Why does "memset(arr, -1, sizeof(arr)/sizeof(int))" not clear an integer array to -1?
arrays, c, integer, memory, memset
Solution
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.
Problem
Is it not possible to use `memset` on an array of integers? I tried the following `memset` call and didn't get the correct integer values in the `int` array. ``` int arr[5]; memset (arr, -1, sizeof(arr)/sizeof(int)); ``` Values I got are: ``` arr[0] = -1 arr[1] = 255 arr[2] = 0 arr[3] = 0 arr[4] = 0 ```