how does array 100 0 set the entire array to 0

Question

How does the compiler fill values in char array 100     0    What s the magic behind it   I wanted to know how internally compiler initializes

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If your compiler is GCC you can also use following syntax:

int array[256] = {[0 ... 255] = 0};

Please look at http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Designated-Inits.html#Designated-Inits, and note that this is a compiler-specific feature.

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Implementation is up to compiler developers   If your question is  what will happen with such declaration  - compiler will set first array element to the value you ve provided  0  and all others will be set to zero because it is a default value for omitted array elements

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It depends where you put this initialisation    If the array is static as in  char array 100     0    int main void            then it is the compiler that reserves the 100 0 bytes in the data segement of the program  In this case you could have omitted the initialiser    If your array is auto  then it is another story   int foo void    char array 100     0           In this case at every call of the function foo you will have a hidden memset   The code above is equivalent to  int foo void     char array 100    memset array  0  sizeof array             and if you omit the initializer your array will contain random data  the data of the stack    If your local array is declared static like in  int foo void     static char array 100     0           then it is technically the same case as the first one

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It s not magic    The behavior of this code in C is described in section 6 7 8 21 of the C specification  online draft of C spec   for the elements that don t have a specified value  the compiler initializes pointers to NULL and arithmetic types to zero  and recursively applies this to aggregates     The behavior of this code in C   is described in section 8 5 1 7 of the C   specification  online draft of C   spec   the compiler aggregate-initializes the elements that don t have a specified value   Also  note that in C    but not C   you can use an empty initializer list  causing the compiler to aggregate-initialize all of the elements of the array   char array 100          As for what sort of code the compiler might generate when you do this  take a look at this question  Strange assembly from array 0-initialization

[c++] how does array[100] = {0} set the entire array to 0?

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