Concepts

ABI artifacts

An ABI artifact is a relevant part of the ABI of a shared library or program. Examples of ABI artifacts are exported types, variables, functions, or ELF symbols exported by a shared library.

The set of ABI artifact for a binary is called an ABI Corpus.

Harmful changes

A change in the diff report is considered harmful if it might cause ABI compatibility issues. That is, it might prevent an application dynamically linked against a given version of a library to keep working with the changed subsequent versions of the same library.

Harmless changes

A change in the diff report is considered harmless if it will not cause any ABI compatibility issue. That is, it will not prevent an application dynamically linked against given version of a library to keep working with the changed subsequent versions of the same library.

By default, abidiff filters harmless changes from the diff report.

Suppression specifications

Definition

A suppression specification file is a way for a user to instruct abidiff, abipkgdiff or any other relevant libabigail tool to avoid emitting reports for changes involving certain ABI artifacts.

It contains directives (or specifications) that describe the set of ABI artifacts to avoid emitting change reports about.

Introductory examples

Its syntax is based on a simplified and customized form of Ini File Syntax. For instance, to specify that change reports on a type named FooPrivateType should be suppressed, one could write this suppression specification:

[suppress_type]
  name = FooPrivateType

If we want to ensure that only change reports about structures named FooPrivateType should be suppressed, we could write:

[suppress_type]
  type_kind = struct
  name = FooPrivateType

But we could also want to suppress change reports avoid typedefs named FooPrivateType. In that case we would write:

[suppress_type]
  type_kind = typedef
  name = FooPrivateType

Or, we could want to suppress change reports about all struct which names end with the string “PrivateType”:

[suppress_type]
  type_kind = struct
  name_regexp = ^.*PrivateType

Let’s now look at the generic syntax of suppression specification files.

Syntax

Properties

More generally, the format of suppression lists is organized around the concept of property. Every property has a name and a value, delimited by the = sign. E.g:

name = value

Leading and trailing white spaces are ignored around property names and values.

Regular expressions

The value of some properties might be a regular expression. In that case, they must comply with the syntax of extended POSIX regular expressions. Note that Libabigail uses the regular expression engine of the GNU C Library.

Escaping a character in a regular expression

When trying to match a string that contains a * character, like in the pointer type int*, one must be careful to notice that the character * is a special character in the extended POSIX regular expression syntax. And that character must be escaped for the regular expression engine. Thus the regular expression that would match the string int* in a suppression file should be

int\\*

Wait; but then why the two \ characters? Well, because the \ character is a special character in the Ini File Syntax used for specifying suppressions. So it must be escaped as well, so that the Ini File parser leaves a \ character intact in the data stream that is handed to the regular expression engine. Hence the \\ targeted at the Ini File parser.

So, in short, to escape a character in a regular expression, always prefix the character with the \\ sequence.

Modus operandi

Suppression specifications can be applied at two different points of the processing pipeline of libabigail.

In the default operating mode called “late suppression mode”, suppression specifications are applied to the result of comparing the in-memory internal representations of two ABIs. In this mode, if an ABI artifact matches a suppression specification, its changes are not mentioned in the ABI change report. The internal representation of the “suppressed” changed ABI artifact is still present in memory; it is just not mentioned in the ABI change report. The change report can still mention statistics about the number of changed ABI artifacts that were suppressed.

There is another operating mode called the “early suppression mode” where suppression specifications are applied during the construction of the in-memory internal representation of a given ABI. In that mode, if an ABI artifact matches a suppression specification, no in-memory internal representation is built for it. As a result, no change about the matched ABI artifact is going to be mentioned in the ABI change report and no statistic about the number of suppressed ABI changes is available. Also, please note that because suppressed ABI artifacts are removed from the in-memory internal representation in this mode, the amount memory used by the internal representation is potentially smaller than the memory consumption in the late suppression mode.

Sections

Properties are then grouped into arbitrarily named sections that shall not be nested. The name of the section is on a line by itself and is surrounded by square brackets, i.e:

[section_name]
property1_name = property1_value
property2_name = property2_value

A section might or might not have properties. Sections that expect to have properties and which are found nonetheless empty are just ignored. Properties that are not recognized by the reader are ignored as well.

Section names

Each different section can be thought of as being a directive to suppress ABI change reports for a particular kind of ABI artifact.

[suppress_file]

This directive prevents a given tool from loading a file (binary or not) if its file name matches certain properties. Thus, if the tool is meant to compare the ABIs of two files, and if the directive prevents it from loading either one of the files, then no comparison is performed.

Note that for the [suppress_file] directive to work, at least one of the following properties must be provided:

file_name_regexp, file_name_not_regexp.

The potential properties of this sections are listed below:

  • file_name_regexp

    Usage:

    file_name_regexp = <regular-expression>

    Prevents the system from loading the file which name matches the regular expression specified as value of this property.

  • file_name_not_regexp

    Usage:

    file_name_not_regexp = <regular-expression>

    Prevents the system from loading the file which name does not match the regular expression specified as value of this property.

  • label

Usage:

label = <some-value>

Define a label for the section. A label is just an informative string that might be used by the tool to refer to a type suppression in error messages.

[suppress_type]

This directive suppresses report messages about a type change.

Note that for the [suppress_type] directive to work, at least one of the following properties must be provided:

file_name_regexp, file_name_not_regexp, soname_regexp, soname_not_regexp, name, name_regexp, type_kind, source_location_not_in, source_location_not_regexp.

If none of the following properties are provided, then the [suppress_type] directive is simply ignored.

The potential properties of this sections are listed below:

  • file_name_regexp

    Usage:

    file_name_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a binary file which name matches the regular expression specified as value of this property.

  • file_name_not_regexp

    Usage:

    file_name_not_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a binary file which name does not match the regular expression specified as value of this property.

  • soname_regexp

    Usage:

    soname_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property matches the regular expression specified as value of this property.

  • soname_not_regexp

    Usage:

    soname_not_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property does not match the regular expression specified as value of this property.

  • name_regexp

Usage:

name_regexp = <regular-expression>

Suppresses change reports involving types whose name matches the regular expression specified as value of this property.

  • name

Usage:

name = <a-value>

Suppresses change reports involving types whose name equals the value of this property.

  • type_kind

Usage:

type_kind = class | struct | union | enum |
array | typedef | builtin

Suppresses change reports involving a certain kind of type. The kind of type to suppress change reports for is specified by the possible values listed above:

  • class: suppress change reports for class types. Note that

    even if class types don’t exist for C, this value still triggers the suppression of change reports for struct types, in C. In C++ however, it should do what it suggests.

  • struct: suppress change reports for struct types in C or C++.

    Note that the value class above is a super-set of this one.

  • union: suppress change reports for union types.

  • enum: suppress change reports for enum types.

  • array: suppress change reports for array types.

  • typedef: suppress change reports for typedef types.

  • builtin: suppress change reports for built-in (or native) types. Example of built-in types are char, int, unsigned int, etc.

  • source_location_not_in

Usage:

source_location_not_in = <list-of-file-paths>

Suppresses change reports involving a type which is defined in a file which path is NOT listed in the value list-of-file-paths. Note that the value is a comma-separated list of file paths e.g, this property

source_location_not_in = libabigail/abg-ir.h, libabigail/abg-dwarf-reader.h

suppresses change reports about all the types that are NOT defined in header files whose path end up with the strings libabigail/abg-ir.h or libabigail/abg-dwarf-reader.h.

  • source_location_not_regexp

Usage:

source_location_not_regexp = <regular-expression>

Suppresses change reports involving a type which is defined in a file which path does NOT match the regular expression provided as value of the property. E.g, this property

source_location_not_regexp = libabigail/abg-.*\\.h

suppresses change reports involving all the types that are NOT defined in header files whose path match the regular expression provided a value of the property.

  • has_data_member_inserted_at

Usage:

has_data_member_inserted_at = <offset-in-bit>

Suppresses change reports involving a type which has at least one data member inserted at an offset specified by the property value offset-in-bit. The value offset-in-bit is either:

  • an integer value, expressed in bits, which denotes the offset of the insertion point of the data member, starting from the beginning of the relevant structure or class.
  • the keyword end which is a named constant which value equals the offset of the end of the of the structure or class.
  • the function call expression offset_of(data-member-name) where data-member-name is the name of a given data member of the relevant structure or class. The value of this function call expression is an integer that represents the offset of the data member denoted by data-member-name.
  • the function call expression offset_after(data-member-name) where data-member-name is the name of a given data member of the relevant structure or class. The value of this function call expression is an integer that represents the offset of the point that comes right after the region occupied by the data member denoted by data-member-name.
  • has_data_member_inserted_between

Usage:

has_data_member_inserted_between = {<range-begin>, <range-end>}

Suppresses change reports involving a type which has at least one data mber inserted at an offset that is comprised in the range between range-begin`` and range-end. Please note that each of the lues range-begin and range-end can be of the same form as the has_data_member_inserted_at property above.

Usage examples of this properties are:

has_data_member_inserted_between = {8, 64}

or:

has_data_member_inserted_between = {16, end}

or:

has_data_member_inserted_between = {offset_after(member1), end}
  • has_data_members_inserted_between

Usage:

has_data_members_inserted_between = {<sequence-of-ranges>}

Suppresses change reports involving a type which has multiple data member inserted in various offset ranges. A usage example of this property is, for instance:

has_data_members_inserted_between = {{8, 31}, {72, 95}}

This usage example suppresses change reports involving a type which has data members inserted in bit offset ranges [8 31] and [72 95]. The length of the sequence of ranges or this has_data_members_inserted_between is not bounded; it can be as long as the system can cope with. The values of the boundaries of the ranges are of the same kind as for the has_data_member_inserted_at property above.

Another usage example of this property is thus:

has_data_members_inserted_between =
  {
       {offset_after(member0), offset_of(member1)},
       {72, end}
  }
  • accessed_through

Usage:

accessed_through = <some-predefined-values>

Suppress change reports involving a type which is referred to either directly or through a pointer or a reference. The potential values of this property are the predefined keywords below:

  • direct

    So if the [suppress_type] contains the property description:

    accessed_through = direct
    

    then changes about a type that is referred-to directly (i.e, not through a pointer or a reference) are going to be suppressed.

  • pointer

    If the accessed_through property is set to the value pointer then changes about a type that is referred-to through a pointer are going to be suppressed.

  • reference

    If the accessed_through property is set to the value reference then changes about a type that is referred-to through a reference are going to be suppressed.

  • reference-or-pointer

    If the accessed_through property is set to the value reference-or-pointer then changes about a type that is referred-to through either a reference or a pointer are going to be suppressed.

For an extensive example of how to use this property, please check out the example below about suppressing change reports about types accessed either directly or through pointers.

  • drop

Usage:

drop = yes | no

If a type is matched by a suppression specification which contains the “drop” property set to “yes” (or to “true”) then the type is not even going to be represented in the internal representation of the ABI being analyzed. This property makes its enclosing suppression specification to be applied in the early suppression specification mode. The net effect is that it potentially reduces the memory used to represent the ABI being analyzed.

Please note that for this property to be effective, the enclosing suppression specification must have at least one of the following properties specified: name_regexp, name, name_regexp, source_location_not_in or source_location_not_regexp.

  • label

Usage:

label = <some-value>

Define a label for the section. A label is just an informative string that might be used by a tool to refer to a type suppression in error messages.

[suppress_function]

This directive suppresses report messages about changes on a set of functions.

Note that for the [suppress_function] directive to work, at least one of the following properties must be provided:

label, file_name_regexp, file_name_not_regexp, soname_regexp, soname_not_regexp, name, name_regexp, name_not_regexp, parameter, return_type_name,

symbol_name, symbol_name_regexp, symbol_version,

symbol_version_regexp.

If none of the following properties are provided, then the [suppress_function] directive is simply ignored.

The potential properties of this sections are:

  • label

Usage:

label = <some-value>

This property is the same as the label property defined above.

  • file_name_regexp

    Usage:

    file_name_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a binary file which name matches the regular expression specified as value of this property.

  • file_name_not_regexp

    Usage:

    file_name_not_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a binary file which name does not match the regular expression specified as value of this property.

  • soname_regexp

    Usage:

    soname_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property matches the regular expression specified as value of this property.

  • soname_not_regexp

    Usage:

    soname_not_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property does not match the regular expression specified as value of this property.

  • name

Usage:

name = <some-value>

Suppresses change reports involving functions whose name equals the value of this property.

  • name_regexp

Usage:

name_regexp = <regular-expression>

Suppresses change reports involving functions whose name matches the regular expression specified as value of this property.

Let’s consider the case of functions that have several symbol names. This happens when the underlying symbol for the function has aliases. Each symbol name is actually one alias name.

In this case, if the regular expression matches the name of at least one of the aliases names, then it must match the names of all of the aliases of the function for the directive to actually suppress the diff reports for said function.

  • name_not_regexp

Usage:

name_not_regexp = <regular-expression>

Suppresses change reports involving functions whose names don’t match the regular expression specified as value of this property.

The rules for functions that have several symbol names are the same rules as for the name_regexp property above.

  • change_kind

Usage:

change_kind = <predefined-possible-values>

Specifies the kind of changes this suppression specification should apply to. The possible values of this property as well as their meaning are listed below:

  • function-subtype-change

    This suppression specification applies to functions that which have at least one sub-type that has changed.

  • added-function

    This suppression specification applies to functions that have been added to the binary.

  • deleted-function

    This suppression specification applies to functions that have been removed from the binary.

  • all

    This suppression specification applies to functions that have all of the changes above. Note that not providing the change_kind property at all is equivalent to setting it to the value all.

  • parameter

Usage:

parameter = <function-parameter-specification>

Suppresses change reports involving functions whose parameters match the parameter specification indicated as value of this property.

The format of the function parameter specification is:

' <parameter-index> <space> <type-name-or-regular-expression>

That is, an apostrophe followed by a number that is the index of the parameter, followed by one of several spaces, followed by either the name of the type of the parameter, or a regular expression describing a family of parameter type names.

If the parameter type name is designated by a regular expression, then said regular expression must be enclosed between two slashes; like /some-regular-expression/.

The index of the first parameter of the function is zero. Note that for member functions (methods of classes), the this is the first parameter that comes after the implicit “this” pointer parameter.

Examples of function parameter specifications are:

'0 int

Which means, the parameter at index 0, whose type name is int.

'4 unsigned char*

Which means, the parameter at index 4, whose type name is unsigned char*.

'2 /^foo.*&/

Which means, the parameter at index 2, whose type name starts with the string “foo” and ends with an ‘&’. In other words, this is the third parameter and it’s a reference on a type that starts with the string “foo”.

  • return_type_name

Usage:

return_type_name = <some-value>

Suppresses change reports involving functions whose return type name equals the value of this property.

  • return_type_regexp

Usage:

return_type_regexp = <regular-expression>

Suppresses change reports involving functions whose return type name matches the regular expression specified as value of this property.

  • symbol_name

Usage:

symbol_name = <some-value>

Suppresses change reports involving functions whose symbol name equals the value of this property.

  • symbol_name_regexp

Usage:

symbol_name_regexp = <regular-expression>

Suppresses change reports involving functions whose symbol name matches the regular expression specified as value of this property.

Let’s consider the case of functions that have several symbol names. This happens when the underlying symbol for the function has aliases. Each symbol name is actually one alias name.

In this case, the regular expression must match the names of all of the aliases of the function for the directive to actually suppress the diff reports for said function.

  • symbol_version

Usage:

symbol_version = <some-value>

Suppresses change reports involving functions whose symbol version equals the value of this property.

  • symbol_version_regexp

Usage:

symbol_version_regexp = <regular-expression>

Suppresses change reports involving functions whose symbol version matches the regular expression specified as value of this property.

  • drop

Usage:

drop = yes | no

If a function is matched by a suppression specification which contains the “drop” property set to “yes” (or to “true”) then the function is not even going to be represented in the internal representation of the ABI being analyzed. This property makes its enclosing suppression specification to be applied in the early suppression specification mode. The net effect is that it potentially reduces the memory used to represent the ABI being analyzed.

Please note that for this property to be effective, the enclosing suppression specification must have at least one of the following properties specified: name_regexp, name, name_regexp, source_location_not_in or source_location_not_regexp.

[suppress_variable]

This directive suppresses report messages about changes on a set of variables.

Note that for the [suppress_variable] directive to work, at least one of the following properties must be provided:

label, file_name_regexp, file_name_not_regexp, soname_regexp, soname_not_regexp, name, name_regexp, symbol_name, symbol_name_regexp, symbol_version, symbol_version_regexp.

If none of the following properties are provided, then the [suppres_variable] directive is simply ignored.

The potential properties of this sections are:

  • label

Usage:

label = <some-value>

This property is the same as the label property defined above.

  • file_name_regexp

    Usage:

    file_name_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a binary file which name matches the regular expression specified as value of this property.

  • file_name_not_regexp

    Usage:

    file_name_not_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a binary file which name does not match the regular expression specified as value of this property.

  • soname_regexp

    Usage:

    soname_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property matches the regular expression specified as value of this property.

  • soname_not_regexp

    Usage:

    soname_not_regexp = <regular-expression>

    Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property does not match the regular expression specified as value of this property.

  • name

Usage:

name = <some-value>

Suppresses change reports involving variables whose name equals the value of this property.

  • name_regexp

Usage:

name_regexp = <regular-expression>

Suppresses change reports involving variables whose name matches the regular expression specified as value of this property.

  • change_kind

Usage:

change_kind = <predefined-possible-values>

Specifies the kind of changes this suppression specification should apply to. The possible values of this property as well as their meaning are the same as when it’s used in the [suppress_function] section.

  • symbol_name

Usage:

symbol_name = <some-value>

Suppresses change reports involving variables whose symbol name equals the value of this property.

  • symbol_name_regexp

Usage:

symbol_name_regexp = <regular-expression>

Suppresses change reports involving variables whose symbol name matches the regular expression specified as value of this property.

  • symbol_version

Usage:

symbol_version = <some-value>

Suppresses change reports involving variables whose symbol version equals the value of this property.

  • symbol_version_regexp

Usage:

symbol_version_regexp = <regular-expression>

Suppresses change reports involving variables whose symbol version matches the regular expression specified as value of this property.

  • type_name

Usage:

type_name = <some-value>

Suppresses change reports involving variables whose type name equals the value of this property.

  • type_name_regexp

Usage:

type_name_regexp = <regular-expression>

Suppresses change reports involving variables whose type name matches the regular expression specified as value of this property.

Comments

; or # ASCII character at the beginning of a line indicates a comment. Comment lines are ignored.

Code examples

  1. Suppressing change reports about types.

    Suppose we have a library named libtest1-v0.so which contains this very useful code:

    $ cat -n test1-v0.cc
         1  // A forward declaration for a type considered to be opaque to
         2  // function foo() below.
         3  struct opaque_type;
         4
         5  // This function cannot touch any member of opaque_type.  Hence,
         6  // changes to members of opaque_type should not impact foo, as far as
         7  // ABI is concerned.
         8  void
         9  foo(opaque_type*)
        10  {
        11  }
        12
        13  struct opaque_type
        14  {
        15    int member0;
        16    char member1;
        17  };
    $

Let’s change the layout of struct opaque_type by inserting a data member around line 15, leading to a new version of the library, that we shall name libtest1-v1.so:

$ cat -n test1-v1.cc
     1  // A forward declaration for a type considered to be opaque to
     2  // function foo() below.
     3  struct opaque_type;
     4
     5  // This function cannot touch any member of opaque_type;  Hence,
     6  // changes to members of opaque_type should not impact foo, as far as
     7  // ABI is concerned.
     8  void
     9  foo(opaque_type*)
    10  {
    11  }
    12
    13  struct opaque_type
    14  {
    15    char added_member; // <-- a new member got added here now.
    16    int member0;
    17    char member1;
    18  };
$

Let’s compile both examples. We shall not forget to compile them with debug information generation turned on:

$ g++ -shared -g -Wall -o libtest1-v0.so test1-v0.cc
$ g++ -shared -g -Wall -o libtest1-v1.so test1-v1.cc

Let’s ask abidiff which ABI differences it sees between libtest1-v0.so and libtest1-v1.so:

$ abidiff libtest1-v0.so libtest1-v1.so
Functions changes summary: 0 Removed, 1 Changed, 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

1 function with some indirect sub-type change:

  [C]'function void foo(opaque_type*)' has some indirect sub-type changes:
    parameter 0 of type 'opaque_type*' has sub-type changes:
      in pointed to type 'struct opaque_type':
        size changed from 64 to 96 bits
        1 data member insertion:
          'char opaque_type::added_member', at offset 0 (in bits)
        2 data member changes:
         'int opaque_type::member0' offset changed from 0 to 32
         'char opaque_type::member1' offset changed from 32 to 64

So abidiff reports that the opaque_type’s layout has changed in a significant way, as far as ABI implications are concerned, in theory. After all, a sub-type (struct opaque_type) of an exported function (foo()) has seen its layout change. This might have non negligible ABI implications. But in practice here, the programmer of the litest1-v1.so library knows that the “soft” contract between the function foo() and the type struct opaque_type is to stay away from the data members of the type. So layout changes of struct opaque_type should not impact foo().

Now to teach abidiff about this soft contract and have it avoid emitting what amounts to false positives in this case, we write the suppression specification file below:

$ cat test1.suppr
[suppress_type]
  type_kind = struct
  name = opaque_type

Translated in plain English, this suppression specification would read: “Do not emit change reports about a struct which name is opaque_type”.

Let’s now invoke abidiff on the two versions of the library again, but this time with the suppression specification:

$ abidiff --suppressions test1.suppr libtest1-v0.so libtest1-v1.so
Functions changes summary: 0 Removed, 0 Changed (1 filtered out), 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

As you can see, abidiff does not report the change anymore; it tells us that it was filtered out instead.

Suppressing change reports about types with data member insertions

Suppose the first version of a library named libtest3-v0.so has this source code:

/* Compile this with:
     gcc -g -Wall -shared -o libtest3-v0.so test3-v0.c
 */

struct S
{
  char member0;
  int member1; /*
                  between member1 and member2, there is some padding,
                  at least on some popular platforms.  On
                  these platforms, adding a small enough data
                  member into that padding shouldn't change
                  the offset of member1.  Right?
                */
};

int
foo(struct S* s)
{
  return s->member0 + s->member1;
}

Now, suppose the second version of the library named libtest3-v1.so has this source code in which a data member has been added in the padding space of struct S and another data member has been added at its end:

/* Compile this with:
     gcc -g -Wall -shared -o libtest3-v1.so test3-v1.c
 */

struct S
{
  char member0;
  char inserted1; /* <---- A data member has been added here...  */
  int member1;
  char inserted2; /* <---- ... and another one has been added here.  */
};

int
foo(struct S* s)
{
  return s->member0 + s->member1;
}

In libtest3-v1.so, adding char data members S::inserted1 and S::inserted2 can be considered harmless (from an ABI compatibility perspective), at least on the x86 platform, because that doesn’t change the offsets of the data members S::member0 and S::member1. But then running abidiff on these two versions of library yields:

$ abidiff libtest3-v0.so libtest3-v1.so
Functions changes summary: 0 Removed, 1 Changed, 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

1 function with some indirect sub-type change:

  [C]'function int foo(S*)' has some indirect sub-type changes:
    parameter 0 of type 'S*' has sub-type changes:
      in pointed to type 'struct S':
        type size changed from 64 to 96 bits
        2 data member insertions:
          'char S::inserted1', at offset 8 (in bits)
          'char S::inserted2', at offset 64 (in bits)
$

That is, abidiff shows us the two changes, even though we (the developers of that very involved library) know that these changes are harmless in this particular context.

Luckily, we can devise a suppression specification that essentially tells abidiff to filter out change reports about adding a data member between S::member0 and S::member1, and adding a data member at the end of struct S. We have written such a suppression specification in a file called test3-1.suppr and it unsurprisingly looks like:

[suppress_type]
  name = S
  has_data_member_inserted_between = {offset_after(member0), offset_of(member1)}
  has_data_member_inserted_at = end

Now running abidiff with this suppression specification yields:

$ ../build/tools/abidiff --suppressions test3-1.suppr libtest3-v0.so libtest3-v1.so
Functions changes summary: 0 Removed, 0 Changed (1 filtered out), 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

$

Hooora! \o/ (I guess)

Suppressing change reports about types accessed either directly or through pointers

Suppose we have a first version of an object file which source code is the file widget-v0.cc below:

// Compile with: g++ -g -c widget-v0.cc

struct widget
{
  int x;
  int y;

  widget()
    :x(), y()
  {}
};

void
fun0(widget*)
{
  // .. do stuff here.
}

void
fun1(widget&)
{
  // .. do stuff here ..
}

void
fun2(widget w)
{
  // ... do other stuff here ...
}

Now suppose in the second version of that file, named widget-v1.cc, we have added some data members at the end of the type struct widget; here is what the content of that file would look like:

// Compile with: g++ -g -c widget-v1.cc

struct widget
{
  int x;
  int y;
  int w; // We have added these two new data members here ..
  int h; // ... and here.

  widget()
    : x(), y(), w(), h()
  {}
};

void
fun0(widget*)
{
  // .. do stuff here.
}

void
fun1(widget&)
{
  // .. do stuff here ..
}

void
fun2(widget w)
{
  // ... do other stuff here ...
}

When we invoke abidiff on the object files resulting from the compilation of the two file above, here is what we get:

 $ abidiff widget-v0.o widget-v1.o
 Functions changes summary: 0 Removed, 2 Changed (1 filtered out), 0 Added functions
 Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

 2 functions with some indirect sub-type change:

   [C]'function void fun0(widget*)' has some indirect sub-type changes:
     parameter 1 of type 'widget*' has sub-type changes:
       in pointed to type 'struct widget':
         type size changed from 64 to 128 bits
         2 data member insertions:
           'int widget::w', at offset 64 (in bits)
           'int widget::h', at offset 96 (in bits)

   [C]'function void fun2(widget)' has some indirect sub-type changes:
     parameter 1 of type 'struct widget' has sub-type changes:
       details were reported earlier
$

I guess a little bit of explaining is due here. abidiff detects that two data member got added at the end of struct widget. it also tells us that the type change impacts the exported function fun0() which uses the type struct widget through a pointer, in its signature.

Careful readers will notice that the change to struct widget also impacts the exported function fun1(), that uses type struct widget through a reference. But then abidiff doesn’t tell us about the impact on that function fun1() because it has evaluated that change as being redundant with the change it reported on fun0(). It has thus filtered it out, to avoid cluttering the output with noise.

Redundancy detection and filtering is fine and helpful to avoid burying the important information in a sea of noise. However, it must be treated with care, by fear of mistakenly filtering out relevant and important information.

That is why abidiff tells us about the impact that the change to struct widget has on function fun2(). In this case, that function uses the type struct widget directly (in its signature). It does not use it via a pointer or a reference. In this case, the direct use of this type causes fun2() to be exposed to a potentially harmful ABI change. Hence, the report about fun2() is not filtered out, even though it’s about that same change on struct widget.

To go further in suppressing reports about changes that are harmless and keeping only those that we know are harmful, we would like to go tell abidiff to suppress reports about this particular struct widget change when it impacts uses of struct widget through a pointer or reference. In other words, suppress the change reports about fun0() and fun1(). We would then write this suppression specification, in file widget.suppr:

[suppress_type]
  name = widget
  type_kind = struct
  has_data_member_inserted_at = end
  accessed_through = reference-or-pointer

  # So this suppression specification says to suppress reports about
  # the type 'struct widget', if this type was added some data member
  # at its end, and if the change impacts uses of the type through a
  # reference or a pointer.

Invoking abidiff on widget-v0.o and widget-v1.o with this suppression specification yields:

$ abidiff --suppressions widget.suppr widget-v0.o widget-v1.o
Functions changes summary: 0 Removed, 1 Changed (2 filtered out), 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

1 function with some indirect sub-type change:

  [C]'function void fun2(widget)' has some indirect sub-type changes:
    parameter 1 of type 'struct widget' has sub-type changes:
      type size changed from 64 to 128 bits
      2 data member insertions:
        'int widget::w', at offset 64 (in bits)
        'int widget::h', at offset 96 (in bits)
$

As expected, I guess.

Suppressing change reports about functions.

Suppose we have a first version a library named libtest2-v0.so whose source code is:

 $ cat -n test2-v0.cc

  1     struct S1
  2     {
  3       int m0;
  4
  5       S1()
  6         : m0()
  7       {}
  8     };
  9
 10     struct S2
 11     {
 12       int m0;
 13
 14       S2()
 15         : m0()
 16       {}
 17     };
 18
 19     struct S3
 20     {
 21       int m0;
 22
 23       S3()
 24         : m0()
 25       {}
 26     };
 27
 28     int
 29     func(S1&)
 30     {
 31       // suppose the code does something with the argument.
 32       return 0;
 33
 34     }
 35
 36     char
 37     func(S2*)
 38     {
 39       // suppose the code does something with the argument.
 40       return 0;
 41     }
 42
 43     unsigned
 44     func(S3)
 45     {
 46       // suppose the code does something with the argument.
 47       return 0;
 48     }
$

And then we come up with a second version libtest2-v1.so of that library; the source code is modified by making the structures S1, S2, S3 inherit another struct:

$ cat -n test2-v1.cc
      1 struct base_type
      2 {
      3   int m_inserted;
      4 };
      5
      6 struct S1 : public base_type // <--- S1 now has base_type as its base
      7                              // type.
      8 {
      9   int m0;
     10
     11   S1()
     12     : m0()
     13   {}
     14 };
     15
     16 struct S2 : public base_type // <--- S2 now has base_type as its base
     17                              // type.
     18 {
     19   int m0;
     20
     21   S2()
     22     : m0()
     23   {}
     24 };
     25
     26 struct S3 : public base_type // <--- S3 now has base_type as its base
     27                              // type.
     28 {
     29   int m0;
     30
     31   S3()
     32     : m0()
     33   {}
     34 };
     35
     36 int
     37 func(S1&)
     38 {
     39   // suppose the code does something with the argument.
     40   return 0;
     41
     42 }
     43
     44 char
     45 func(S2*)
     46 {
     47   // suppose the code does something with the argument.
     48   return 0;
     49 }
     50
     51 unsigned
     52 func(S3)
     53 {
     54   // suppose the code does something with the argument.
     55   return 0;
     56 }
 $

Now let’s build the two libraries:

g++ -Wall -g -shared -o libtest2-v0.so test2-v0.cc
g++ -Wall -g -shared -o libtest2-v0.so test2-v0.cc

Let’s look at the output of abidiff:

$ abidiff libtest2-v0.so libtest2-v1.so
Functions changes summary: 0 Removed, 3 Changed, 0 Added functions
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

3 functions with some indirect sub-type change:

  [C]'function unsigned int func(S3)' has some indirect sub-type changes:
    parameter 0 of type 'struct S3' has sub-type changes:
      size changed from 32 to 64 bits
      1 base class insertion:
        struct base_type
      1 data member change:
       'int S3::m0' offset changed from 0 to 32

  [C]'function char func(S2*)' has some indirect sub-type changes:
    parameter 0 of type 'S2*' has sub-type changes:
      in pointed to type 'struct S2':
        size changed from 32 to 64 bits
        1 base class insertion:
          struct base_type
        1 data member change:
         'int S2::m0' offset changed from 0 to 32

  [C]'function int func(S1&)' has some indirect sub-type changes:
    parameter 0 of type 'S1&' has sub-type changes:
      in referenced type 'struct S1':
        size changed from 32 to 64 bits
        1 base class insertion:
          struct base_type
        1 data member change:
         'int S1::m0' offset changed from 0 to 32
$

Let’s tell abidiff to avoid showing us the differences on the overloads of func that takes either a pointer or a reference. For that, we author this simple suppression specification:

$ cat -n libtest2.suppr
     1 [suppress_function]
     2   name = func
     3   parameter = '0 S1&
     4
     5 [suppress_function]
     6   name = func
     7   parameter = '0 S2*
$

And then let’s invoke abidiff with the suppression specification:

$ ../build/tools/abidiff --suppressions libtest2.suppr libtest2-v0.so libtest2-v1.so
Functions changes summary: 0 Removed, 1 Changed (2 filtered out), 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

1 function with some indirect sub-type change:

       [C]'function unsigned int func(S3)' has some indirect sub-type changes:
         parameter 0 of type 'struct S3' has sub-type changes:
           size changed from 32 to 64 bits
           1 base class insertion:
             struct base_type
           1 data member change:
            'int S3::m0' offset changed from 0 to 32

The suppression specification could be reduced using regular expressions:

$ cat -n libtest2-1.suppr
          1   [suppress_function]
          2     name = func
          3     parameter = '0 /^S.(&|\\*)/
$

$ ../build/tools/abidiff --suppressions libtest2-1.suppr libtest2-v0.so libtest2-v1.so
Functions changes summary: 0 Removed, 1 Changed (2 filtered out), 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

1 function with some indirect sub-type change:

       [C]'function unsigned int func(S3)' has some indirect sub-type changes:
         parameter 0 of type 'struct S3' has sub-type changes:
           size changed from 32 to 64 bits
           1 base class insertion:
             struct base_type
           1 data member change:
            'int S3::m0' offset changed from 0 to 32

$