NAME

acid, acidtypes - debugger

SYNOPSIS

acid [ -l library ] [ -wq ] [ -m machine ] [ pid | core ] [ textfile ]

acidtypes [ -p prefix ] file ...

DESCRIPTION

Acid is a programmable symbolic debugger. It can inspect one or more processes that share an address space. A program to be debugged may be specified by the process id of a running or defunct process, or by the name of the program's text file (a.out by default). At the prompt, acid will store function definitions or print the value of expressions. Options are

-w: Allow the textfile to be modified.
-q: Print variable renamings at startup.
-l library: Load from library at startup; see below.
-m machine: Assume instructions are for the given CPU type (see instead of using the executable header to select the CPU type.
-k: Debug the kernel state for the process, rather than the user state.

At startup, acid obtains standard function definitions from the library file /acid/port, architecture-dependent functions from /acid/$objtype, user-specified functions from $HOME/lib/acid, and further functions from -l files. Definitions in any file may override previously defined functions. If the function acidinit() is defined, it will be invoked after all modules have been loaded. Then the function acidmap() will be invoked if defined. /acid/port provides a definition of acidmap that attaches all the shared libraries being used by the target process and then runs acidtypes (q.v.) to create acid functions for examining data structures.

Language

Symbols of the program being debugged become integer variables whose values are addresses. Contents of addresses are obtained by indirection. Local variables are qualified by function name, for example main:argv. When program symbols conflict with acid words, distinguishing $ signs are prefixed. Such renamings are reported at startup; option -q suppresses them.

Variable types (integer, float, list, string) and formats are inferred from assignments. Truth values false/true are attributed to zero/nonzero integers or floats and to empty/nonempty lists or strings. Lists are sequences of expressions surrounded by {} and separated by commas.

Expressions are much as in C, but yield both a value and a format. Casts to complex types are allowed. Lists admit the following operators, with subscripts counted from 0.

: head list
tail list
append list, element
delete list, subscript

Format codes are the same as in Formats may be attached to (unary) expressions with \, e.g. (32*7)\D. There are two indirection operators, * to address a core image, @ to address a text file. The type and format of the result are determined by the format of the operand, whose type must be integer.

Statements are

: if expr then statement [ else statement ]
while expr do statement
loop expr, expr do statement
defn name(args) { statement }
defn name
name(args)
builtin name(args)
local name
return expr
whatis [ name ]

The statement defn name clears the definition for name. A defn may override a built-in function; prefixing a function call with builtin ignores any overriding defn, forcing the use of the built-in function.

Here is a partial list of functions; see the manual for a complete list.

stk(): Print a stack trace for current process.
lstk(): Print a stack trace with values of local variables.
gpr(): Print general registers. Registers can also be accessed by name, for example *R0.
spr(): Print special registers such as program counter and stack pointer.
fpr(): Print floating-point registers.
regs(): Same as spr();gpr().
fmt(expr,format): Expression expr with format given by the character value of expression format.
src(address): Print 10 lines of source around the program address.
Bsrc(address): Get the source line for the program address into a window of a running and select it.
line(address): Print source line nearest to the program address.
source(): List current source directories.
addsrcdir(string): Add a source directory to the list.
filepc(where): Convert a string of the form sourcefile:linenumber to a machine address.
pcfile(address): Convert a machine address to a source file name.
pcline(address): Convert a machine address to a source line number.
bptab(): List breakpoints set in the current process.
bpset(address): Set a breakpoint in the current process at the given address. (Doesn't work on Unix yet.)
bpdel(address): Delete a breakpoint from the current process.
cont(): Continue execution of current process and wait for it to stop.
step(): Execute a single machine instruction in the current process. (Doesn't work on Unix yet.)
func(): Step repeatedly until after a function return.
stopped(pid): This replaceable function is called automatically when the given process stops. It normally prints the program counter and returns to the prompt.
asm(address): Disassemble 30 machine instructions beginning at the given address.
mem(address,string): Print a block of memory interpreted according to a string of format codes.
dump(address,n,string): Like mem(), repeated for n consecutive blocks.
print(expr,...): Print the values of the expressions.
newproc(arguments): Start a new process with arguments given as a string and halt at the first instruction.
new(): Like newproc(), but take arguments (except argv[0]) from string variable progargs.
win(): Like new(), but run the process in a separate window.
start(pid): Start a stopped process.
kill(pid): Kill the given process.
setproc(pid): Make the given process current.
rc(string): Escape to the shell, to execute the command string.
include(string): Read acid commands from the named file.
includepipe(string): Run the command string, reading its standard output as acid commands.

Shared library segments

When a pid or core file is specified on the command line, acid will, as part of its startup, determine the set of shared libraries in use by the process image and map those at appropriate locations. If acid is started without a pid or core file and is subsequently attached to a process via setproc, the shared library maps can be initialized by calling dynamicmap().

Type information

Unix compilers conventionally include detailed type information in the debugging symbol section of binaries. The external program acidtypes extracts this information and formats it as acid program text. Once the shared libraries have been mapped, the default acid startup invokes acidtypes (via includepipe) on the set of currently mapped text files. The function acidtypes() can be called to rerun the command after changing the set of mapped text files.

Acid Libraries

There are a number of acid `libraries' that provide higher-level debugging facilities. One notable example is trump, which uses acid to trace memory allocation. Trump requires starting acid on the program, either by attaching to a running process or by executing new() on a binary (perhaps after setting progargs), stopping the process, and then running trump() to execute the program under the scaffolding. The output will be a trace of the memory allocation and free calls executed by the program. When finished tracing, stop the process and execute untrump() followed by cont() to resume execution.

EXAMPLES

Start to debug /bin/ls; set some breakpoints; run up to the first one (this example doesn't work on Unix yet):

% acid /bin/ls
/bin/ls: mips plan 9 executable
/sys/lib/acid/port
/sys/lib/acid/mips
acid: new()
70094: system call	_main	ADD	$-0x14,R29
70094: breakpoint	main+0x4	MOVW	R31,0x0(R29)
acid: pid
70094
acid: argv0 = **main:argv\s
acid: whatis argv0
integer variable format s
acid: *argv0
/bin/ls
acid: bpset(ls)
acid: cont()
70094: breakpoint  ls	ADD	$-0x16c8,R29
acid:

Display elements of a linked list of structures:

complex Str { 'D' 0 val; 'X' 4 next; };
s = *headstr;
while s != 0 do{
	complex Str s;
	print(s.val, "\n");
	s = s.next;
}

Note the use of the . operator instead of ->.

Display an array of bytes declared in C as char array[].

*(array\s)

This example gives array string format, then prints the string beginning at the address (in acid notation) *array.

Trace the system calls executed by (neither does this one):

% acid -l truss /bin/ls
/bin/ls:386 plan 9 executable
/sys/lib/acid/port
/sys/lib/acid/kernel
/sys/lib/acid/truss
/sys/lib/acid/386
acid: progargs = "-l lib/profile"
acid: new()
acid: truss()
open("#c/pid", 0)
	return value: 3
pread(3, 0x7fffeeac, 20, -1)
	return value: 12
	data: "        166 "
...
stat("lib/profile", 0x0000f8cc, 113)
	return value: 65
open("/env/timezone", 0)
	return value: 3
pread(3, 0x7fffd7c4, 1680, -1)
	return value: 1518
	data: "EST -18000 EDT -14400
   9943200   25664400   41392800   57718800   73447200   89168400
 104896800  ..."
close(3)
	return value: 0
pwrite(1, "--rw-rw-r-- M 9 rob rob 2519 Mar 22 10:29 lib/profile
", 54, -1)
--rw-rw-r-- M 9 rob rob 2519 Mar 22 10:29 lib/profile
	return value: 54
...
166: breakpoint	_exits+0x5	INTB	$0x40
acid: cont()

FILES

/acid/$objtype
/acid/port
/acid/kernel
/acid/trump
/acid/truss
$HOME/lib/acid

SOURCE

/src/cmd/acid

DIAGNOSTICS

At termination, kill commands are proposed for processes that are still active.

BUGS

There is no way to redirect the standard input and standard output of a new process.

Source line selection near the beginning of a file may pick an adjacent file.

With the extant stepping commands, one cannot step through instructions outside the text segment and it is hard to debug across process forks.

Breakpoints do not work yet. Therefore, commands such as step, new, and truss do not work either. New in particular will need some help to cope with dynamic libraries.