# bufPrint bug workaround **URL:** \ **Category:** Help\ **Tags:** language, standard-library\ **Created:** [May 18, 2024, 6:12pm UTC](https://ziggit.dev/t/bufprint-bug-workaround/4366 "2024-05-18T18:12:43Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Av1d](https://ziggit.dev/user_avatar/ziggit.dev/av1d/32/2383_2.png) [@Av1d](https://ziggit.dev/u/Av1d)\ **Post date:** [May 18, 2024, 6:12pm UTC](https://ziggit.dev/t/bufprint-bug-workaround/4366/1 "2024-05-18T18:12:43Z")
Hi guys! I’m building a language parser and trying to make the resulting AST printable for inspection and testing. Each expression has a `string()` method, that uses `std.fmt.bufPrint()` method to represent the expression as a string. Everything is fine until I try to use return value of one `bufPrint` in another `bufPrint`. If there are 2 placeholders, it outputs some gibberish, and if I leave only one placeholder I get a panic. Here is the code. The issue is in the `.pref` branch. ```zig pub const Expression = union(enum) { ident: Identifier, int: Integer, pref: Prefix, inf: Infix, pub fn string(self: @This(), buf: []u8) ![]u8 { return switch (self) { .ident => |ident| bufPrint(buf, "{s}", .{ident.token.get_value().?}), .int => |int| bufPrint(buf, "{d}", .{int.value}), .pref => |pref| bufPrint(buf, "({s}{s})", .{ @tagName(pref.token), try pref.right.string(buf), }), .inf => unreachable, }; } }; ``` Given expressions “!5” and “-15” expected output: “(bang5)” and “(minus15)” got: “(bang()” and “(minus(m)” Panic on removing the first placeholder and value: _thread 322558 panic: @memcpy arguments alias_ _Unwind error at address `exe:0x8051d33` (error.MissingFDE), trace may be incomplete_ Looking at the following issue it seems to be a bug with bufPrint, so I don’t think I’m doing something wrong. The question is if there is some workaround to be able to continue using bufPrint, or is there a good alternative that can be used for this task. > > > \### Zig Version > > 0.11.0-dev.3298+5744ceedb > > \### Steps to Reproduce and Observed …Behavior > > \`\`\`zig > const std = @import("std"); > const bufPrint = std.fmt.bufPrint; > > test "bufPrint arguments alias" { > var buf: \[10\]u8 = undefined; > const foo = try bufPrint(&buf, "{s}", .{"foo"}); > const foobar = try bufPrint(&buf, "{s}{s}", .{ foo, "bar" }); > \_ = foobar; > } > \`\`\` > \`\`\` > $ zig test test.zig > Test \[1/1\] test.bufPrint aliased arguments... thread 19632 panic: @memcpy arguments alias > /snap/zig/7524/lib/std/io/fixed\_buffer\_stream.zig:70:57: 0x2112bf in write (test) > @memcpy(self.buffer\[self.pos..\]\[0..n\], bytes\[0..n\]); > ^ > /snap/zig/7524/lib/std/io/writer.zig:17:27: 0x21bc4b in write (test) > return writeFn(self.context, bytes); > ^ > /snap/zig/7524/lib/std/io/writer.zig:23:40: 0x212dc2 in writeAll (test) > index += try self.write(bytes\[index..\]); > ^ > /snap/zig/7524/lib/std/fmt.zig:1055:28: 0x2127f8 in formatBuf\_\_anon\_3582 (test) > try writer.writeAll(buf); > ^ > /snap/zig/7524/lib/std/fmt.zig:656:45: 0x210208 in formatType\_\_anon\_1453 (test) > return formatBuf(value, options, writer); > ^ > /snap/zig/7524/lib/std/fmt.zig:184:23: 0x2100c6 in format\_\_anon\_1440 (test) > try formatType( > ^ > /snap/zig/7524/lib/std/fmt.zig:1995:15: 0x20f54c in bufPrint\_\_anon\_1144 (test) > try format(fbs.writer(), fmt, args); > ^ > /home/jayschwa/Desktop/test.zig:7:32: 0x20f267 in test.bufPrint aliased arguments (test) > const foobar = try bufPrint(&buf, "{s}{s}", .{ foo, "bar" }); > ^ > \`\`\` > > \### Expected Behavior > > This concatenative use of \`bufPrint\` worked not long ago (within the last month, I think). If the new behavior is working as intended, then documentation for the function probably ought to mention it. ---
**Author:** ![dimdin](https://ziggit.dev/user_avatar/ziggit.dev/dimdin/32/1457_2.png) [@dimdin](https://ziggit.dev/u/dimdin)\ **Post date:** [May 18, 2024, 6:25pm UTC](https://ziggit.dev/t/bufprint-bug-workaround/4366/2 "2024-05-18T18:25:54Z")
Hi @Av1d Welcome to ziggit 🙂 > [@Av1d](#): > > ```zig > bufPrint(buf, "({s}{s})", .{ > @tagName(pref.token), > try pref.right.string(buf), > }) > > ``` Here `buf` is both output and input. There is no guarantee that `bufPrint` is not going to override the input stored in the output buffer. To solve this you need different buffers. ---
**Author:** ![Sze](https://ziggit.dev/user_avatar/ziggit.dev/sze/32/496_2.png) [@Sze](https://ziggit.dev/u/Sze)\ **Post date:** [May 18, 2024, 6:44pm UTC](https://ziggit.dev/t/bufprint-bug-workaround/4366/3 "2024-05-18T18:44:10Z")
I think I would define and use a format function instead and then just print the object in whatever way is appropriate. With the writer you don’t have to pre-decide whether you want to use a buffer or not and your code stays agnostic. This post is a good description: > [@Idiomatic Complex Formatting](https://ziggit.dev/t/idiomatic-complex-formatting/3735/5): > > The idiomatic way of implementing custom formatting would be to implement a format function. [std.fmt.format](https://ziglang.org/documentation/master/std/#std.fmt.format), which std.debug.print and other formatting functions that take a format string with placeholders are derived from, has this to say in its doc comment: If a formatted user type contains a function of the type pub fn format( value: ?, comptime fmt: []const u8, options: std.fmt.FormatOptions, writer: anytype, ) !void with ? being the type formatted, this function will be call… ---
**Author:** ![Av1d](https://ziggit.dev/user_avatar/ziggit.dev/av1d/32/2383_2.png) [@Av1d](https://ziggit.dev/u/Av1d)\ **Post date:** [May 20, 2024, 3:34pm UTC](https://ziggit.dev/t/bufprint-bug-workaround/4366/4 "2024-05-20T15:34:18Z")
Thank you all! That was quite helpful. I’ve actually tried @dimdin 's solution before, but decided that it’s too hacky. Now though, it does not look _too_ bad to me. The `std.fmt.format` stuff is really interesting. Didn’t know Zig has such things in std. I’ll try to make use of it. Appreciate the help. Cheers!