memset() or value initialization to zero out a struct?
Solution 1
Those two constructs a very different in their meaning. The first one uses a memset
function, which is intended to set a buffer of memory to certain value. The second to initialize an object. Let me explain it with a bit of code:
Lets assume you have a structure that has members only of POD types ("Plain Old Data" - see What are POD types in C++?)
struct POD_OnlyStruct
{
int a;
char b;
};
POD_OnlyStruct t = {}; // OK
POD_OnlyStruct t;
memset(&t, 0, sizeof t); // OK as well
In this case writing a POD_OnlyStruct t = {}
or POD_OnlyStruct t; memset(&t, 0, sizeof t)
doesn't make much difference, as the only difference we have here is the alignment bytes being set to zero-value in case of memset
used. Since you don't have access to those bytes normally, there's no difference for you.
On the other hand, since you've tagged your question as C++, let's try another example, with member types different from POD:
struct TestStruct
{
int a;
std::string b;
};
TestStruct t = {}; // OK
{
TestStruct t1;
memset(&t1, 0, sizeof t1); // ruins member 'b' of our struct
} // Application crashes here
In this case using an expression like TestStruct t = {}
is good, and using a memset
on it will lead to crash. Here's what happens if you use memset
- an object of type TestStruct
is created, thus creating an object of type std::string
, since it's a member of our structure. Next, memset
sets the memory where the object b
was located to certain value, say zero. Now, once our TestStruct object goes out of scope, it is going to be destroyed and when the turn comes to it's member std::string b
you'll see a crash, as all of that object's internal structures were ruined by the memset
.
So, the reality is, those things are very different, and although you sometimes need to memset
a whole structure to zeroes in certain cases, it's always important to make sure you understand what you're doing, and not make a mistake as in our second example.
My vote - use memset
on objects only if it is required, and use the default initialization x = {}
in all other cases.
Solution 2
Depending on the structure members, the two variants are not necessarily equivalent. memset
will set the structure to all-bits-zero whereas value initialization will initialize all members to the value zero. The C standard guarantees these to be the same only for integral types, not for floating-point values or pointers.
Also, some APIs require that the structure really be set to all-bits-zero. For instance, the Berkeley socket API uses structures polymorphically, and there it is important to really set the whole structure to zero, not just the values that are apparent. The API documentation should say whether the structure really needs to be all-bits-zero, but it might be deficient.
But if neither of these, or a similar case, applies, then it's up to you. I would, when defining the structure, prefer value initialization, as that communicates the intent more clearly. Of course, if you need to zeroize an existing structure, memset
is the only choice (well, apart from initializing each member to zero by hand, but that wouldn't normally be done, especially for large structures).
Solution 3
If your struct contains things like :
int a;
char b;
int c;
Then bytes of padding will be inserted between "b" and "c". memset() will zero those, the other way will not, so there will be 3 bytes of garbage (if your ints are 32 bits). If you intend to use your struct to read/write from a file, this might be important.
Solution 4
I would use value initialization because it looks clean and less error prone as you mentioned. I don't see any drawback in doing it.
You might rely on memset
to zero out the struct after it has been used though.
Solution 5
not that it's common, but I guess the second way also has the benefit of initializing floats to zero. While doing a memset would certainly not
sharptooth
Updated on October 20, 2020Comments
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sharptooth over 3 years
In Win32 API programming it's typical to use C
struct
s with multiple fields. Usually only a couple of them have meaningful values and all others have to be zeroed out. This can be achieved in either of the two ways:STRUCT theStruct; memset( &theStruct, 0, sizeof( STRUCT ) );
or
STRUCT theStruct = {};
The second variant looks cleaner - it's a one-liner, it doesn't have any parameters that could be mistyped and lead to an error being planted.
Does it have any drawbacks compared to the first variant? Which variant to use and why?
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qrdl over 14 yearsValues are not initialised at compile time. Compiler generates startup code that initialise all globals during program start thus at runtime. For stack variables initialisation is performed on function entry - again at runtime.
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Gregory Pakosz over 14 yearsout of curiosity, on which platform a float with all bits to zeros is not the positive zero?
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Andrew McGregor over 14 yearsSeveral old pre-IEEE-754 CPUs had strange float zeros. Non-754 math might come back yet, you never know, so it's better not to write those bugs.
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sbk over 14 years
while doing a memset would certainly not
- not entirely true. Actually, on x86 and x64 memsetting a float/double to zero will set it to zero. Sure, this is not in the C/C++ standard, but it works on the most popular platforms. -
Toad over 14 yearsDoesn't matter. The C standard doesn't specify what float format is used. So even if it works now for IEEE 754, it might not work on a different float implementation (future or past)
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Toad over 14 yearssbk: for now... who knows what floating point implementation they might start using. IEEE 754 is not defined for the compiler. So even if it might work now, it is just lucky for you, but can give problems later.
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JaakkoK over 14 yearsI would guess not many nowadays, with IEEE being so common, but they used to be more prevalent. I understand software FP implementations are typical examples where zero was not all-bits-zero. So you probably won't get in trouble, but still, C doesn't mandate IEEE, so unless the zero initialization is a bottleneck, the "safer" way doesn't really cost anything.
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Prof. Falken over 14 years@qrdl, depends on the compiler and target. For ROM-able code, values are sometimes set at compile time.
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Martin York over 14 years@qrdl: Let me re-phrase that. Value initialization may allow (under certain situations) the compiler to do the initialization at compile time (rather than run-time). So POD only globals can be initialized at compile time.
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jamesdlin over 14 yearsHow does it help in future-proofing? If you're assuming that the client code isn't recompiled, it'd end up calling
memset
with the wrong structure size. If the client code is recompiled, it'd need access to the updated header file with the struct definition for eithermemset
or value initialization to work. (The client and the library do need to have a consistent notion of how the null pointer is represented, however, so if the API recommendsmemset
, it ought to be checking against all-bits-zero, not against NULL.) -
jamesdlin over 14 yearsAlso, if the struct is part of a public API, then maybe one should consider an opaque structure with an initialization function instead.
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supercat over 12 years@qrdl: On many platforms, if "foo" is an Int32_t of static storage class, run-time statement "foo=0x12345678;" will generate code to store 0x12345678 in foo; that code would likely be at least ten bytes long, some microcontrollers would require as many as 32 bytes. A declaration "Int32_t foo=0x12345678;" would on many platforms cause the variable to be linked in an initialized-data segment and add 4 bytes to an initialization list. On some systems, "Int32_t foo;" would be four bytes more cheaper than "Int32_t foo=0;", the latter forcing foo to the initialized-data segment.
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fmuecke almost 12 yearsInitializing each member to zero will not make every member to zero, but you'll miss the padding bytes. Therefore memset is your only choice.
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supercat over 9 years@AndrewMcGregor: Some systems may also use something other than all-bits-zero for a null pointer. For example, if a 32-bit system traps reads or writes of invalid memory addresses, and all-bits-zero is valid but memory addresses with the MSB set aren't, it might be better to use 0x80000000 or 0xC0000000 rather than 0x0000000 as
NULL
[if a system used 0xFFFFFFFF, then accessingp->bar
whenp
is null might refer to a seemingly-valid address, but using 0xC0000000 would guard against that]. -
Jody Bruchon over 9 yearsThis actually bit me hard recently. I was working on a custom piece of compression code and was initializing some large structures at declaration time using
struct something foo = { x, y, z }
and cachegrind showed that 70% of my program's "work" was inmemset
because the structs were zeroed on EVERY function call. -
MayurK over 7 yearsHi Dimity! I have a structure which has some members and I tried the first option of memsetting: "struct stVar={}". But I am getting "-Wmissing-field-initializers" warning. Is it an issue?
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Al.G. over 6 yearsIn this case, by POD do you mean actually a trivially constructible object (i.e. object without user-provided c-tor)? I don't think it should be constrained to POD.
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Kyle Strand almost 6 yearsThis doesn't appear to be true. From CppReference: "If T is an non-union class type, all base classes and non-static data members are zero-initialized, and all padding is initialized to zero bits. The constructors, if any, are ignored." en.cppreference.com/w/cpp/language/zero_initialization
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Kyle Strand almost 6 yearsThis won't crash: coliru.stacked-crooked.com/a/4b3dbf0b8761bc9b It's technically undefined behavior because the struct is not trivially assignable (hence the compiler warning). However, I doubt there's any common platform where zeroed-bytes are an invalid value for
std::string
. -
Clément over 4 yearsI think this answer is outdated. In C++11, padding bits are guaranteed to be zero-initialized:
if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits;
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SomeWittyUsername over 4 years"Of course, if you need to zeroize an existing structure, memset is the only choice (well, apart from initializing each member to zero by hand, but that wouldn't normally be done, especially for large structures)". What prevents from doing the same as in init? Works fine: ideone.com/AzRQWC
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syockit almost 4 yearsProbably only applies to C and not C++.
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mab0189 over 2 yearsLearned my lesson the hard way with the Berkeley socket API...