U_ILLEGAL_ARGUMENT_ERROR

U_ILLEGAL_ARGUMENT_ERROR

Error message

icupkg: \"%s\" is not recognized as a package filename (must end with .dat)\n

What it means

In extractPackageName (package.cpp/icupkg), the supplied filename's basename is checked: its length minus 4 must be positive and the last 4 chars must be '.dat'. If not, icupkg rejects the name as not a package file and exits with U_ILLEGAL_ARGUMENT_ERROR. The package name is the basename without the .dat extension.

Source

Thrown at deps/icu-small/source/tools/toolutil/package.cpp:186

        return -1;
    }

    return makeTypeEnum(pInfo->charsetFamily, static_cast<UBool>(pInfo->isBigEndian));
}

// file handling ----------------------------------------------------------- ***

static void
extractPackageName(const char *filename,
                   char pkg[], int32_t capacity) {
    const char *basename;
    int32_t len;

    basename=findBasename(filename);
    len = static_cast<int32_t>(strlen(basename)) - 4; /* -4: subtract the length of ".dat" */

    if(len<=0 || 0!=strcmp(basename+len, ".dat")) {
        fprintf(stderr, "icupkg: \"%s\" is not recognized as a package filename (must end with .dat)\n",
                         basename);
        exit(U_ILLEGAL_ARGUMENT_ERROR);
    }

    if(len>=capacity) {
        fprintf(stderr, "icupkg: the package name \"%s\" is too long (>=%ld)\n",
                         basename, static_cast<long>(capacity));
        exit(U_ILLEGAL_ARGUMENT_ERROR);
    }

    memcpy(pkg, basename, len);
    pkg[len]=0;
}

static int32_t
getFileLength(FILE *f) {
    int32_t length;

View on GitHub (pinned to 1b2de5e052)

Solutions

Ensure the argument ends with '.dat' (e.g. icudt67l.dat).
Rename or regenerate the package file so it carries the .dat suffix.
Double-check the script that builds the icupkg command line.
If packaging custom data, use icupkg -a to build a correctly-named .dat.

Example fix

# before: wrong extension
icupkg -l mydata.res

# after: .dat package filename
icupkg -l mydata.dat

Defensive patterns

Strategy: type-guard

Validate before calling

// Reject filenames that won't pass extractPackageName before calling icupkg.
#include <string.h>
bool isDatPackage(const char *fn) {
    size_t n = strlen(fn);
    return n > 4 && strcmp(fn + n - 4, ".dat") == 0;
}

Type guard

// Type guard: narrows a string to a valid icupkg .dat package filename.
function isIcuDatPackage(fn: string): fn is string {
  return fn.length > 4 && fn.endsWith('.dat');
}

Prevention

Always pass a .dat filename to icupkg.
Use icupkg -a to build packages with the correct extension.
Guard the filename with a suffix check before invoking icupkg.
Double-check generated command lines for dropped extensions.

When it happens

Trigger: Calling icupkg with a filename whose basename does not end in '.dat', or whose name is exactly '.dat' or shorter (len<=0 after subtracting 4). The check runs before any package content is read.

Common situations: Passing a .zip, .res, or source file to icupkg by mistake; a script that drops the .dat extension; a typo in the filename argument.

Related errors

AI-assisted analysis of nodejs/node@1b2de5e052 (2026-08-13). Data as JSON: /api/errors/52c913049eb88ce6. Report an issue: GitHub.