In C array of structs, what are structs initialized to?

Question

In a dynamically created array of structs, what does every entry of the struct get initialized to?

Details:
If we create a dynamic array of floats like so:

float* arr = ( float* ) malloc ( 100 * sizeof ( float ) );

then the array can be populated by anything (see here). But I am having trouble wrapping my head around what happens when we substitute structs in for floats like so

typedef struct
{
   float x = 123.456;
} foo;

foo* arr = ( foo* ) malloc ( 100 * sizeof ( foo ) );

Are all the entries of the array arr created with fully initialized foo structs or do I have to go around and manually update the x value?

Was that edit really necessary? It comes across as an attempt to gain rep points. — puk, Jul 04 '12 at 00:17

score 3 · Accepted Answer · answered Jul 03 '12 at 16:32

3

typedef struct
{
   float x = 123.456;
} foo;

You cannot have default values in structure types. This is not valid C code.

Objects allocated by malloc have an indeterminate value.

answered Jul 03 '12 at 16:32

ouah

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This is tagged as C but even in C11 it is not possible. – ouah Jul 03 '12 at 16:35
Thank you I was not aware of that. – puk Jul 03 '12 at 16:54

score 1 · Answer 2 · answered Jul 03 '12 at 16:33

1

There is no difference between floats and structs. You need to manually initialize every dynamically allocated memory region.

answered Jul 03 '12 at 16:33

perreal

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score 1 · Answer 3 · answered Jul 03 '12 at 16:39

The fact that the float case can be "initialized" to anything should provide the answer you need - it's not in fact initialized, but it may contain anything, because no initialization is done. The same is true with a struct - no initialization is done on allocation, so the (quite likely) garbage contents of the memory segment returned is what you'll get. If you care about the contents of your newly allocated memory (and you probably should), you need to initialize it explicitly yourself.

In C array of structs, what are structs initialized to?

3 Answers3