C char array initialization

Question

I m not sure what will be in the char array after initialization in the following ways       1 char buf 10        2  char buf 10         3  char buf 10     a      For case 2  I think buf 0  should be      buf 1  should be   0   and from buf 2  to buf 9  will be random content  For case 3  I think buf 0  should be  a   buf 1  should be   0   and from buf 2  to buf 9  will be random content   Is that correct   And for the case 1  what will be in the buf  buf 0       0  and from buf 1  to buf 9  will be random content

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Edit  OP  or an editor  silently changed some of the single quotes in the original question to double quotes at some point after I provided this answer    Your code will result in compiler errors  Your first code fragment   char buf 10    buf        is doubly illegal  First  in C  there is no such thing as an empty char  You can use double quotes to designate an empty string  as with   char  buf          That will give you a pointer to a NUL string  i e   a single-character string with only the NUL character in it  But you cannot use single quotes with nothing inside them--that is undefined  If you need to designate the NUL character  you have to specify it   char buf     0     The backslash is necessary to disambiguate from character  0     char buf   0    accomplishes the same thing  but the former is a tad less ambiguous to read  I think    Secondly  you cannot initialize arrays after they have been defined    char buf 10     declares and defines the array  The array identifier buf is now an address in memory  and you cannot change where buf points through assignment  So   buf          anything on RHS   is illegal  Your second and third code fragments are illegal for this reason   To initialize an array  you have to do it at the time of definition   char buf  10           will give you a 10-character array with the first char being the space   040  and the rest being NUL  i e     0   When an array is declared and defined with an initializer  the array elements  if any  past the ones with specified initial values are automatically padded with 0  There will not be any  random content    If you declare and define the array but don t initialize it  as in the following   char buf  10     you will have random content in all the elements

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This is not how you initialize an array  but for    The first declaration   char buf 10          is equivalent to  char buf 10     0  0  0  0  0  0  0  0  0  0    The second declaration   char buf 10           is equivalent to  char buf 10          0  0  0  0  0  0  0  0  0    The third declaration   char buf 10     a     is equivalent to  char buf 10      a   0  0  0  0  0  0  0  0  0      As you can see  no random content  if there are fewer initializers  the remaining of the array is initialized with 0  This the case even if the array is declared inside a function

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These are equivalent  char buf 10        char buf 10     0   char buf 10     0  0  0  0  0  0  0  0  0  0    These are equivalent  char buf 10         char buf 10           char buf 10          0  0  0  0  0  0  0  0  0    These are equivalent  char buf 10     a   char buf 10      a    char buf 10      a   0  0  0  0  0  0  0  0  0

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The relevant part of C11 standard draft n1570 6 7 9 initialization says      14   An array of character type may be initialized by a character string literal or UTF-8 string        literal  optionally enclosed in braces  Successive bytes of the string literal  including the        terminating null character if there is room or if the array is of unknown size  initialize the        elements of the array    and     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    Thus  the   0  is appended  if there is enough space  and the remaining characters are initialized with the value that a static char c  would be initialized within a function   Finally       10   If an object that has automatic storage duration is not initialized explicitly  its value is        indeterminate  If an object that has static or thread storage duration is not initialized        explicitly  then        --          if it has arithmetic type  it is initialized to  positive or unsigned  zero           --    Thus  char being an arithmetic type the remainder of the array is also guaranteed to be initialized with zeroes

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Interestingly enough  it is possible to initialize arrays in any way at any time in the program  provided they are members of a struct or union   Example program    include  lt stdio h gt   struct ccont     char array 32       struct icont     int array 32       int main       int  cnt    char carray 32       A   66  6 11 1           A    B    C     0     0         int  iarray 32      67  42  25       struct ccont cc     0      struct icont ic     0           these don t work   carray      0  1                 expected expression before     token   carray      0     31  1           likewise    carray    char 32     0  3       incompatible types when assigning to type  char 32   from type  char      iarray    int 32    1             likewise  but s char int g             but these perfectly work      cc    struct ccont    array  a                a     0     0     0            the following is a gcc extension     cc    struct ccont    array    0     2   a            a    a    a     0     0         ic    struct icont    array   42 67              42  67  0  0  0           index ranges can overlap  the latter override the former       no compiler warning with -Wall -Wextra    ic    struct icont    array    0     1  42   1     2  67         42  67  67  0  0         for  cnt 0  cnt lt 5  cnt        printf   2d  c  2d  c n  iarray cnt   carray cnt  ic array cnt  cc array cnt       return 0

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I m not sure but I commonly initialize an array to    in that case I don t need worry about the null end of the string   main         void something char         char s 100             something s       printf   s   s      void something char s                 do something  pass the output to s        no need to add s i      0   because all unused slot is already set to   0

[c] C char array initialization

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