9 @@ pub fn defaultPanic(
                 stderr.print("{s}\n", .{msg}) catch break :trace;
 
                 if (@errorReturnTrace()) |t| if (t.index > 0) {
-                    stderr.writeStreamingAll("error return context:\n") catch break :trace;
+                    stderr.writeAll("error return context:\n") catch break :trace;
                     writeStackTrace(t, stderr, tty_config) catch break :trace;
-                    stderr.writeStreamingAll("\nstack trace:\n") catch break :trace;
+                    stderr.writeAll("\nstack trace:\n") catch break :trace;
                 };
                 writeCurrentStackTrace(.{
                     .first_address = first_trace_addr orelse @returnAddress(),
@@ -617,6 +617,8 @@ pub const StackUnwindOptions = struct {
 ///
 /// See `writeCurrentStackTrace` to immediately print the trace instead of capturing it.
 pub noinline fn captureCurrentStackTrace(options: StackUnwindOptions, addr_buf: []usize) StackTrace {
+    var threaded: Io.Threaded = .init_single_threaded;
+    const io = threaded.ioBasic();
     const empty_trace: StackTrace = .{ .index = 0, .instruction_addresses = &.{} };
     if (!std.options.allow_stack_tracing) return empty_trace;
     var it: StackIterator = .init(options.context);
@@ -628,7 +630,7 @@ pub noinline fn captureCurrentStackTrace(options: StackUnwindOptions, addr_buf:
     // Ideally, we would iterate the whole stack so that the `index` in the returned trace was
     // indicative of how many frames were skipped. However, this has a significant runtime cost
     // in some cases, so at least for now, we don't do that.
-    while (index < addr_buf.len) switch (it.next()) {
+    while (index < addr_buf.len) switch (it.next(io)) {
         .switch_to_fp => if (!it.stratOk(options.allow_unsafe_unwind)) break,
         .end => break,
         .frame => |ret_addr| {
@@ -684,7 +686,7 @@ pub noinline fn writeCurrentStackTrace(options: StackUnwindOptions, writer: *Wri
     var total_frames: usize = 0;
     var wait_for = options.first_address;
     var printed_any_frame = false;
-    while (true) switch (it.next()) {
+    while (true) switch (it.next(io)) {
         .switch_to_fp => |unwind_error| {
             switch (StackIterator.fp_usability) {
                 .useless, .unsafe => {},
@@ -1196,54 +1198,26 @@ fn printLineFromFile(io: Io, writer: *Writer, source_location: SourceLocation) !
     // Need this to always block even in async I/O mode, because this could potentially
     // be called from e.g. the event loop code crashing.
     const cwd: Io.Dir = .cwd();
-    var f = try cwd.openFile(io, source_location.file_name, .{});
-    defer f.close(io);
+    var file = try cwd.openFile(io, source_location.file_name, .{});
+    defer file.close(io);
     // TODO fstat and make sure that the file has the correct size
 
-    var buf: [4096]u8 = undefined;
-    var amt_read = try f.read(buf[0..]);
-    const line_start = seek: {
-        var current_line_start: usize = 0;
-        var next_line: usize = 1;
-        while (next_line != source_location.line) {
-            const slice = buf[current_line_start..amt_read];
-            if (mem.findScalar(u8, slice, '\n')) |pos| {
-                next_line += 1;
-                if (pos == slice.len - 1) {
-                    amt_read = try f.read(buf[0..]);
-                    current_line_start = 0;
-                } else current_line_start += pos + 1;
-            } else if (amt_read < buf.len) {
-                return error.EndOfFile;
-            } else {
-                amt_read = try f.read(buf[0..]);
-                current_line_start = 0;
-            }
+    var buffer: [4096]u8 = undefined;
+    var file_reader: File.Reader = .init(file, io, &buffer);
+    const r = &file_reader.interface;
+    var line_index: usize = 0;
+    while (r.takeDelimiterExclusive('\n')) |line| {
+        line_index += 1;
+        if (line_index == source_location.line) {
+            // TODO delete hard tabs from the language
+            mem.replaceScalar(u8, line, '\t', ' ');
+            try writer.writeAll(line);
+            // Make sure printing last line of file inserts extra newline.
+            try writer.writeByte('\n');
+            return;
         }
-        break :seek current_line_start;
-    };
-    const slice = buf[line_start..amt_read];
-    if (mem.findScalar(u8, slice, '\n')) |pos| {
-        const line = slice[0 .. pos + 1];
-        mem.replaceScalar(u8, line, '\t', ' ');
-        return writer.writeAll(line);
-    } else { // Line is the last inside the buffer, and requires another read to find delimiter. Alternatively the file ends.
-        mem.replaceScalar(u8, slice, '\t', ' ');
-        try writer.writeAll(slice);
-        while (amt_read == buf.len) {
-            amt_read = try f.read(buf[0..]);
-            if (mem.findScalar(u8, buf[0..amt_read], '\n')) |pos| {
-                const line = buf[0 .. pos + 1];
-                mem.replaceScalar(u8, line, '\t', ' ');
-                return writer.writeAll(line);
-            } else {
-                const line = buf[0..amt_read];
-                mem.replaceScalar(u8, line, '\t', ' ');
-                try writer.writeAll(line);
-            }
-        }
-        // Make sure printing last line of file inserts extra newline
-        try writer.writeByte('\n');
+    } else |err| {
+        return err;
     }
 }
 
@@ -1598,7 +1572,7 @@ pub fn defaultHandleSegfault(addr: ?usize, name: []const u8, opt_ctx: ?CpuContex
             // We're still holding the mutex but that's fine as we're going to
             // call abort().
             const stderr, _ = lockStderrWriter(&.{});
-            stderr().writeAll("aborting due to recursive panic\n") catch {};
+            stderr.writeAll("aborting due to recursive panic\n") catch {};
         },
         else => {}, // Panicked while printing the recursive panic message.
     }
diff --git a/lib/std/debug/ElfFile.zig b/lib/std/debug/ElfFile.zig
index 203ee8effb1b74c19cc4f247ef70878e4d29c5d6..a101309d22c52f4d324fbb43452bef23cd0e958a 100644
--- a/lib/std/debug/ElfFile.zig
+++ b/lib/std/debug/ElfFile.zig
@@ -123,6 +123,7 @@ pub const LoadError = error{
 
 pub fn load(
     gpa: Allocator,
+    io: Io,
     elf_file: Io.File,
     opt_build_id: ?[]const u8,
     di_search_paths: *const DebugInfoSearchPaths,
@@ -131,7 +132,7 @@ pub fn load(
     errdefer arena_instance.deinit();
     const arena = arena_instance.allocator();
 
-    var result = loadInner(arena, elf_file, null) catch |err| switch (err) {
+    var result = loadInner(arena, io, elf_file, null) catch |err| switch (err) {
         error.CrcMismatch => unreachable, // we passed crc as null
         else => |e| return e,
     };
@@ -156,7 +157,7 @@ pub fn load(
             if (build_id.len < 3) break :build_id;
 
             for (di_search_paths.global_debug) |global_debug| {
-                if (try loadSeparateDebugFile(arena, &result, null, "{s}/.build-id/{x}/{x}.debug", .{
+                if (try loadSeparateDebugFile(arena, io, &result, null, "{s}/.build-id/{x}/{x}.debug", .{
                     global_debug,
                     build_id[0..1],
                     build_id[1..],
@@ -164,7 +165,7 @@ pub fn load(
             }
 
             if (di_search_paths.debuginfod_client) |components| {
-                if (try loadSeparateDebugFile(arena, &result, null, "{s}{s}/{x}/debuginfo", .{
+                if (try loadSeparateDebugFile(arena, io, &result, null, "{s}{s}/{x}/debuginfo", .{
                     components[0],
                     components[1],
                     build_id,
@@ -181,18 +182,18 @@ pub fn load(
 
             const exe_dir = di_search_paths.exe_dir orelse break :debug_link;
 
-            if (try loadSeparateDebugFile(arena, &result, debug_crc, "{s}/{s}", .{
+            if (try loadSeparateDebugFile(arena, io, &result, debug_crc, "{s}/{s}", .{
                 exe_dir,
                 debug_filename,
             })) |mapped| break :load_di mapped;
-            if (try loadSeparateDebugFile(arena, &result, debug_crc, "{s}/.debug/{s}", .{
+            if (try loadSeparateDebugFile(arena, io, &result, debug_crc, "{s}/.debug/{s}", .{
                 exe_dir,
                 debug_filename,
             })) |mapped| break :load_di mapped;
             for (di_search_paths.global_debug) |global_debug| {
                 // This looks like a bug; it isn't. They really do embed the absolute path to the
                 // exe's dirname, *under* the global debug path.
-                if (try loadSeparateDebugFile(arena, &result, debug_crc, "{s}/{s}/{s}", .{
+                if (try loadSeparateDebugFile(arena, io, &result, debug_crc, "{s}/{s}/{s}", .{
                     global_debug,
                     exe_dir,
                     debug_filename,
@@ -378,7 +379,7 @@ fn loadSeparateDebugFile(
     const elf_file = Io.Dir.cwd().openFile(io, path, .{}) catch return null;
     defer elf_file.close(io);
 
-    const result = loadInner(arena, elf_file, opt_crc) catch |err| switch (err) {
+    const result = loadInner(arena, io, elf_file, opt_crc) catch |err| switch (err) {
         error.OutOfMemory => |e| return e,
         error.CrcMismatch => return null,
         else => return null,
@@ -423,13 +424,14 @@ const LoadInnerResult = struct {
 };
 fn loadInner(
     arena: Allocator,
+    io: Io,
     elf_file: Io.File,
     opt_crc: ?u32,
 ) (LoadError || error{ CrcMismatch, Streaming, Canceled })!LoadInnerResult {
     const mapped_mem: []align(std.heap.page_size_min) const u8 = mapped: {
         const file_len = std.math.cast(
             usize,
-            elf_file.getEndPos() catch |err| switch (err) {
+            elf_file.length(io) catch |err| switch (err) {
                 error.PermissionDenied => unreachable, // not asking for PROT_EXEC
                 else => |e| return e,
             },
diff --git a/lib/std/debug/MachOFile.zig b/lib/std/debug/MachOFile.zig
index 18126a1c297171fe3f95e97fdf6d1d7dbc675c06..9e81fb8911d759993f99b980125476b0a20229dd 100644
--- a/lib/std/debug/MachOFile.zig
+++ b/lib/std/debug/MachOFile.zig
@@ -520,7 +520,7 @@ fn mapDebugInfoFile(io: Io, path: []const u8) ![]align(std.heap.page_size_min) c
 
     const file_len = std.math.cast(
         usize,
-        file.getEndPos() catch return error.ReadFailed,
+        file.length(io) catch return error.ReadFailed,
     ) orelse return error.ReadFailed;
 
     return posix.mmap(
diff --git a/lib/std/debug/SelfInfo/Elf.zig b/lib/std/debug/SelfInfo/Elf.zig
index 6ed18bcb80ad08fd5201d1affe4fb1bab6826f4b..be76f3a8c24fa92e256588359350d65767b031aa 100644
--- a/lib/std/debug/SelfInfo/Elf.zig
+++ b/lib/std/debug/SelfInfo/Elf.zig
@@ -326,7 +326,7 @@ const Module = struct {
         const load_result = if (mod.name.len > 0) res: {
             var file = Io.Dir.cwd().openFile(io, mod.name, .{}) catch return error.MissingDebugInfo;
             defer file.close(io);
-            break :res std.debug.ElfFile.load(gpa, file, mod.build_id, &.native(mod.name));
+            break :res std.debug.ElfFile.load(gpa, io, file, mod.build_id, &.native(mod.name));
         } else res: {
             const path = std.process.executablePathAlloc(io, gpa) catch |err| switch (err) {
                 error.OutOfMemory => |e| return e,
@@ -335,7 +335,7 @@ const Module = struct {
             defer gpa.free(path);
             var file = Io.Dir.cwd().openFile(io, path, .{}) catch return error.MissingDebugInfo;
             defer file.close(io);
-            break :res std.debug.ElfFile.load(gpa, file, mod.build_id, &.native(path));
+            break :res std.debug.ElfFile.load(gpa, io, file, mod.build_id, &.native(path));
         };
 
         var elf_file = load_result catch |err| switch (err) {
diff --git a/lib/std/debug/SelfInfo/MachO.zig b/lib/std/debug/SelfInfo/MachO.zig
index db8e5334e65d1ba978f8a9a40f0ecb37ed2d7222..714ff539f1c42073a7f9e5582a0d39311e1c0b49 100644
--- a/lib/std/debug/SelfInfo/MachO.zig
+++ b/lib/std/debug/SelfInfo/MachO.zig
@@ -622,7 +622,7 @@ fn mapDebugInfoFile(io: Io, path: []const u8) ![]align(std.heap.page_size_min) c
     };
     defer file.close(io);
 
-    const file_end_pos = file.getEndPos() catch |err| switch (err) {
+    const file_end_pos = file.length(io) catch |err| switch (err) {
         error.Unexpected => |e| return e,
         else => return error.ReadFailed,
     };
diff --git a/lib/std/os/uefi/protocol/file.zig b/lib/std/os/uefi/protocol/file.zig
index 9e3e7cc081f9bfc4a340e996126582a93cb3abcb..9b371916a71bb4974e3e9e7627b270a3d5fb308f 100644
--- a/lib/std/os/uefi/protocol/file.zig
+++ b/lib/std/os/uefi/protocol/file.zig
@@ -163,15 +163,6 @@ pub const File = extern struct {
         }
     }
 
-    fn getEndPos(self: *File) SeekError!u64 {
-        const start_pos = try self.getPosition();
-        // ignore error
-        defer self.setPosition(start_pos) catch {};
-
-        try self.setPosition(end_of_file);
-        return self.getPosition();
-    }
-
     pub fn setPosition(self: *File, position: u64) SeekError!void {
         switch (self._set_position(self, position)) {
             .success => {},