set CGO_CFLAGS_DISALLOW to a regular expression matching arguments
that must be disallowed. In both cases the regular expression must match
a full argument: to allow -mfoo=bar, use CGO_CFLAGS_ALLOW='-mfoo.*',
not just CGO_CFLAGS_ALLOW='-mfoo'. Similarly named variables control
the allowed CPPFLAGS, CXXFLAGS, FFLAGS, and LDFLAGS.

Also for security reasons, only a limited set of characters are

	octBits := uint(n-1) * 3
	return x >= 0 && (n >= 22 || x < 1<<octBits)
}

// parsePAXTime takes a string of the form %d.%d as described in the PAX
// specification. Note that this implementation allows for negative timestamps,
// which is allowed for by the PAX specification, but not always portable.
func parsePAXTime(s string) (time.Time, error) {
	const maxNanoSecondDigits = 9

	// Split string into seconds and sub-seconds parts.

	dataAddr      map[string]int64 // Most recent address for DATA for this symbol.
	isJump        bool             // Instruction being assembled is a jump.
	allowABI      bool             // Whether ABI selectors are allowed.
	pkgPrefix     string           // Prefix to add to local symbols.
	errorWriter   io.Writer
}

type Patch struct {
	addr  *obj.Addr
	label string
}

	// Check desired format.
	if wantFormat := h.Format; wantFormat != FormatUnknown {
		if wantFormat.has(FormatPAX) && !preferPAX {
			wantFormat.mayBe(FormatUSTAR) // PAX implies USTAR allowed too
		}
		format.mayOnlyBe(wantFormat) // Set union of formats allowed and format wanted
	}
	if format == FormatUnknown {
		switch h.Format {
		case FormatUSTAR:
			err = headerError{"Format specifies USTAR", whyNoUSTAR, whyOnlyPAX, whyOnlyGNU}

//	sub-second times  |     no |       yes |        no
//	sparse files      |     no |       yes |       yes
//
// The table's upper portion shows the [Header] fields, where each format reports
// the maximum number of bytes allowed for each string field and
// the integer type used to store each numeric field
// (where timestamps are stored as the number of seconds since the Unix epoch).
//

The [GODEBUG History](#history) gives the exact defaults for each Go toolchain version.
For example, Go 1.21 introduces the `panicnil` setting,
controlling whether `panic(nil)` is allowed;
it defaults to `panicnil=0`, making `panic(nil)` a run-time error.
Using `panicnil=1` restores the behavior of Go 1.20 and earlier.

When compiling a work module or workspace that declares

</p>

<p>
Several backslash escapes allow arbitrary values to be encoded as
ASCII text.  There are four ways to represent the integer value
as a numeric constant: <code>\x</code> followed by exactly two hexadecimal
digits; <code>\u</code> followed by exactly four hexadecimal digits;
<code>\U</code> followed by exactly eight hexadecimal digits, and a
plain backslash <code>\</code> followed by exactly three octal digits.

</p>

<p>
In Go object files and binaries, the full name of a symbol is the
package path followed by a period and the symbol name:
<code>fmt.Printf</code> or <code>math/rand.Int</code>.
Because the assembler's parser treats period and slash as punctuation,
those strings cannot be used directly as identifier names.
Instead, the assembler allows the middle dot character U+00B7
and the division slash U+2215 in identifiers and rewrites them to

The patterns used by [ServeMux] allow multiple spaces matching

			for _, spec := range d.Specs {
				if s, ok := spec.(*ast.ImportSpec); ok && s.Path.Value == `"C"` {
					// Replace "C" with _ "unsafe", to keep program valid.
					// (Deleting import statement or clause is not safe if it is followed
					// in the source by an explicit semicolon.)
					f.Edit.Replace(f.offset(s.Path.Pos()), f.offset(s.Path.End()), `_ "unsafe"`)
				}
			}
		}
	}

	// Accumulate pointers to uses of C.x.