Initializing a struct to 0

Question

If I have a struct like this   typedef struct       unsigned char c1      unsigned char c2    myStruct    What would be the easiest way to initialize this struct to 0  Would the following suffice   myStruct  m1    0     or Would I need to explicitly init each member to 0   myStruct  m2    0 0

User · Answer

If the data is a static or global variable, it is zero-filled by default, so just declare it myStruct _m;

If the data is a local variable or a heap-allocated zone, clear it with memset like:

memset(&m, 0, sizeof(myStruct));

Current compilers (e.g. recent versions of gcc) optimize that quite well in practice. This works only if all zero values (include null pointers and floating point zero) are represented as all zero bits, which is true on all platforms I know about (but the C standard permits implementations where this is false; I know no such implementation).

You could perhaps code myStruct m = {}; or myStruct m = {0}; (even if the first member of myStruct is not a scalar).

My feeling is that using memset for local structures is the best, and it conveys better the fact that at runtime, something has to be done (while usually, global and static data can be understood as initialized at compile time, without any cost at runtime).

User · Answer

See   6 7 9 Initialization      21 If there are fewer initializers in a brace-enclosed list than there are elements or members   of an aggregate  or fewer characters in a string literal used to initialize an array of known   size than there are elements in the array  the remainder of the aggregate shall be   initialized implicitly the same as objects that have static storage duration    So  yes both of them work  Note that in C99 a new way of initialization  called designated initialization can be used too   myStruct  m1     c2   0   c1   1

User · Answer

I also thought this would work but it s misleading  myStruct  m1    0    When I tried this  myStruct  m1    0xff    Only the 1st byte was set to 0xff  the remaining ones were set to 0  So I wouldn t get into the habit of using this

User · Answer

The first is easiest involves less typing   and it is guaranteed to work  all members will be set to 0 Ref 1   The second is more readable        The choice depends on user preference or the one which your coding standard mandates    Ref 1  Reference C99 Standard 6 7 8 21          If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate  or fewer characters in a string literal used to initialize an array of known size than there are elements in the array  the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration    Good Read  C and C     Partial initialization of automatic structure

[c] Initializing a struct to 0

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