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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  // IntegerType is here for the purposes of documentation only and is not actually
    18  // part of the unsafe package. It represents any arbitrary integer type.
    19  type IntegerType int
    20  
    21  // Pointer represents a pointer to an arbitrary type. There are four special operations
    22  // available for type Pointer that are not available for other types:
    23  //	- A pointer value of any type can be converted to a Pointer.
    24  //	- A Pointer can be converted to a pointer value of any type.
    25  //	- A uintptr can be converted to a Pointer.
    26  //	- A Pointer can be converted to a uintptr.
    27  // Pointer therefore allows a program to defeat the type system and read and write
    28  // arbitrary memory. It should be used with extreme care.
    29  //
    30  // The following patterns involving Pointer are valid.
    31  // Code not using these patterns is likely to be invalid today
    32  // or to become invalid in the future.
    33  // Even the valid patterns below come with important caveats.
    34  //
    35  // Running "go vet" can help find uses of Pointer that do not conform to these patterns,
    36  // but silence from "go vet" is not a guarantee that the code is valid.
    37  //
    38  // (1) Conversion of a *T1 to Pointer to *T2.
    39  //
    40  // Provided that T2 is no larger than T1 and that the two share an equivalent
    41  // memory layout, this conversion allows reinterpreting data of one type as
    42  // data of another type. An example is the implementation of
    43  // math.Float64bits:
    44  //
    45  //	func Float64bits(f float64) uint64 {
    46  //		return *(*uint64)(unsafe.Pointer(&f))
    47  //	}
    48  //
    49  // (2) Conversion of a Pointer to a uintptr (but not back to Pointer).
    50  //
    51  // Converting a Pointer to a uintptr produces the memory address of the value
    52  // pointed at, as an integer. The usual use for such a uintptr is to print it.
    53  //
    54  // Conversion of a uintptr back to Pointer is not valid in general.
    55  //
    56  // A uintptr is an integer, not a reference.
    57  // Converting a Pointer to a uintptr creates an integer value
    58  // with no pointer semantics.
    59  // Even if a uintptr holds the address of some object,
    60  // the garbage collector will not update that uintptr's value
    61  // if the object moves, nor will that uintptr keep the object
    62  // from being reclaimed.
    63  //
    64  // The remaining patterns enumerate the only valid conversions
    65  // from uintptr to Pointer.
    66  //
    67  // (3) Conversion of a Pointer to a uintptr and back, with arithmetic.
    68  //
    69  // If p points into an allocated object, it can be advanced through the object
    70  // by conversion to uintptr, addition of an offset, and conversion back to Pointer.
    71  //
    72  //	p = unsafe.Pointer(uintptr(p) + offset)
    73  //
    74  // The most common use of this pattern is to access fields in a struct
    75  // or elements of an array:
    76  //
    77  //	// equivalent to f := unsafe.Pointer(&s.f)
    78  //	f := unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f))
    79  //
    80  //	// equivalent to e := unsafe.Pointer(&x[i])
    81  //	e := unsafe.Pointer(uintptr(unsafe.Pointer(&x[0])) + i*unsafe.Sizeof(x[0]))
    82  //
    83  // It is valid both to add and to subtract offsets from a pointer in this way.
    84  // It is also valid to use &^ to round pointers, usually for alignment.
    85  // In all cases, the result must continue to point into the original allocated object.
    86  //
    87  // Unlike in C, it is not valid to advance a pointer just beyond the end of
    88  // its original allocation:
    89  //
    90  //	// INVALID: end points outside allocated space.
    91  //	var s thing
    92  //	end = unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Sizeof(s))
    93  //
    94  //	// INVALID: end points outside allocated space.
    95  //	b := make([]byte, n)
    96  //	end = unsafe.Pointer(uintptr(unsafe.Pointer(&b[0])) + uintptr(n))
    97  //
    98  // Note that both conversions must appear in the same expression, with only
    99  // the intervening arithmetic between them:
   100  //
   101  //	// INVALID: uintptr cannot be stored in variable
   102  //	// before conversion back to Pointer.
   103  //	u := uintptr(p)
   104  //	p = unsafe.Pointer(u + offset)
   105  //
   106  // Note that the pointer must point into an allocated object, so it may not be nil.
   107  //
   108  //	// INVALID: conversion of nil pointer
   109  //	u := unsafe.Pointer(nil)
   110  //	p := unsafe.Pointer(uintptr(u) + offset)
   111  //
   112  // (4) Conversion of a Pointer to a uintptr when calling syscall.Syscall.
   113  //
   114  // The Syscall functions in package syscall pass their uintptr arguments directly
   115  // to the operating system, which then may, depending on the details of the call,
   116  // reinterpret some of them as pointers.
   117  // That is, the system call implementation is implicitly converting certain arguments
   118  // back from uintptr to pointer.
   119  //
   120  // If a pointer argument must be converted to uintptr for use as an argument,
   121  // that conversion must appear in the call expression itself:
   122  //
   123  //	syscall.Syscall(SYS_READ, uintptr(fd), uintptr(unsafe.Pointer(p)), uintptr(n))
   124  //
   125  // The compiler handles a Pointer converted to a uintptr in the argument list of
   126  // a call to a function implemented in assembly by arranging that the referenced
   127  // allocated object, if any, is retained and not moved until the call completes,
   128  // even though from the types alone it would appear that the object is no longer
   129  // needed during the call.
   130  //
   131  // For the compiler to recognize this pattern,
   132  // the conversion must appear in the argument list:
   133  //
   134  //	// INVALID: uintptr cannot be stored in variable
   135  //	// before implicit conversion back to Pointer during system call.
   136  //	u := uintptr(unsafe.Pointer(p))
   137  //	syscall.Syscall(SYS_READ, uintptr(fd), u, uintptr(n))
   138  //
   139  // (5) Conversion of the result of reflect.Value.Pointer or reflect.Value.UnsafeAddr
   140  // from uintptr to Pointer.
   141  //
   142  // Package reflect's Value methods named Pointer and UnsafeAddr return type uintptr
   143  // instead of unsafe.Pointer to keep callers from changing the result to an arbitrary
   144  // type without first importing "unsafe". However, this means that the result is
   145  // fragile and must be converted to Pointer immediately after making the call,
   146  // in the same expression:
   147  //
   148  //	p := (*int)(unsafe.Pointer(reflect.ValueOf(new(int)).Pointer()))
   149  //
   150  // As in the cases above, it is invalid to store the result before the conversion:
   151  //
   152  //	// INVALID: uintptr cannot be stored in variable
   153  //	// before conversion back to Pointer.
   154  //	u := reflect.ValueOf(new(int)).Pointer()
   155  //	p := (*int)(unsafe.Pointer(u))
   156  //
   157  // (6) Conversion of a reflect.SliceHeader or reflect.StringHeader Data field to or from Pointer.
   158  //
   159  // As in the previous case, the reflect data structures SliceHeader and StringHeader
   160  // declare the field Data as a uintptr to keep callers from changing the result to
   161  // an arbitrary type without first importing "unsafe". However, this means that
   162  // SliceHeader and StringHeader are only valid when interpreting the content
   163  // of an actual slice or string value.
   164  //
   165  //	var s string
   166  //	hdr := (*reflect.StringHeader)(unsafe.Pointer(&s)) // case 1
   167  //	hdr.Data = uintptr(unsafe.Pointer(p))              // case 6 (this case)
   168  //	hdr.Len = n
   169  //
   170  // In this usage hdr.Data is really an alternate way to refer to the underlying
   171  // pointer in the string header, not a uintptr variable itself.
   172  //
   173  // In general, reflect.SliceHeader and reflect.StringHeader should be used
   174  // only as *reflect.SliceHeader and *reflect.StringHeader pointing at actual
   175  // slices or strings, never as plain structs.
   176  // A program should not declare or allocate variables of these struct types.
   177  //
   178  //	// INVALID: a directly-declared header will not hold Data as a reference.
   179  //	var hdr reflect.StringHeader
   180  //	hdr.Data = uintptr(unsafe.Pointer(p))
   181  //	hdr.Len = n
   182  //	s := *(*string)(unsafe.Pointer(&hdr)) // p possibly already lost
   183  //
   184  type Pointer *ArbitraryType
   185  
   186  // Sizeof takes an expression x of any type and returns the size in bytes
   187  // of a hypothetical variable v as if v was declared via var v = x.
   188  // The size does not include any memory possibly referenced by x.
   189  // For instance, if x is a slice, Sizeof returns the size of the slice
   190  // descriptor, not the size of the memory referenced by the slice.
   191  // The return value of Sizeof is a Go constant.
   192  func Sizeof(x ArbitraryType) uintptr
   193  
   194  // Offsetof returns the offset within the struct of the field represented by x,
   195  // which must be of the form structValue.field. In other words, it returns the
   196  // number of bytes between the start of the struct and the start of the field.
   197  // The return value of Offsetof is a Go constant.
   198  func Offsetof(x ArbitraryType) uintptr
   199  
   200  // Alignof takes an expression x of any type and returns the required alignment
   201  // of a hypothetical variable v as if v was declared via var v = x.
   202  // It is the largest value m such that the address of v is always zero mod m.
   203  // It is the same as the value returned by reflect.TypeOf(x).Align().
   204  // As a special case, if a variable s is of struct type and f is a field
   205  // within that struct, then Alignof(s.f) will return the required alignment
   206  // of a field of that type within a struct. This case is the same as the
   207  // value returned by reflect.TypeOf(s.f).FieldAlign().
   208  // The return value of Alignof is a Go constant.
   209  func Alignof(x ArbitraryType) uintptr
   210  
   211  // The function Add adds len to ptr and returns the updated pointer
   212  // Pointer(uintptr(ptr) + uintptr(len)).
   213  // The len argument must be of integer type or an untyped constant.
   214  // A constant len argument must be representable by a value of type int;
   215  // if it is an untyped constant it is given type int.
   216  // The rules for valid uses of Pointer still apply.
   217  func Add(ptr Pointer, len IntegerType) Pointer
   218  
   219  // The function Slice returns a slice whose underlying array starts at ptr
   220  // and whose length and capacity are len.
   221  // Slice(ptr, len) is equivalent to
   222  //
   223  //	(*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
   224  //
   225  // except that, as a special case, if ptr is nil and len is zero,
   226  // Slice returns nil.
   227  //
   228  // The len argument must be of integer type or an untyped constant.
   229  // A constant len argument must be non-negative and representable by a value of type int;
   230  // if it is an untyped constant it is given type int.
   231  // At run time, if len is negative, or if ptr is nil and len is not zero,
   232  // a run-time panic occurs.
   233  func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
   234  

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