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Incorrect std.fs.File.Writer output #30149

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opened 2025年12月08日 19:09:30 +01:00 by sergzz · 3 comments

Zig Version

0.16.0-dev.1574+5d96a58f1

Steps to Reproduce and Observed Behavior

run zig build-exe on the following code and then run the compiled code:

const std = @import("std");
const Writer = std.fs.File.Writer;
const Allocator = std.mem.Allocator;

fn printTest(allocator: Allocator, min: usize, max: usize, writer: *Writer) !void {
for (min..max) |i| {
const s = try std.fmt.allocPrint(allocator, "{d:016}", .{i});
_ = try writer.interface.write(s);
_ = try writer.interface.write("\n");
}
}

pub fn main() !void {
// Get allocator
var a = std.heap.ArenaAllocator.init(std.heap.page_allocator);
const allocator = a.allocator();
defer _ = a.deinit();

var buffer: [1024]u8 = undefined;
var writer = std.fs.File.stdout().writer(&buffer);
try printTest(allocator, 1, 65, &writer);
try writer.interface.flush();

}

I see the following output ( line 61 is absent):
0000000000000001
0000000000000002
0000000000000003
0000000000000004
0000000000000005
0000000000000006
0000000000000007
0000000000000008
0000000000000009
0000000000000010
0000000000000011
0000000000000012
0000000000000013
0000000000000014
0000000000000015
0000000000000016
0000000000000017
0000000000000018
0000000000000019
0000000000000020
0000000000000021
0000000000000022
0000000000000023
0000000000000024
0000000000000025
0000000000000026
0000000000000027
0000000000000028
0000000000000029
0000000000000030
0000000000000031
0000000000000032
0000000000000033
0000000000000034
0000000000000035
0000000000000036
0000000000000037
0000000000000038
0000000000000039
0000000000000040
0000000000000041
0000000000000042
0000000000000043
0000000000000044
0000000000000045
0000000000000046
0000000000000047
0000000000000048
0000000000000049
0000000000000050
0000000000000051
0000000000000052
0000000000000053
0000000000000054
0000000000000055
0000000000000056
0000000000000057
0000000000000058
0000000000000059
0000000000000060

0000000000000062
0000000000000063
0000000000000064

Expected Behavior

line 61 is present

### Zig Version 0.16.0-dev.1574+5d96a58f1 ### Steps to Reproduce and Observed Behavior run zig build-exe on the following code and then run the compiled code: const std = @import("std"); const Writer = std.fs.File.Writer; const Allocator = std.mem.Allocator; fn printTest(allocator: Allocator, min: usize, max: usize, writer: *Writer) !void { for (min..max) |i| { const s = try std.fmt.allocPrint(allocator, "{d:016}", .{i}); _ = try writer.interface.write(s); _ = try writer.interface.write("\n"); } } pub fn main() !void { // Get allocator var a = std.heap.ArenaAllocator.init(std.heap.page_allocator); const allocator = a.allocator(); defer _ = a.deinit(); var buffer: [1024]u8 = undefined; var writer = std.fs.File.stdout().writer(&buffer); try printTest(allocator, 1, 65, &writer); try writer.interface.flush(); } I see the following output ( line 61 is absent): 0000000000000001 0000000000000002 0000000000000003 0000000000000004 0000000000000005 0000000000000006 0000000000000007 0000000000000008 0000000000000009 0000000000000010 0000000000000011 0000000000000012 0000000000000013 0000000000000014 0000000000000015 0000000000000016 0000000000000017 0000000000000018 0000000000000019 0000000000000020 0000000000000021 0000000000000022 0000000000000023 0000000000000024 0000000000000025 0000000000000026 0000000000000027 0000000000000028 0000000000000029 0000000000000030 0000000000000031 0000000000000032 0000000000000033 0000000000000034 0000000000000035 0000000000000036 0000000000000037 0000000000000038 0000000000000039 0000000000000040 0000000000000041 0000000000000042 0000000000000043 0000000000000044 0000000000000045 0000000000000046 0000000000000047 0000000000000048 0000000000000049 0000000000000050 0000000000000051 0000000000000052 0000000000000053 0000000000000054 0000000000000055 0000000000000056 0000000000000057 0000000000000058 0000000000000059 0000000000000060 0000000000000062 0000000000000063 0000000000000064 ### Expected Behavior line 61 is present
Contributor
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write probably does a small write. If you change the write to writeAll functions, the 61 exists.

`write` probably does a small write. If you change the `write` to `writeAll` functions, the `61` exists.

Interessting. I can repoduce this problem. seems that the terminal swallows, when the buffer gets full (1021 Bytes written).

If you redirect the output to a file. everything is correct, and nothing gets swallowed

fox@m1lx:~/coding/zig$ zig build-exe t.zig
fox@m1lx:~/coding/zig$ diff <(./t) <(./t >tmp.txt; cat tmp.txt)
61c61
<
---
> 0000000000000061
Interessting. I can repoduce this problem. seems that the terminal swallows, when the buffer gets full (1021 Bytes written). If you redirect the output to a file. everything is correct, and nothing gets swallowed ``` fox@m1lx:~/coding/zig$ zig build-exe t.zig fox@m1lx:~/coding/zig$ diff <(./t) <(./t >tmp.txt; cat tmp.txt) 61c61 < --- > 0000000000000061 ```
Author
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Seems related to Linux terminal behavior related to unterminated lines (without new line character).

Seems related to Linux terminal behavior related to unterminated lines (without new line character).
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