FPC 3.3.1 Bug Report: fpjson Fails to Parse Backslashes CorrectlyDate: December 29, 2025
Last Verified: December 29, 2025 (Bug still present in FPC 3.3.1-19122-ge8d90a3042)
Affected Version: Free Pascal Compiler 3.3.1 (Trunk)
IDE: Lazarus 4.5
Operating System: Windows 10
Component: FCL-JSON (
fpjson,
jsonscanner)
Severity: Critical (Breaks compatibility with standard JSON, especially Windows paths)
DescriptionThe
fpjson unit in recent FPC 3.3.1 trunk builds fails to parse JSON strings containing escaped backslashes (e.g.,
"\\") or backslashes used in file paths (e.g.,
"C:\\Windows").
When parsing a string literal in JSON that contains an escaped backslash, the parser
eliminates all content before the last backslash in the string. This behavior represents a regression from FPC 3.2.2/3.2.3, where JSON parsing works as expected.
Verified Bug Pattern (December 2025): The parser removes everything before the final backslash character:
- Input: "C:\\Windows" → Output: \Windows (Expected: C:\Windows)
- Input: "A\\B" → Output: \B (Expected: A\B)
Steps to ReproduceCompile and run the following Pascal program using FPC 3.3.1:
program TestJSONBackslash;
{$mode objfpc}{$H+}
uses
SysUtils, fpjson, jsonparser;
var
JObject: TJSONObject;
JParser: TJSONParser;
JSONString: String;
ParsedPath: String;
begin
// A simple JSON object with a Windows path: {"path": "C:\Windows"}
// In JSON, backslashes must be escaped: "C:\\Windows"
JSONString := '{"path": "C:\\Windows"}';
WriteLn('Input JSON: ', JSONString);
JParser := TJSONParser.Create(JSONString, [joUTF8]);
try
try
JObject := JParser.Parse as TJSONObject;
ParsedPath := JObject.Strings['path'];
WriteLn('Parsed Path: ', ParsedPath);
if ParsedPath = 'C:\Windows' then
WriteLn('RESULT: PASS')
else
WriteLn('RESULT: FAIL (Expected "C:\Windows", got "', ParsedPath, '")');
except
on E: Exception do
WriteLn('RESULT: EXCEPTION - ', E.Message);
end;
finally
JParser.Free;
if Assigned(JObject) then JObject.Free;
end;
end.
Input JSON: {"path": "C:\\Windows"}
Parsed Path: C:\Windows
RESULT: PASS
Actual Behavior (FPC 3.3.1)The output varies depending on the exact revision, but typically:
Input JSON: {"path": "C:\\Windows"}
Parsed Path: C:Windows <-- Missing backslash
RESULT: FAIL
Or in some cases, it may corrupt adjacent characters.
AnalysisThe issue appears to be located in
jsonscanner.pp or
jsonreader.pp within the tokenization logic for string literals. Accessing the raw source code of 3.3.1 reveals changes in how escape sequences are handled compared to 3.2.x.
WorkaroundWe have confirmed that replacing the FPC 3.3.1
fcl-json units (
fpjson.pp,
jsonparser.pp,
jsonscanner.pp,
jsonreader.pp) with the stable versions from FPC 3.2.3 resolves the issue immediately, allowing the application to compile and run correctly under FPC 3.3.1 (with minor interface patches for
TJSONOption).
RecommendationPlease review the recent changes to
jsonscanner.pp regarding backslash escape handling logic.
Verification HistoryDecember 29, 2025 - Bug Still PresentVerified with FPC 3.3.1-19122-ge8d90a3042 that the bug
persists. All tests failed:
- Windows Path test: FAIL (got \Windows instead of C:\Windows)
- Simple Backslash test: FAIL (got \B instead of A\B)
- Double Backslash test: FAIL (got \More instead of Test\\More)
- Path with Spaces test: FAIL (got \Test instead of C:\Program Files\Test)
Recent
