It depends. If the data is coming from a pipe (like core_pattern) then yes you have to check for runs of zeroes. If it's coming from a filesystem, then there are various system calls that let you skip them (specifically SEEK_HOLE and SEEK_DATA flags of lseek(2)).
Also if the data is being copied into userspace anyway, then it's quite fast to check that memory is zero. There's no C "primitive" for this, but all C compilers can turn a simple loop into relatively efficient assembler[1].
If you're using an API that never copies the data into userspace and you have to read from a pipe, then yes sparse detection will be much more expensive.
In either case it should save disk space for core files which are highly sparse.
The easiest way to handle things like sparse files correctly is to invoke a program like GNU dd that already has this feature built in. GNU cp handles it, too, but it doesn't accept input from stdin.
Comments
It depends. If the data is coming from a pipe (like core_pattern) then yes you have to check for runs of zeroes. If it's coming from a filesystem, then there are various system calls that let you skip them (specifically SEEK_HOLE and SEEK_DATA flags of lseek(2)).
Also if the data is being copied into userspace anyway, then it's quite fast to check that memory is zero. There's no C "primitive" for this, but all C compilers can turn a simple loop into relatively efficient assembler[1].
If you're using an API that never copies the data into userspace and you have to read from a pipe, then yes sparse detection will be much more expensive.
In either case it should save disk space for core files which are highly sparse.
[1] https://stackoverflow.com/a/1494021
The easiest way to handle things like sparse files correctly is to invoke a program like GNU dd that already has this feature built in. GNU cp handles it, too, but it doesn't accept input from stdin.