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

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

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