def add_hello():
exec "def hello(): print 'hello'"
hello()
add_hello() # "hello"
hello() # NameError: name 'hello' is not defined
def add_hello():
exec "def hello(): print 'hello'" in locals(), globals()
hello()
add_hello() # "hello"
hello() # "hello"
$ ltrace getent hosts 127.0.0.1 2>&1 | grep gethostby
gethostbyaddr("\177", 4, 2) = 0xb7ed0aa0
$ strace getent hosts 127.0.0.1 2>&1 | grep "/etc/hosts"
open("/etc/hosts", O_RDONLY|O_CLOEXEC) = 3
$ strace host 127.0.0.1 2>&1 | grep "/etc/hosts"
$ ltrace host 127.0.0.1 2>&1 | grep gethostby#include <stdio.h>
int main(void)
{
printf("hello, world!\n");
puts("hello, world!\n");
printf("hello, world! %d\n", 3);
return 0;
}
$ ltrace ./f > /dev/null __libc_start_main(0x80483f4, 1, 0xbf812ab4, 0x8048450, 0x8048440puts("hello, world!") = 14 puts("hello, world!\n") = 15 printf("hello, world! %d\n", 3) = 16 +++ exited (status 0) +++
$ printf '#include <stdio.h>\n#include <assert.h>\nvoid gethostbyaddr(void) { assert(0); }\n' > t.c
$ gcc -fPIC -shared t.c -o t.so
$ LD_PRELOAD=./t.so getent hosts 127.0.0.1
getent: t.c:3: gethostbyaddr: Assertion `0' failed.
Aborted
$ LD_PRELOAD=./t.so host 127.0.0.1
1.0.0.127.in-addr.arpa domain name pointer localhost.
1 Executable programs or shell commands
2 System calls (functions provided by the kernel)
3 Library calls (functions within program libraries)
4 Special files (usually found in /dev)
5 File formats and conventions eg /etc/passwd
6 Games
7 Miscellaneous (including macro packages and conven-
tions), e.g. man(7), groff(7)
8 System administration commands (usually only for root)
9 Kernel routines [Non standard]
2012-01-30 更新
見 tag strace 下的新文章, 了解比較詳細的 strace 使用例子。
sudo aptitude install manpages-dev
1.2.3.4 myhost.org.tw myhost這樣就不會去問 DNS 等個五秒才放棄。
import socket
print socket.gethostbyaddr('127.0.0.1')This is a common problem that newbies run into. Answer: host(1) and nslookup(1) does not call gethostbyname(3) or the IPv6 safe getnameinfo(3) but issue DNS requests directly.
What you want is getent(1) from glibc:
getent hosts 127.0.0.1
(defmacro backwards (expr) (reverse expr))
CL-USER> (backwards ("hello, world" t format))
hello, world
NIL
CL-USER> (macroexpand-1 '(backwards ("hello, world" t format)))
(FORMAT T "hello, world")
Tthe Lisp compiler passes the arguments, unevaluated, to the macro code, which returns a new Lisp expression that is then evaluated in place of the original macro call.