C struct padding on initialization

Question

I have a struct such as

typedef struct
{
    int a;  // Let's say this ends up being 4 bytes
    int b;  // 4 bytes
    char text[10]; // 10 bytes
} blah_t;

static blah_t myvar;

Suppose the sum of fields’ sizes is 18 bytes in blah_t, but sizeof(blah_t) is 20 due to padding.
The myvar is static, therefore it will be zero initialized.

Question:

Are padding bytes 19 and 20 guaranteed to be 0 for a static variable? If not, I would need to do memset(&myvar, 0, sizeof(blah_t)) for any memcmp of the struct to be valid – even for a static variable.
What about calloc(1, sizeof(blah_t))? Are bytes 19 and 20 guaranteed to be zero? I believe this is the case.

Community · Accepted Answer · 2017-05-23 10:29:03Z

5

From ISO C99 standard: "When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values."

Looking further, this appears to discuss some additional issues: Comparing structures in C vs C++

EDIT: not a duplicate of this question per se, but many common issues with detailed responses.

edited May 23, 2017 at 10:29

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answered Mar 13, 2013 at 0:05

Randy Howard

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6 Comments

Jonathan Leffler Over a year ago

Good to quote the standard; can you give 'chapter and verse' (which section, which paragraph)? In C2011 (ISO/IEC 9899:2011), it is in §6.2.6 Representations of types, and sub-section §6.2.6.1 General, para 6.

Randy Howard Over a year ago

Good point, I meant to and left it out. 6.2.6.1 Representations of types, paragraph 6.

B. Nadolson Over a year ago

I don't see any duplication with the C++ question you mention. C++ classes are a different animal. What I'm hoping to find out ... if somewhere in the specification it describes how static variables are initialized for this particular case.

Randy Howard Over a year ago

It explicitly says that padding values have unspecified values, so you can't really trust memcmp() to "do the right thing" portably.

afk Over a year ago

This answer could be updated for C11 changes. The padding bits are set to zero when a static variable is not initialized explicitly. I added an answer

|

afk · Accepted Answer · 2019-08-14 20:10:04Z

Randy Howard's accepted answer isn't quite accurate anymore with C11.

Are padding bytes 19 and 20 guaranteed to be 0 for a static variable?

In C11, yes, the padding bits are set to zero when a static variable is not initialized explicitly. C11 Standard, Subclause 6.7.9, paragraph 10:

. . . If an object that has static or thread storage duration is not initialized explicitly, then: . . . if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits . . .

Yes, calloc will zero the entire sizeof (including the padding) of the structure.

2 Comments

B. Nadolson Over a year ago

I want to memcmp a static struct versus a struct that has been allocated via calloc. I'm aware the behavior of padding is undefined when it comes to storage, but a statically allocated struct may or may not be considered "storage" in specification. Very technical distinction I'm looking for, yes.

Nick Desaulniers Over a year ago

doesn't matter for portability, but can matter for security. Information can be leaked from the stack for automatically allocated variables whose padding was unintialized.

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