CODEWITHSUNDEEP
c++
Rom Grk
Rom Grk

Reputation: 536

Why can't I assign an int to the union member of a struct member?

I'm trying to assign a value to the member of an union which is member of a struct, but the value doesn't seem to be assigned. I've stepped through the program, and right after the assignation, len_param.data value seems to be some sort of pointer (see GDB output at the end of the post)

Also, if I printf, it runs normally:

len_param.data.v_int = 4;

// If I uncomment this line, IT RUNS FINE! WHY?
// printf("len_param.data.v_int: %i \n", len_param.data.v_int);

The type of len_param is as follow:

struct Parameter {
    enum {
        NORMAL, ARRAY, SKIP,
    } type;

    GIDirection direction;

    GIArgument data;
};

And GIArgument definition:

union _GIArgument
{
  gboolean v_boolean;
  gint8    v_int8;
  guint8   v_uint8;
  gint16   v_int16;
  guint16  v_uint16;
  gint32   v_int32;
  guint32  v_uint32;
  gint64   v_int64;
  guint64  v_uint64;
  gfloat   v_float;
  gdouble  v_double;
  gshort   v_short;
  gushort  v_ushort;
  gint     v_int;
  guint    v_uint;
  glong    v_long;
  gulong   v_ulong;
  gssize   v_ssize;
  gsize    v_size;
  gchar *  v_string;
  gpointer v_pointer;
};
typedef _GIArgument GIArgument;

The complete file can be found here: https://gist.github.com/romgrk/642388914a9ff412eb5683fca44009d7#file-function-cc-L255

And my GDB output when I step at that line is:

Thread 1 "node" hit Breakpoint 1, GNodeJS::FunctionInvoker (info=...) at ../src/function.cc:256
256                     len_param.data.v_int = GetV8ArrayLength(info[in_arg]);
(gdb) step
GNodeJS::GetV8ArrayLength (value=...) at ../src/function.cc:25
25  static int GetV8ArrayLength (Local<Value> value) {
(gdb) finish
Run till exit from #0  GNodeJS::GetV8ArrayLength (value=...) at ../src/function.cc:25
GNodeJS::FunctionInvoker (info=...) at ../src/function.cc:260
260                     callable_arg_values[length_i].v_pointer = &len_param.data;
Value returned is $1 = 4
(gdb) p len_param.data.v_int
$2 = -17928
(gdb) p len_param
$3 = {
  type = GNodeJS::Parameter::SKIP, 
  direction = GI_DIRECTION_INOUT, 
  data = {
    v_boolean = -17928, 
    v_int8 = -8 '\370', 
    v_uint8 = 248 '\370', 
    v_int16 = -17928, 
    v_uint16 = 47608, 
    v_int32 = -17928, 
    v_uint32 = 4294949368, 
    v_int64 = 140737488337400, 
    v_uint64 = 140737488337400, 
    v_float = -nan(0x7fb9f8), 
    v_double = 6.9533558069492434e-310, 
    v_short = -17928, 
    v_ushort = 47608, 
    v_int = -17928, 
    v_uint = 4294949368, 
    v_long = 140737488337400, 
    v_ulong = 140737488337400, 
    v_ssize = 140737488337400, 
    v_size = 140737488337400, 
    v_string = 0x7fffffffb9f8 "\t\357\304\303J\a", 
    v_pointer = 0x7fffffffb9f8
  }
}

Upvotes: 1

Views: 188

Answers (1)

Michael Burr
Michael Burr

Reputation: 340218

The problem might be that nothing is ever done with len_param.data except to take its address just before it goes out of scope and the lifetime of len_param expires. So the compiler figures there's no point storing anything there.

Here's the code snippet where len_param is defined, used and dies (with unnecessary code elided and some annotation comments added):

if (param.type == Parameter::ARRAY) {

    // ... 

    Parameter len_param = call_parameters[length_i];    // len_param is defined

    if (len_param.direction == GI_DIRECTION_IN) {
        // ...
    }
    else if (len_param.direction == GI_DIRECTION_INOUT) {

        len_param.data.v_int = GetV8ArrayLength(info[in_arg]);

        callable_arg_values[length_i].v_pointer = &len_param.data;
    }
}       // len_param goes out of scope, so it's no longer alive
        // and the pointer that got placed in `callable_arg_values[length_i].v_pointer`
        // is pointing to garbage

The root problem is that the pointer in callable_arg_values[length_i].v_pointer isn't valid moments after it gets stored.

Upvotes: 1

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