Why does "memset(arr, -1, sizeof(arr)/sizeof(int))" not clear an integer array to -1?

Just change to memset (arr, -1, sizeof(arr));

Note that for other values than 0 and -1 this would not work since memset sets the byte values for the block of memory that starts at the variable indicated by *ptr for the following num bytes.

void * memset ( void * ptr, int value, size_t num );

And since int is represented on more than one byte, you will not get the desired value for the integers in your array.

Exceptions:

• 0 is an exception since, if you set all the bytes to 0, the value will be zero
  
• -1 is another exception since, as Patrick highlighted -1 is 0xff (=255) in int8_t and 0xffffffff in int32_t

The reason you got:

arr[0] = -1arr[1] = 255arr[2] = 0arr[3] = 0arr[4] = 0

Is because, in your case, the length of an int is 4 bytes (32 bit representation), the length of your array in bytes being 20 (=5*4), and you only set 5 bytes to -1 (=255) instead of 20.

Don't use memset to initialize anything else than single-byte data types.

At first sight, it might appear that it should work for initializing an int to 0 or -1 (and on many systems it will work), but then you're not taking into account the possibility that you might generate a trap representation, causing undefined behavior, or the fact that the integer representation is not necessarily two's complement.

The correct way to initialize an array of int to -1, is to loop over the array, and set each value explicitly.

gcc provides a good array initialization shortcut

int arr[32] = {[0 ... 10] = 3, [11 ... 31] = 4}

mind the space before and after ...