ltrace.conf.5 revision 8978a1f7e605a876ce186479175dcfebc7362045 
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 Copyright (c) 2012 Petr Machata, Red Hat Inc.
 Copyright (c) 1997-2005 Juan Cespedes <cespedes@debian.org>

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 ltrace.conf "5" "October 2012" "" "ltrace configuration file"
 "NAME"

ltrace.conf - Configuration file for ltrace(1).


 DESCRIPTION

This manual page describes ltrace.conf, a file that describes
prototypes of functions in binaries for ltrace(1) to use.
Ltrace needs this information to display function call arguments.

Each line of a configuration file describes at most a single item.
Lines composed entirely of white space are ignored, as are lines
starting with semicolon character (comment lines). Described items
can be either function prototypes, or definitions of type aliases.


 PROTOTYPES

A prototype describes return type and parameter types of a single
function. The syntax is as follows:



LENS NAME ([LENS{,LENS}]);



NAME is the (mangled) name of a symbol. In the elementary case,
LENS is simply a type. Both lenses and types are described
below. For example, a simple function prototype might look like this:



 int kill(int,int); 


Despite the apparent similarity with C, ltrace.conf is really
its own language that's only somewhat inspired by C.


 TYPES

Ltrace understands a range of primitive types. Those are interpreted
according to C convention native on a given architecture.
E.g. ulong is interpreted as 4-byte unsigned integer on 32-bit
GNU/Linux machine, but 8-byte unsigned integer on 64-bit GNU/Linux
machine.



 void Denotes that a function does not return anything. Can be also used to
construct a generic pointer, i.e. pointer-sized number formatted in
hexadecimal format.


 char 8-bit quantity rendered as a character


 ushort,short Denotes unsigned or signed short integer.


 uint,int Denotes unsigned or signed integer.


 ulong,long Denotes unsigned or signed long integer.


 float Denotes floating point number with single precision.


 double Denotes floating point number with double precision.



Besides primitive types, the following composed types are possible:



 struct([LENS{,LENS}]) Describes a structure with given types as fields,
e.g. struct(int,int,float).

Alignment is computed as customary on the architecture. Custom
alignment (e.g. packed structs) and bit-fields are not supported.
It's also not possible to differentiate between structs and non-POD
C++ classes, for arches where it makes a difference.



 array(LENS,EXPR) Describes array of length EXPR, which is composed of types
described by LENS, e.g. array(int, 6).

Note that in C, arrays in role of function argument decay into
pointers. Ltrace currently handles this automatically, but for full
formal correctness, any such arguments should be described as pointers
to arrays.



 LENS* Describes a pointer to a given type, e.g. char* or int***.
Note that the former example actually describes a pointer to a
character, not a string. See below for string lens, which is
applicable to these cases.


 LENSES

Lenses change the way that types are described. In the simplest case,
a lens is directly a type. Otherwise a type is decorated by the lens.
Ltrace understands the following lenses:



 octal(TYPE) The argument, which should be an integer type, is formatted in base-8.



 hex(TYPE) The argument, which should be an integer type, is formatted in
base-16.



 hide(TYPE) The argument is not shown in argument list.



 bool(TYPE) Arguments with zero value are shown as "false", other are shown as
"true".



 string(TYPE) 

 string[EXPR] 

 string 

The first form of the argument is canonical, the latter two are
syntactic sugar. In the canonical form, the function argument is
formatted as string. The TYPE shall be either a char*, or
array(char,EXPR), or array(char,EXPR)*. If an
array is given, the length will typically be a zero expression
(but doesn't have to be). Using argument that is plain array
(i.e. not a pointer to array) makes sense e.g. in C structs, in cases
like struct(string(array(char, 6))), which describes the C
type struct {char s[6];}.

Because simple C-like strings are pretty common, there are two
shorthand forms. The first shorthand form (with brackets) means the
same as string(array(char, EXPR)*). Plain string
without an argument is then taken to mean the same as
string[zero].

Note that char* by itself describes a pointer to a char. Ltrace
will dereference the pointer, and read and display the single
character that it points to.



 enum(NAME[=VALUE]{,NAME[=VALUE]}) 

 enum[TYPE](NAME[=VALUE]{,NAME[=VALUE]}) 

This describes an enumeration lens. If an argument has any of the
given values, it is instead shown as the corresponding NAME. If
a VALUE is omitted, the next consecutive value following after
the previous VALUE is taken instead. If the first VALUE
is omitted, it's 0 by default.

TYPE, if given, is the underlying type. It is thus possible to
create enums over shorts or longs\(emarguments that are themselves
plain, non-enum types in C, but whose value can be meaningfully
described as enumerations. If omitted, TYPE is taken to be
int.




 TYPE ALIASES

A line in config file can, instead of describing a prototype, create a
type alias. Instead of writing the same enum or struct on many places
(and possibly updating when it changes), one can introduce a name for
such type, and later just use that name:



typedef NAME = TYPE;



Note that TYPE currently cannot be a lens, but must really be a
type.


 EXPRESSIONS

Ltrace has support for some elementary expressions. Each expression
can be either of the following:



 NUM An integer number.



 argNUM Value of NUM-th argument. The expression has the same value as
the corresponding argument. arg1 refers to the first argument,
arg0 to the return value of the given function.

 retval Return value of function, same as arg0.



 emtNUM Value of NUM-th element of the surrounding structure type. E.g.
struct(ulong,array(int,elt1)) describes a structure whose first
element is a length, and second element an array of ints of that
length.



 zero 

 zero(EXPR) 

Describes array which extends until the first element, whose each byte
is 0. If and expression is given, that is the maximum length of that
array. If NUL terminator is not found earlier, that's where the array
ends.




 PARAMETER PACKS

Sometimes the actual function prototype varies slightly depending on
the exact parameters given. For example, the number and types of
printf parameters are not known in advance, but ltrace might be able
to determine them in runtime. This feature has wider applicability,
but currently the only parameter pack that ltrace supports is
printf-style format string itself:



 format When format is seen in the parameter list, the underlying string
argument is parsed, and GNU-style format specifiers are used to
determine what the following actual arguments are. E.g. if the format
string is "%s %d\\n", it's as if the format was replaced by
string, string, int.


 RETURN ARGUMENTS

C functions often use one or more arguments for returning values back
to the caller. The caller provides a pointer to storage, which the
called function initializes. Ltrace has some support for this idiom.

When a traced binary hits a function call, ltrace first fetches all
arguments. It then displays left portion of the argument list.
Only when the function returns does ltrace display right portion
as well. Typically, left portion takes up all the arguments, and
right portion only contains return value. But ltrace allows you to
configure where exactly to put the dividing line by means of a +
operator placed in front of an argument:



 int asprintf(+string*, format); 


Here, the first argument to asprintf is denoted as return argument,
which means that displaying the whole argument list is delayed until
the function returns:



a.out->asprintf( <unfinished ...>


libc.so.6->malloc(100) = 0x245b010


[... more calls here ...]


<... asprintf resumed> "X=1", "X=%d", 1) = 5



It is currently not possible to have an "inout" argument that passes
information in both directions.


 EXAMPLES

In the following, the first is the C prototype, and following that is
ltrace configuration line.



 void func_charp_string(char str[]);  void func_charp_string(string); 


 enum e_foo {RED, GREEN, BLUE}; 

 void func_enum(enum e_foo bar); 

 void func_enum(enum(RED,GREEN,BLUE)); 

- or -


 typedef e_foo = enum(RED,GREEN,BLUE); 

 void func_enum(e_foo); 




 void func_arrayi(int arr[], int len);  void func_arrayi(array(int,arg2)*,int); 


 struct S1 {float f; char a; char b;}; 

 struct S2 {char str[6]; float f;}; 

 struct S1 func_struct(int a, struct S2, double d); 

 struct(float,char,char) func_struct_2(int, struct(string(array(char, 6)),float), double); 



 AUTHOR
Petr Machata <pmachata@redhat.com>