As the gcc suggestion, previous version miss required headers and use implict-function-declaration to pass
xc-tutor.c:1238:37: error: implicit declaration of function ‘open’; did you mean ‘popen’? [-Wimplicit-function-declaration] 1238 | else if (op == OPEN) { ax = open((char *)sp[1], sp[0]); } | ^~~~ | popen xc-tutor.c:1239:37: error: implicit declaration of function ‘close’; did you mean ‘pclose’? [-Wimplicit-function-declaration] 1239 | else if (op == CLOS) { ax = close(*sp);} | ^~~~~ | pclose xc-tutor.c:1240:37: error: implicit declaration of function ‘read’; did you mean ‘fread’? [-Wimplicit-function-declaration] 1240 | else if (op == READ) { ax = read(sp[2], (char *)sp[1], *sp); }
According to the linux man page, open is in <fcntl.h>, and read, close are in <unistd.h>
So I made this pr