Why does calloc exist?
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Why does calloc exist?
1–10 of 141 posts
Re: Why does calloc exist?
#2> So basically, calloc exists because it lets the memory allocator and kernel engage in a sneaky conspiracy to make your code faster and use less memory. You should let it! Don't use malloc+memset!
On the flip side, if your critical metric is latency then these tricks of calloc's and the OS's are exactly what you try to avoid. memset() the buffer, and if you have the privileges you should mlock() it to prevent it from being paged out. Of course, this all presumes that it's not an ephemeral buffer to begin with. Best to change your design to leverage a long-lived resource if possible.
Re: Why does calloc exist?
#3That's a nice alternative history fiction.
Here's an early implementation: https://github.com/dspinellis/unix-history-repo/blob/Researc...
Re: Why does calloc exist?
#4The real reason "calloc" exists was that it was really easy to hit 16-bit overflow back in the PDP-11 days.
Re: Why does calloc exist?
#5That's a nice alternative history fiction. Here's an early implementation: https://github.com/dspinellis/unix-history-repo/blob/Researc...
It's a good explanation of why calloc still exists and is useful. Otherwise it would have been dropped from the standard like cfree was.
Re: Why does calloc exist?
#6> So basically, calloc exists because it lets the memory allocator and kernel engage in a sneaky conspiracy to make your code faster and use less memory. You should let it! Don't use malloc+memset! On the flip side, if your critical metric is latency then these tricks of calloc's and the OS's are exactly what you try to avoid. memset() the buffer, and if you have the privileges you should mlock() it to prevent it fro…
Best to change your design to leverage a long-lived
resource if possible.
On the flip side, if your critical metric is latency
then these tricks [...] are exactly what you try to avoid
Re: Why does calloc exist?
#7I always thought it was because of padding. An array of M structures each N bytes long could require more than M*N bytes (certainly has on some architectures I've worked with). But I guess that's not it after all.
Re: Why does calloc exist?
#8I always thought it was because of padding. An array of M structures each N bytes long could require more than M*N bytes (certainly has on some architectures I've worked with). But I guess that's not it after all.
calloc doesn't know what it's allocating for, so it has to hand out m * n.
(Despite the syntax, the same goes for operator new in C++! Placement vector new in particular is a trap.)
Re: Why does calloc exist?
#9I always thought it was because of padding. An array of M structures each N bytes long could require more than M*N bytes (certainly has on some architectures I've worked with). But I guess that's not it after all.
Padding is included in sizeof.
Re: Why does calloc exist?
#10I always thought it was because of padding. An array of M structures each N bytes long could require more than M*N bytes (certainly has on some architectures I've worked with). But I guess that's not it after all.
C accounts for padding in the size of the individual type. By the time you do sizeof, it's already rounded up to where you can just do M*N. For example:
struct S {
long a;
char b;
};