What is the fastest way to overwrite an entire file with zeros in C?

Question

What I need to do is to fill the entire file contents with zeros in the fastest way. I know some linux commands like cp actually gets what is the best block siz

Answer 1

With mmap (and without error checking):

stat(filename,&stat_buf);
len=stat_buf.st_size;
fd=open(filename,O_RDWR);
ptr=mmap(NULL,len,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);
memset(ptr,0,len);
munmap(ptr,len);
close(fd);

This should use the kernel's idea of block size, so you don't need to worry about it. Unless the file is larger than your address space.