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Source file src/unsafe/unsafe.go

Documentation: unsafe

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  /*
     6  	Package unsafe contains operations that step around the type safety of Go programs.
     7  
     8  	Packages that import unsafe may be non-portable and are not protected by the
     9  	Go 1 compatibility guidelines.
    10  */
    11  package unsafe
    12  
    13  // ArbitraryType is here for the purposes of documentation only and is not actually
    14  // part of the unsafe package. It represents the type of an arbitrary Go expression.
    15  type ArbitraryType int
    16  
    17  // Pointer represents a pointer to an arbitrary type. There are four special operations
    18  // available for type Pointer that are not available for other types:
    19  //	- A pointer value of any type can be converted to a Pointer.
    20  //	- A Pointer can be converted to a pointer value of any type.
    21  //	- A uintptr can be converted to a Pointer.
    22  //	- A Pointer can be converted to a uintptr.
    23  // Pointer therefore allows a program to defeat the type system and read and write
    24  // arbitrary memory. It should be used with extreme care.
    25  //
    26  // The following patterns involving Pointer are valid.
    27  // Code not using these patterns is likely to be invalid today
    28  // or to become invalid in the future.
    29  // Even the valid patterns below come with important caveats.
    30  //
    31  // Running "go vet" can help find uses of Pointer that do not conform to these patterns,
    32  // but silence from "go vet" is not a guarantee that the code is valid.
    33  //
    34  // (1) Conversion of a *T1 to Pointer to *T2.
    35  //
    36  // Provided that T2 is no larger than T1 and that the two share an equivalent
    37  // memory layout, this conversion allows reinterpreting data of one type as
    38  // data of another type. An example is the implementation of
    39  // math.Float64bits:
    40  //
    41  //	func Float64bits(f float64) uint64 {
    42  //		return *(*uint64)(unsafe.Pointer(&f))
    43  //	}
    44  //
    45  // (2) Conversion of a Pointer to a uintptr (but not back to Pointer).
    46  //
    47  // Converting a Pointer to a uintptr produces the memory address of the value
    48  // pointed at, as an integer. The usual use for such a uintptr is to print it.
    49  //
    50  // Conversion of a uintptr back to Pointer is not valid in general.
    51  //
    52  // A uintptr is an integer, not a reference.
    53  // Converting a Pointer to a uintptr creates an integer value
    54  // with no pointer semantics.
    55  // Even if a uintptr holds the address of some object,
    56  // the garbage collector will not update that uintptr's value
    57  // if the object moves, nor will that uintptr keep the object
    58  // from being reclaimed.
    59  //
    60  // The remaining patterns enumerate the only valid conversions
    61  // from uintptr to Pointer.
    62  //
    63  // (3) Conversion of a Pointer to a uintptr and back, with arithmetic.
    64  //
    65  // If p points into an allocated object, it can be advanced through the object
    66  // by conversion to uintptr, addition of an offset, and conversion back to Pointer.
    67  //
    68  //	p = unsafe.Pointer(uintptr(p) + offset)
    69  //
    70  // The most common use of this pattern is to access fields in a struct
    71  // or elements of an array:
    72  //
    73  //	// equivalent to f := unsafe.Pointer(&s.f)
    74  //	f := unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f))
    75  //
    76  //	// equivalent to e := unsafe.Pointer(&x[i])
    77  //	e := unsafe.Pointer(uintptr(unsafe.Pointer(&x[0])) + i*unsafe.Sizeof(x[0]))
    78  //
    79  // It is valid both to add and to subtract offsets from a pointer in this way.
    80  // It is also valid to use &^ to round pointers, usually for alignment.
    81  // In all cases, the result must continue to point into the original allocated object.
    82  //
    83  // Unlike in C, it is not valid to advance a pointer just beyond the end of
    84  // its original allocation:
    85  //
    86  //	// INVALID: end points outside allocated space.
    87  //	var s thing
    88  //	end = unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Sizeof(s))
    89  //
    90  //	// INVALID: end points outside allocated space.
    91  //	b := make([]byte, n)
    92  //	end = unsafe.Pointer(uintptr(unsafe.Pointer(&b[0])) + uintptr(n))
    93  //
    94  // Note that both conversions must appear in the same expression, with only
    95  // the intervening arithmetic between them:
    96  //
    97  //	// INVALID: uintptr cannot be stored in variable
    98  //	// before conversion back to Pointer.
    99  //	u := uintptr(p)
   100  //	p = unsafe.Pointer(u + offset)
   101  //
   102  // (4) Conversion of a Pointer to a uintptr when calling syscall.Syscall.
   103  //
   104  // The Syscall functions in package syscall pass their uintptr arguments directly
   105  // to the operating system, which then may, depending on the details of the call,
   106  // reinterpret some of them as pointers.
   107  // That is, the system call implementation is implicitly converting certain arguments
   108  // back from uintptr to pointer.
   109  //
   110  // If a pointer argument must be converted to uintptr for use as an argument,
   111  // that conversion must appear in the call expression itself:
   112  //
   113  //	syscall.Syscall(SYS_READ, uintptr(fd), uintptr(unsafe.Pointer(p)), uintptr(n))
   114  //
   115  // The compiler handles a Pointer converted to a uintptr in the argument list of
   116  // a call to a function implemented in assembly by arranging that the referenced
   117  // allocated object, if any, is retained and not moved until the call completes,
   118  // even though from the types alone it would appear that the object is no longer
   119  // needed during the call.
   120  //
   121  // For the compiler to recognize this pattern,
   122  // the conversion must appear in the argument list:
   123  //
   124  //	// INVALID: uintptr cannot be stored in variable
   125  //	// before implicit conversion back to Pointer during system call.
   126  //	u := uintptr(unsafe.Pointer(p))
   127  //	syscall.Syscall(SYS_READ, uintptr(fd), u, uintptr(n))
   128  //
   129  // (5) Conversion of the result of reflect.Value.Pointer or reflect.Value.UnsafeAddr
   130  // from uintptr to Pointer.
   131  //
   132  // Package reflect's Value methods named Pointer and UnsafeAddr return type uintptr
   133  // instead of unsafe.Pointer to keep callers from changing the result to an arbitrary
   134  // type without first importing "unsafe". However, this means that the result is
   135  // fragile and must be converted to Pointer immediately after making the call,
   136  // in the same expression:
   137  //
   138  //	p := (*int)(unsafe.Pointer(reflect.ValueOf(new(int)).Pointer()))
   139  //
   140  // As in the cases above, it is invalid to store the result before the conversion:
   141  //
   142  //	// INVALID: uintptr cannot be stored in variable
   143  //	// before conversion back to Pointer.
   144  //	u := reflect.ValueOf(new(int)).Pointer()
   145  //	p := (*int)(unsafe.Pointer(u))
   146  //
   147  // (6) Conversion of a reflect.SliceHeader or reflect.StringHeader Data field to or from Pointer.
   148  //
   149  // As in the previous case, the reflect data structures SliceHeader and StringHeader
   150  // declare the field Data as a uintptr to keep callers from changing the result to
   151  // an arbitrary type without first importing "unsafe". However, this means that
   152  // SliceHeader and StringHeader are only valid when interpreting the content
   153  // of an actual slice or string value.
   154  //
   155  //	var s string
   156  //	hdr := (*reflect.StringHeader)(unsafe.Pointer(&s)) // case 1
   157  //	hdr.Data = uintptr(unsafe.Pointer(p))              // case 6 (this case)
   158  //	hdr.Len = n
   159  //
   160  // In this usage hdr.Data is really an alternate way to refer to the underlying
   161  // pointer in the string header, not a uintptr variable itself.
   162  //
   163  // In general, reflect.SliceHeader and reflect.StringHeader should be used
   164  // only as *reflect.SliceHeader and *reflect.StringHeader pointing at actual
   165  // slices or strings, never as plain structs.
   166  // A program should not declare or allocate variables of these struct types.
   167  //
   168  //	// INVALID: a directly-declared header will not hold Data as a reference.
   169  //	var hdr reflect.StringHeader
   170  //	hdr.Data = uintptr(unsafe.Pointer(p))
   171  //	hdr.Len = n
   172  //	s := *(*string)(unsafe.Pointer(&hdr)) // p possibly already lost
   173  //
   174  type Pointer *ArbitraryType
   175  
   176  // Sizeof takes an expression x of any type and returns the size in bytes
   177  // of a hypothetical variable v as if v was declared via var v = x.
   178  // The size does not include any memory possibly referenced by x.
   179  // For instance, if x is a slice, Sizeof returns the size of the slice
   180  // descriptor, not the size of the memory referenced by the slice.
   181  func Sizeof(x ArbitraryType) uintptr
   182  
   183  // Offsetof returns the offset within the struct of the field represented by x,
   184  // which must be of the form structValue.field. In other words, it returns the
   185  // number of bytes between the start of the struct and the start of the field.
   186  func Offsetof(x ArbitraryType) uintptr
   187  
   188  // Alignof takes an expression x of any type and returns the required alignment
   189  // of a hypothetical variable v as if v was declared via var v = x.
   190  // It is the largest value m such that the address of v is always zero mod m.
   191  // It is the same as the value returned by reflect.TypeOf(x).Align().
   192  // As a special case, if a variable s is of struct type and f is a field
   193  // within that struct, then Alignof(s.f) will return the required alignment
   194  // of a field of that type within a struct. This case is the same as the
   195  // value returned by reflect.TypeOf(s.f).FieldAlign().
   196  func Alignof(x ArbitraryType) uintptr
   197  

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