月份：2015年11月

probe后面有时会跟?或是!字符：

kernel.function("no_such_function") ?
module("awol").function("no_such_function") !

对此，man手册的解释如下：

However, a probe point may be followed by a “?” character, to indicate that it is optional, and that no error should result if it fails to resolve. Optionalness passes down through all levels of alias/wildcard expansion. Alternately, a probe point may be followed by a “!” character, to indicate that it is both optional and sufficient. (Think vaguely of the prolog cut operator.) If it does resolve, then no further probe points in the same comma-separated list will be resolved. Therefore, the “!” sufficiency mark only makes sense in a list of probe point alternatives.

?表明probe是可选的，即使不存在相应的probe，也不会导致命令出错，而是继续解析其它的probe。!表明probe一旦解析成功，则不会继续解析后面的probe。因此!只在存在probe列表的情况下才有效。

当指针是一个void *类型，或是保存为整数后，可以使用cast运算符指定指针的数据类型：

@cast(p, "type_name"[, "module"])->member

可选的module参数用来指定从哪里得到type_name（The optional module tells the translator where to look for information about that type. Multiple modules may be specified as a list with : separators. If the module is not specified, it will default either to the probe module for dwarf probes, or to “kernel” for functions and all other probes types.）。

另外，translator可以从头文件中创建type信息：

The translator can create its own module with type information from a header surrounded by angle brackets, in case normal debuginfo is not available. For kernel headers, prefix it with “kernel” to use the appropriate build system. All other headers are build with default GCC parameters into a user module. Multiple headers may be specified in sequence to resolve a codependency.

@cast(tv, “timeval”, “<sys/time.h>”)->tvsec
@cast(task, “taskstruct”, “kernel<linux/sched.h>”)->tgid
@cast(task, “taskstruct”, “kernel<linux/sched.h><linux/fsstruct.h>”)->fs->umask

SystemTap可以为target variable生成一系列可打印的字符串：

$$vars

Expands to a character string that is equivalent to sprintf(“parm1=%x … parmN=%x var1=%x … varN=%x”, parm1, …, parmN, var1, …, varN) for each variable in scope at the probe point. Some values may be printed as “=?” if their run-time location cannot be found.

$$locals

Expands to a subset of $$vars containing only the local variables.

$$parms

Expands to a subset of $$vars containing only the function parameters.

$$return

Is available in return probes only. It expands to a string that is equivalent to sprintf(“return=%x”, $return) if the probed function has a return value, or else an empty string.

参考下面例子：

# stap -e 'probe kernel.function("do_dentry_open") {printf("%s\n", $$vars); exit(); }'
f=0xffff880022ec6080 open=0x0 cred=0xffff880030d483c0 empty_fops={...} inode=? error=?
# stap -e 'probe kernel.function("do_dentry_open") {printf("%s\n", $$parms); exit(); }'
f=0xffff880030d453c0 open=0x0 cred=0xffff880030d483c0
# stap -e 'probe kernel.function("do_dentry_open") {printf("%s\n", $$locals); exit(); }'
empty_fops={...} inode=? error=?

在上述变量后加上$或$$可以打印更详细的结构体信息。参考下例：

# stap -e 'probe kernel.function("do_dentry_open") {printf("%s\n", $$parms$); exit(); }'
f={.f_u={...}, .f_path={...}, .f_inode=0x0, .f_op=0x0, .f_lock={...}, .f_count={...}, .f_flags=32768, .f_mode=0, .f_pos=0, .f_owner={...}, .f_cred=0xffff880030d483c0, .f_ra={...}, .f_version=0, .f_security=0xffff880012982b80, .private_data=0x0, .f_ep_links={...}, .f_tfile_llink={...}, .f_mapping=0x0} open=<function>:0x0 cred={.usage={...}, .uid={...}, .gid={...}, .suid={...}, .sgid={...}, .euid={...}, .egid={...}, .fsuid={...}, .fsgid={...}, .securebits=0, .cap_inheritable={...}, .cap_permitted={...}, .cap
# stap -e 'probe kernel.function("do_dentry_open") {printf("%s\n", $$parms$$); exit(); }'
f={.f_u={.fu_llist={.next=0x0}, .fu_rcuhead={.next=0x0, .func=0x0}}, .f_path={.mnt=0xffff8800337510e0, .dentry=0xffff8800318b82d8}, .f_inode=0x0, .f_op=0x0, .f_lock={<union>={.rlock={.raw_lock={.head_tail=0, <class>={.tickets={.head=0, .tail=0}, .owner=0}}}}}, .f_count={.counter=1}, .f_flags=32768, .f_mode=0, .f_pos=0, .f_owner={.lock={.raw_lock={.lock=1048576, .write=1048576}}, .pid=0x0, .pid_type=0, .uid={.val=0}, .euid={.val=0}, .signum=0}, .f_cred=0xffff880030d89300, .f_ra={.start=0, .size=0, .async_si