The POSIX standard as of 2003 requires only the option -u to be implemented, which prevents cat from buffering its output - on some systems, cat buffers its output in 512-byte blocks (McIlroy), similarly to dd’s default as defined by POSIX (“dd(1p)”), though most currently popular cat implementations do this by default and ignore the -u flag altogether (busybox, GNU coreutils). POSIX doesn’t mandate buffering by default - specifically, -u has to guarantee that the output is unbuffered, but cat doesn't have to buffer it in the first place and can ignore -u in that case.
This is a POSIX-compatible implementation of UNIX cat with no additional features nor buffered output in C:
+
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
@@ -164,7 +164,7 @@ main(int argc, char *argv[]){
It’s worth noting that this concept of cat as a utility that sequentially prints given files to standard output means cat can be replaced by a simple shell script that does the same using dd and printf; cat as defined by POSIX is actually totally redundant to other POSIX utilities. Here’s the shell script:
-
+
#!/bin/sh
# dd_ is used so that dd can easily be re-defined