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

#cgo LDFLAGS: -framework CoreFoundation -framework Security
#include <TargetConditionals.h>
#include <CoreFoundation/CoreFoundation.h>
#include <Security/Security.h>
#if TARGET_OS_IPHONE == 0 && __MAC_OS_X_VERSION_MAX_ALLOWED < 101200
  typedef CFStringRef SecKeyAlgorithm;
  static CFDataRef SecKeyCreateSignature(SecKeyRef key, SecKeyAlgorithm algorithm, CFDataRef dataToSign, CFErrorRef *error){return NULL;}

	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
}

type eofReader struct {
	buf []byte
}

func (r *eofReader) Read(p []byte) (int, error) {
	read := copy(p, r.buf)
	r.buf = r.buf[read:]

	switch read {
	case 0, len(r.buf):
		// As allowed in the documentation, this will return io.EOF
		// in the same call that consumes the last of the data.
		// https://godoc.org/io#Reader
		return read, io.EOF
	}

	return read, nil
}

</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.

	MOVQ (AX), M0  // 0f6f00
	MOVQ M0, 8(SP) // 0f7f442408
	MOVQ 8(SP), M0 // 0f6f442408
	MOVQ M0, (AX)  // 0f7f00
	MOVQ M0, (BX)  // 0f7f03
	// On non-64bit arch, Go asm allowed uint32 offsets instead of int32.
	// These tests check that property for backwards-compatibility.
	MOVL 2147483648(AX), AX  // 8b8000000080
	MOVL -2147483648(AX), AX // 8b8000000080
	ADDL 2147483648(AX), AX  // 038000000080

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.

	b.Write(buf[0:1])
	var cap0 int
	for i := 0; i < 5<<10; i++ {
		b.Write(buf)
		b.Read(buf)
		if i == 0 {
			cap0 = b.Cap()
		}
	}
	cap1 := b.Cap()
	// (*Buffer).grow allows for 2x capacity slop before sliding,
	// so set our error threshold at 3x.
	if cap1 > cap0*3 {
		t.Errorf("buffer cap = %d; too big (grew from %d)", cap1, cap0)
	}
}

func BenchmarkWriteByte(b *testing.B) {

</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.