File Alignment must be power of 2 from 0x200 to 0x10000

Error message

File Alignment must be power of 2 from 0x200 to 0x10000

What it means

The `/ALI=<n>` (file alignment) switch validates the value in main.cpp:469: it must be a power of two and within [0x200, 0x10000]. The check `(align & (align-1))` detects non-powers-of-two; values outside the PE range are also rejected. When invalid, ilasm prints this and, unless OnErrGo is set, jumps to InvalidOption.

Source

Thrown at src/coreclr/ilasm/main.cpp:472

                                    pAsm->m_wSSVersionMajor = (WORD)major;
                                if((minor >= 0)&&(minor < 0xFFFF))
                                    pAsm->m_wSSVersionMinor = (WORD)minor;
                            } else
                                goto InvalidOption;
                        }
                    }
                    else if (!_stricmp(szOpt, "ALI"))
                    {
                        WCHAR *pStr = EqualOrColon(argv[i]);
                        if(pStr == NULL) goto InvalidOption;
                        pStr++;
                        const CHAR *pFmt = ((*pStr=='0')&&(*(pStr+1) == 'x'))? "%x" : "%d";
                        NarrowForNumberParsing str{pStr};
                        if(sscanf_s(str,pFmt,&g_dwFileAlignment)!=1) goto InvalidOption;
                        if((g_dwFileAlignment & (g_dwFileAlignment-1))
                           || (g_dwFileAlignment < 0x200) || (g_dwFileAlignment > 0x10000))
                        {
                            fprintf(stderr,"\nFile Alignment must be power of 2 from 0x200 to 0x10000\n");
                            if(!pAsm->OnErrGo) goto InvalidOption;
                        }
                    }
                    else if (!_stricmp(szOpt, "FLA"))
                    {
                        WCHAR *pStr = EqualOrColon(argv[i]);
                        if(pStr == NULL) goto InvalidOption;
                        pStr++;
                        const CHAR *pFmt = ((*pStr=='0')&&(*(pStr+1) == 'x'))? "%x" : "%d";
                        NarrowForNumberParsing str{pStr};
                        if(sscanf_s(str,pFmt,&g_dwComImageFlags)!=1) goto InvalidOption;
                    }
                    else if (!_stricmp(szOpt, "BAS"))
                    {
                        WCHAR *pStr = EqualOrColon(argv[i]);
                        if(pStr == NULL) goto InvalidOption;
                        pStr++;
                        const CHAR *pFmt = ((*pStr=='0')&&(*(pStr+1) == 'x'))? "%llx" : "%lld";

View on GitHub (pinned to 290d5ab72c)

Solutions

Use a power-of-two alignment in range, e.g. `/ALI=0x200` (512), `/ALI=0x1000` (4096), or `/ALI=0x10000` (65536).
Omit `/ALI` entirely to accept the default alignment.

Example fix

// before
ilasm /ALI=0x300 foo.il
// after
ilasm /ALI=0x200 foo.il

Defensive patterns

Strategy: validation

Validate before calling

// Validate /ALI value before passing to ilasm.
function validAlignment(v) {
  return v >= 0x200 && v <= 0x10000 && (v & (v - 1)) === 0;
}
if (!validAlignment(align)) throw new Error('Bad file alignment: 0x' + align.toString(16));

Prevention

Default to omitting /ALI unless you need a specific alignment.
Prefer hex power-of-two literals (0x200, 0x1000, 0x10000).
Document the [0x200,0x10000] power-of-two rule next to any /ALI usage.

When it happens

Trigger: Passing `/ALI=` with a non power-of-two value or a value outside 512..65536 (e.g. `/ALI=0x300`, `/ALI=0x80`, `/ALI=0x20000`).

Common situations: Copying a value from another tool's docs; using a decimal where a power-of-two hex was intended; toolchain expecting a specific alignment.

Related errors

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/487e9b30ee0e4990. Report an issue: GitHub.