What causes "APPEND + TRUNCATE not allowed" in MuntashirAkon/AppManager?

Remove the truncate flag from the mode string (use "a" alone to append, or "wt"/"w" alone to truncate then write)

APPEND + TRUNCATE not allowed

Error message

APPEND + TRUNCATE not allowed

What it means

This library's FileUtils.modeToFlag() converts a POSIX-style mode string (e.g. "rw", "rwt") into a Flag object and validates flag combinations. APPEND combined with TRUNCATE is rejected because truncating the file on open contradicts appending to its end — the kernel's O_APPEND|O_TRUNC combination would erase data the caller apparently intended to keep. The library fails fast with IllegalArgumentException rather than producing surprising file contents.

Solutions

Remove the truncate flag from the mode string (use "a" alone to append, or "wt"/"w" alone to truncate then write)
If truncate-then-append is truly wanted, open with "wt", write, then reopen with "a"
Validate mode strings before passing them, rejecting any mode that implies both append and truncate

Example fix

// before
FileUtils.modeToFlag("wat"); // IllegalArgumentException: APPEND + TRUNCATE not allowed
// after
FileUtils.modeToFlag("wa"); // append-only, or "wt" to truncate then write

Defensive patterns

Strategy: validation

Validate before calling

FileUtils.Flag f = FileUtils.modeToFlag(mode); // or pre-check the mode string
if (mode.contains("a") && mode.contains("t")) throw new IllegalArgumentException("mode must not combine append and truncate");

Type guard

boolean isConflictingMode(String mode) { return mode != null && mode.contains("a") && mode.contains("t"); }

Try / catch

try { FileUtils.modeToFlag(mode); } catch (IllegalArgumentException e) { /* fix mode string or prompt user */ }

Prevention

Never combine 'a' with 't' in mode strings
Centralize mode-string construction in one helper that rejects conflicting flags
Unit-test mode parsing for all supported flag combinations

When it happens

Trigger: Calling FileUtils.modeToFlag() (directly or via NIOFactory.openChannel / FileSystemManager openers) with a mode string that parses to both append and truncate, e.g. "ra", "wa", or a mode containing both 'a' and 't' semantics.

Common situations: Constructing mode strings programmatically by concatenating flags; porting code that used "wa" assuming truncate-then-append; users specifying custom open modes in remote file-system calls.

Understand the failure class

Background: Conflicting config options: "cannot be used together" — configuration validation errors across open-source libraries — this error's family across 162 libraries.

Related errors

AI-assisted analysis of MuntashirAkon/AppManager@0152f468fc (2026-09-12). Data as JSON: /api/errors/4fc4d9555546c24f. Report an issue: GitHub.

Appendix: source

Thrown at libcore/io/src/main/java/io/github/muntashirakon/io/FileUtils.java:132

        } else {
            throw new IllegalArgumentException("Bad mode: " + mode);
        }
        if ((mode & MODE_CREATE) == MODE_CREATE) {
            f.create = true;
        }
        if ((mode & MODE_TRUNCATE) == MODE_TRUNCATE) {
            f.truncate = true;
        }
        if ((mode & MODE_APPEND) == MODE_APPEND) {
            f.append = true;
        }

        // Validate flags
        if (f.append && f.read) {
            throw new IllegalArgumentException("READ + APPEND not allowed");
        }
        if (f.append && f.truncate) {
            throw new IllegalArgumentException("APPEND + TRUNCATE not allowed");
        }

        return f;
    }

    @RequiresApi(api = 28)
    static long splice(
            FileDescriptor fdIn, Int64Ref offIn,
            FileDescriptor fdOut, Int64Ref offOut,
            long len, int flags) throws ErrnoException {
        try {
            if (splice == null) {
                splice = Os.class.getMethod("splice",
                        FileDescriptor.class, Int64Ref.class,
                        FileDescriptor.class, Int64Ref.class,
                        long.class, int.class);
            }
            return (long) splice.invoke(null, fdIn, offIn, fdOut, offOut, len, flags);

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