initialize float-type array using memset

Question

I have a pointer float *ptr, after dynamic allocation with length n, I want to initialize this array with zero or one, so I use memset(ptr,0,n*sizeof(float)) or memset(ptr,1,n*sizeof(float)). Is this legal? Because the second argument of memset is int-type, I'm afraid it cannot be applied to float-type.

That argument is the value to set each byte to, not each 32-bit word. 0x0 is float zero, but 0x01010101 isn't 1, it's ~2.3*10**-38 — James Picone
– James Picone, Commented Jun 27, 2018 at 2:32
no they're both safe and defined; it's just that you can't see an array of floats to 1 with memset; the interface doesn't support it. — James Picone
– James Picone, Commented Jun 27, 2018 at 3:17

Jaffrey Joy · Accepted Answer · 2024-11-21 12:15:26Z

3

memset(ptr,1,n*sizeof(float)). Is this legal?

No, not to set the value of the float to 1.0f as the encoding of a float in not the bytes 1,1,1,1 @James Picone

memset(ptr,0,n*sizeof(float)) or better memset(ptr, 0, sizeof *ptr * n) will set every byte to 0. This is certainly the encoding for a float 0.0f.

To set every element of a float array to 1.0f or any value, simply use a loop.

float initial_value = 1.0f;
for (size_t i = 0; i < n; i++) {
   ptr[i] = initial_value;
}

edited Nov 21, 2024 at 12:15

Jaffrey Joy

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answered Jun 27, 2018 at 3:58

chux

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1 Comment

M.M Over a year ago

All bits zero is 0.0f in IEEE754, however the standard doesn't mandate such a representation

John Zwinck · Accepted Answer · 2018-06-27 02:36:15Z

2

Initializing floats to all-bytes-zero is OK (it will produce float 0.0). But all-bytes-1 is not reasonable, because it will produce a "garbage" value (but the same value every time).

answered Jun 27, 2018 at 2:36

John Zwinck

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