gcc static initialization


(gdb) cont Continuing. The addresses are collected in the .ctors section which will be called by the runtime. For object file formats such as COFF or ELF which support multiple sections, GNU C++ will normally arrange to put the addresses of global constructors and destructors into the .ctors and .dtors sections. Perhaps it refers to static variables or static storage duration.). An executable and all its DT_NEEDED transitive dependencies form a dependency graph. Note: the condition is not satisfied on modern systems. You may succeed, but will probably end up with a degenerate form of shared libraries in the end. (also called initialization routines)functions to initialize Priorities from 0 to 100 are reserved for the implementation (-Wprio-ctor-dtor catches violation), e.g. time and exit time. macro properly. There is a way to get around this problem, but it essentially turns your static library into a dynamic library. If you want to trace through initialization using the Cygwin runtime, you will need to set a breakpoint on a function called __main () -- thats two underscores before the word main. The code in drop-tail-queue.cc may look something like, const ClassId DropTailQueue::cid = MakeClassId ("DropTailQueue", Queue::iid); In order to get this constructor to execute under static linkage, you must reference another function in the corresponding compilation unit, i.e., drop-tail-queue.o. GNU C++ runs destructors either by using atexit, or directly from the function exit. Perhaps it would be helpful for me to post a version of the linking command. However, for a program with many shared objects, the checks may be insufficient.

The compiled code for certain languages includes constructors (gdb) break src/node/drop-tail-queue.cc:__static_initialization_and_destruction_0 Breakpoint 2 at 0x2aaaab196b2c: file src/node/drop-tail-queue.cc, line 111. The order that a, b, and C are registered depends on the link order. a's initializations happen before b's below. two variants. parts of crtstuff.c are compiled into that section. As a side effect, in environments that support standard Unix magic numbers, this option also sets the output file's magic number to OMAGIC. The tool of choice to look at DLL dependencies is called cygcheck. The last variant uses neither arbitrary sections nor the GNU linker. Nonetheless, you can get global and static constructors to work if you have the patience, and it does provide an elegant mechanism for system initialization. To use this variant, you must define the INIT_SECTION_ASM_OP FreeBSD added support in 2012-03. While the standards say yesGood practice indicates that you should always initialise variables. In C++, dynamic initializations for non-local variables happen before the first statement of the main function. I have a static variable declared but uninitialized in a function. (I don't know what "static" refers to. the environment). A compilation unit is a particular C++ file. The linker defines DT_INIT_ARRAY and DT_INIT_ARRAYSZ according to the address and size of .init_array. implementations just ensure such dynamic initializations happen before main. OpenBSD added support in 2016-08.

Often bugs relating to static linkage of C++ programs reflect absolutely correct behavior of the linker. Can a timeseries with a clear trend be considered stationary? ( It is (theoretically) possible to use global constructors in C++ with static linkage, but it is a can of worms we would prefer not to open. A better approach is to make sure that you use the compiler to drive the link process. I have recommended that we prepend the characters ns3- to our DLLS. GCC 4.2 and MSVC 7.1 do seem to treat. There is also no ld.so feature altering the order. I assume that the initialization of the crt isn't occurring, but I'm not sure how to fix that. With this GNU ld support, GCC 4.7 made the switch.

Termination functions are handled similarly. The odd syntax (*&) is required to force gdb to correctly interpret the double colons. When you port the compiler to a new In the llvm-project supported toolchains, only MinGW and PlayStation 4 still use .ctors for the latest version. If an object contains both DT_FINI and DT_FINI_ARRAY entries, the functions referenced by the DT_FINI_ARRAY entry are processed before the one referenced by the DT_FINI entry for that object. pointer containing zero. This is preferable when you want to do dynamic linking and when using Assume that we have one object files a.o and b.o with .ctors sections with different priorities, the layout of the .ctors output section is: .dtors is similar: 123456789crtbegin.o:(.dtors) __DTOR_LIST__a.o:(.dtors) b.o:(.dtors)a.o:(.dtors.00001) b.o:(.dtors.00001)a.o:(.dtors.00002) b.o:(.dtors.00002)a.o:(.dtors.65533) b.o:(.dtors.65533)a.o:(.dtors.65534) b.o:(.dtors.65534)crtend.o:(.dtors) __DTOR_LIST_END__. termination functions, called __DTOR_LIST__. Much of the structure is common to all four variations. gcov uses __attribute__((destructor(100))). system, you need to specify how to do this. NetBSD made DT_INIT_ARRAY available for all ports in 2018-12. musl does not support the feature. Geometry Nodes: How to swap/change a material of a specific material slot? code as its value. Does anyone have experience with something similar. The program cygcheck will report this more clearly than a hidden DLL. When you run the program directly, it exits silently. a.o:(.init_array) b.o:(.init_array) has a different order from a.o:(.ctors) b.o:(.ctors). Does gcc automatically initialize static variables to zero? It's unclear whether .fini_array needs to be reversed as well, but it is safe to do so. Why doesn't the compiler give an error when an uninitialized variable is returned? If you are on a Linux system, your library name will be something like libcore.a or libcore.so; if you are on a Cygwin system, your library name will be something like libcore or core.dll. in libgcc2.c and runs the global constructors. The I don't see why that should be the case. We will perform the same examination as we did on the ELF-based system, running the simple-p2p example program, but this time it is simple-p2p.exe since were actually running on Windows. You may find other DLLs hidden depending on your configuration. This continues until the executable contains all of the code required to execute myprintf (there are no unresolved references). For example, consider a situation where you have a static library called libnode.a; and that library is made from a number of object files. &. I filed morestack.S should support .init_array.0 besides .ctors.65535 which was fixed in 2021-10. The __main function is defined Set a breakpoint, continue and step to get there. First of all, you will most likely find thousands of DLLs on your Windows machine and each of them has been made accessible to Windows in some way. For example, >cygcheck ./simple-p2p.exe C:\cygwin\usr\craigdo\repos\ns-3-dev\build-dir\dbg-shared\lib\core.dll C:\cygwin\usr\craigdo\repos\ns-3-dev\build-dir\dbg-shared\lib\simulator.dll C:\cygwin\usr\craigdo\repos\ns-3-dev\build-dir\dbg-shared\lib\common.dll C:\cygwin\usr\craigdo\repos\ns-3-dev\build-dir\dbg-shared\lib\node.dll Error: could not find applications.dll > I had deleted the applications.dll file from the dbg-shared/lib directory. For example, src/node/drop-tail-queue.cc is a compilation unit. Note: ctors_priority = 65535-init_array_priority. It is supposed to be the ns-3 point-to-point topology DLL, also called p2p.dll. Dynamic initialization has three types with different degrees of order guarantee: Basically, in one translation unit, the order of dynamic initializations usually matches the intuition, e.g. In glibc x86-64, sysdeps/x86_64/crti.S and sysdeps/x86_64/crtn.S provide the definitions for crti.o and crtn.o: crti.o calls __gmon_start__ (gmon profiling system) if defined. You can see that there are windows DLLs getting linked in by Cygwin for free, but everything looks great, right?

I thought the difference was just between thetoolchains, but there was a very subtle difference in the link order between how the two versions were being built that lead to an initialization order conflict. Go ahead and continue until you hit the breakpoint (number two in this example). Likewise w w 0 w u R %| n @ %| hu hu ~ %| u 4 ] ] r $o !^ \ }^ ] ] ] @w @w n d ] ] ] w H_ H_ H_ H_ $8 > > Ns-3 Application Note Why Dont Those #@(%!~& Libraries Work? Lu reported --enable-initfini-array should be enabled for cross compiler to Linux and fixed it for GCC 12. The Windows DLL search path has trumped your LD_LIBRARY_PATH and sucked in the wrong DLL. section is available, the absence of INIT_SECTION_ASM_OP causes rev2022.7.21.42639. If no appearance-ordered relationship is defined, we say that two initializations are indeterminately sequenced. TARGET_ASM_CONSTRUCTOR is defined to produce a .stabs Compiling some languages generates destructors (also called Note that if you have the color version of this document, your attention is called to the function by the red color. accumulates all these words into one contiguous .ctors section. DT_PREINIT_ARRAY only applies to the executable. Constructors are called in reverse order of the On a GCC+glibc system, traditionally the section .init in an executable/shared object consisted of four fragments: The linker combines .init input sections and places the fragments into the .init output section. The linker must then recursively search for further unresolved references in the myprintf function and resolve them by (possibly) looking in other libraries. Unfortunately, this results in what is commonly known as, The Static Initialization Order Fiasco. The initialization order is defined by the run-time system and not the language. Since no init An example of this is shown below. ~  = C \ ` a " (gdb) cont Continuing. If you linked to static libraries, your program is self-contained. In this approach the compiler will call the linker and ask it to do (more or less) the right thing. This can lead to all kinds of subtle errors, including our fundamental problem of not calling global or static constructors at all. Just to make sure that we can get you the correct answer, I have moved your question to the Compilers and Libraries forum. Lu proposed that the internal linker script of GNU ld could be changed to place .ctors .ctors.N .init_array .init_array.N input sections into the .init_array output section in RFC: Support mixing .init_array. It turns out that nothing happened. DT_PREINIT_ARRAY: This element holds the address of the array of pointers to pre-initialization functions, discussed in ``Initialization and Termination Functions'' below. However, I found two exceptions. As others have pointed out, it is good practice to always initialise static variables: The reason for doing this is not because some compiler might not always initialise static variables to zero (any compiler that failed to do such initialisation would be terminally broken, and could not claim to be a C or C++ compiler), it is to Say What You Mean - possibly the most basic rule of programming. It will not do to reference a symbol in another execution unit of the library. PE descends from an older Common Object File Format (COFF) developed for VAX / VMS systems. We recommend against attempting to combine systems using global and static constructors with static linkage. If you linked to dynamic libraries, the system will load your program, but will also need to load any required shared libraries into memory and resolve all of the references that were left marked as unresolved by the linker. () #6 0x00000000 in ?? In ld.lld, Rafael Espindola added --shuffle-sections motivated by making tests stabilized. the ordinary linker, but also arranges to include the vectors of This makes cross compilation and testing multiple targets in one build convenient. Use `-Ur' only for the last partial link, and `-r' for the others. initialization and termination functions. In order to ensure that the global or static constructors of a compilation unit are called, you must directly reference some symbol in the compilation unit object file defining the global constructor. See add preinit_array support. If no init section is available, when GCC compiles any function called If a global variable is initialized to 0, will it go to BSS? Since the DLL is not a Cygwin DLL, it didnt have the expected entry point and the exception was raised. this case, TARGET_HAVE_CTORS_DTORS is false, initialization and You never know when you change compiler, or have to compile it on another machine, you want to minimise any potential for unexpected behaviour. The first initialization is for the iostream which was used in drop-tail-queue.cc and thats what you saw when you entered the breakpoint it turns out you were going to the right place after all. This is often called partial linking.

Defining series before enumitem list starts. Depending on the operating system and its executable file format, either .dtors can be seen as undoing .ctors, so its order is the reverse of .ctors, which is the regular order. As an example, lets try and look at the global and static constructor initialization in the drop-tail-queue compilation unit. You can't do that without a process to debug. musl just provides empty crti.o and crtn.o. Second, GCC cross compilers targeting Linux did not enable --enable-initfini-array. File src/node/llc-snap-header.cc: static void __static_initialization_and_destruction_0(int, int); File src/node/drop-tail-queue.cc: static void __static_initialization_and_destruction_0(int, int); File src/node/queue.cc: static void __static_initialization_and_destruction_0(int, int); (gdb) Now you can set a breakpoint on the function, qualified by the file name just like you did for the ELF case as follows. for a.o:(.ctors) b.o:(.ctors), b.o's constructor runs before a.o's. Switching sections needed to consider backward compatibility: how to handle old object files using .ctors sections. and an a.out format must be used. Trending is based off of the highest score sort and falls back to it if no posts are trending. They contain, among other things, code This affected GCC 11 builds by scripts/build-many-glibcs.py. The linker also defines __preinit_array_start and __preinit_array_end if referenced. The runtime references the symbol _init. @R.. Does anyone have experience with something similar? In practice, most static initialization order fiasco bugs are due to the order between two translation units. * and .ctors. the value to a set; the values are accumulated, and are eventually It does not work to use `-Ur' on files that were themselves linked with `-Ur'; once the constructor table has been built, it cannot be added to. Is there a difference between truing a bike wheel and balancing it? ' / p x C G hA hA OJ QJ ^J hZe OJ QJ ^J hZe hf OJ QJ ^J hZe hf hf 6hc@ hf hf OJ QJ ^J hf CJ aJ hZe CJ aJ hf hf CJ aJ hf hA h\ h\ CJ4 aJ4 h\ CJ4 aJ4 h\ / D E F G H I J K L Y t  & $d %d &d 'd N O P Q gdf $a$gd\ $a$gd\ gdA | B O must output something in the assembler code to cause those functions to Why is My Global or Static Constructor not Called?

The linker never sees an unresolved reference to code in that object file and so it assumes that no code from that particular compilation unit is used, and therefore will ignore any global or static constructors in that file as well. program is linked by the gcc driver like this: The prologue of a function (__init) appears in the .init You can now choose to sort by Trending, which boosts votes that have happened recently, helping to surface more up-to-date answers. The linker also defines __init_array_start and __init_array_end if referenced. Unfortunately, some systems don't do this properly meaning objects may be left initialized. Can anyone Identify the make, model and year of this car? If you want all of the global or static constructors to be called in the library, you must reference something in each of the compilation units that have global constructors defined within. All (most?) If the implementation is found in a static library (e.g., libcore.a), the linker copies the implementation of myprintf into the executable. The interesting piece of implicit information here is that the mechanism whereby the global constructors get executed is run-time system dependent. If you must use global or static constructors, make absolutely sure that they are not interdependent, as the initialization order can be broken by changing the order of libraries in the build system. aside for a list of constructors, and another for a list of destructors. This can be seen by stepping through the initialization. This is followed by a series of zero or more function (gdb) You can now go ahead and continue in order to end up at the static constructor initialization code that is associated with the particular file in question. support arbitrary sections, but does support special designated terminates. The compiler must arrange to actually run the code. I dont like typing much, so I usually do an info functions with only a few letters of the desired function in order to get gdb to fill out everything I need. The Cygwin environment bridges the gap between Linux and Windows and so you have a situation where part of your environment is Linux-like and part is Windows-like. Note that these are also the functions that were optimized out in the static linkage case. We've been building this same setup on Linux directly with distro providedarm-linux-gnueabihf toolchains without issue in the past. crtbegin.o and crtend.o do not provide fragments. H.J. Now disassemble and look for a call to a familiar-looking function: (gdb) disassemble movl $0xffffffff,0xffffff88(%ebp) call 0x83c430 cmpl $0xffff,0xc(%ebp) ---Type to continue, or q to quit---q Quit (gdb) We have found the first static constructor in the drop-tail-queue.cc file. The legacy one uses .init/.ctors while the new one uses .init_array. This option may be used more than once. To see this, run the program under the debugger. How do I use extern to share variables between source files? In the example were still pursuing, drop-tail-queue.cc, there is a call to initialize the IO stream subsystem which you get for free, followed by a definition of a debug component and a call to MakeClassId () that initializes DropTailQueue::cid. In order to use the whole-archive mechanism, youll have to convince the build system to do all of the linkage correctly as defined by the ld document. > #` P bjbj5G5G . W- W- s f p p p Z Z Z 8 Z |[ w 2 4\ 4\ 4\ 4\ 4\ ] ] ] w w w w w w w $ x h M{ @w - H_ ] ] H_ H_ @w 4\ 4\ mw $o $o $o H_ 4\ 4\ w $o H_ w $o $o : t , hu 4\ (\ c@ Z i t u In GCC libgcc/crtstuff.c, when __LIBGCC_INIT_ARRAY_SECTION_ASM_OP__ is not defined and __LIBGCC_INIT_SECTION_ASM_OP__ is defined (HAVE_INITFINI_ARRAY_SUPPORT is 1 in $builddir/gcc/auto-host.h), the following scheme is used. ffmpeg compilation problem: avcodec_find_decoder always returns null, Memory allocation for static variable in C, How to initialize a struct in accordance with C programming language standards. As mentioned above, global or static constructors must be executed before any regular function in a shared library is entered. Breakpoint 2, __static_initialization_and_destruction_0 ( __initialize_p=10922, __priority=-1424380912) at src/node/drop-tail-queue.cc:111 111 }; // namespace ns3 At this point you can disassemble and see the actual initialization code if you like. You can see that the ns-3 DLLs are going to be picked up from the dbg-shared build directory as expected. (gdb) At first glance, it looks like our breakpoint is set somewhere else in the middle of nowhere. Everything builds and appears to run, however, the static variables that should be initialized to a value appear to all still be 0. This is a common problem. This must be done before your program is executed, i.e., before main()is called. The C++ language adds a new twist to the initialization game with its use of global constructors (also called static constructors). US to Canada by car with an enhanced driver's license, no passport? What are the "disks" seen on the walls of some NASA space shuttles? If youve gone through the pain and gotten your global and static constructors called, the next issue is that the order in which the global constructors are called is not defined by the C++ standard except within a compilation unit. file formats which the GNU linker does not support, such as ECOFF. Yeah, no. The compiler can easily ignore explicit zero initialisations. At the point the only thought I've had is to see if directly adding crtbegin.o and friends are needed with this setup, but as I haven't had to do that in the past, I'm really not sure what might be going on. Are propositional atoms recoverable from this Boolean algebra structure? .section .init_array.101,"aw",@init_array, ## legacy: .section .ctors.65434,"aw",@progbits, .section .init_array.102,"aw",@init_array, ## legacy: .section .ctors.65433,"aw",@progbits, ## legacy: .section .ctors,"aw",@progbits, GCC crtbegin.o:(.init) # not existent on modern systems, GCC crtend.o:(.init) # not existent on modern systems, GCC crtbegin.o:(.fini) # not existent on modern systems, GCC crtend.o:(.fini) # not existent on modern systems, 0: 48 83 ec 08 subq $8, %rsp, 4: 48 8b 05 00 00 00 00 movq (%rip), %rax # b <_init+0xb>, 0000000000000007: R_X86_64_REX_GOTPCRELX __gmon_start__-0x4, b: 48 85 c0 testq %rax, %rax, e: 74 02 je 0x12 <_init+0x12>, 10: ff d0 callq *%rax, a.o:(.init_array.65533) b.o:(.init_array.65533), a.o:(.init_array.65534) b.o:(.init_array.65534), __attribute__((used,retain)) void fini() { puts("fini"); }. object file that defines an initialization function also puts a word in From the example above, we were interested in the static constructors for the file drop-tail-queue.cc which is part of the node.dll Windows DLL. and with the address of the void function containing the initialization For these object file formats GNU C++ calls constructors from a subroutine __main; a call to __main is automatically inserted into the startup code for main. Each way has two variants. Just Say No Statically linking a program that uses global or static constructors and destructors in C++ can be a confusing and sometimes tricky proposition. I specialized the seed value -1 to mean the deterministic reversed order. termination functions are recognized simply by their names. 12345678910111213# musl% ./aatexitatexit bfinifini b# glibc, FreeBSD% ./aatexitfinifini batexit b.

Connect and share knowledge within a single location that is structured and easy to search. I thought the difference was just between the toolchains, but there was a very subtle difference in the link order between how the two versions were being built that lead to an initialization Static variables failing to initialize using libgcc/libstdc++. You will have more success using shared libraries, however debugging problems with global and static constructors can be difficult since they are executed before your main program starts and are therefore much more difficult to debug than :normal code. Then again the argument against explicitly zero-initializing static variables is that it expands the size of the executable, because they won't live in .bss anymore. It enforces that a dynamic initialization does not touch memory regions of other global variables. If you have a standard compiler, it is initialized by definition. In 2010-12, Mike Hommey filed Replace .ctors/.dtors with .init_array/.fini_array on targets supporting them which I believe was related to his ELF hack work for Firefox. How to Get that Global Constructor Called The first thing to remember is that global constructors are a C++ language concept. Now, we return to global and static constructor initialization. If youre using gdb you can set a breakpoint at the construction function, (gdb) break __do_global_ctors_aux Breakpoint 1 at 0x403e84 (gdb) run At that point youll be able to see the symbols for your dynamic libraries. For example, you might see, (gdb) disassemble Dump of assembler code for function __static_initialization_and_destruction_0: lea 2205373(%rip),%rdi # 0x2aaaab3b1241 callq 0x2aaaab18b560 (gdb) You can also just step into the initialization function. This GDB was configured as "i686-pc-cygwin" (gdb) dll cygwin1.dll (gdb) break *&'dll::init' Breakpoint 1 at 0x61010270 (gdb) run Loaded symbols for node.dll Breakpoint 1, 0x61010270 in dll::init () from /usr/bin/cygwin1.dll (gdb) At this point, you have the symbols for all of your DLLs loaded, but you need to actually load the DLL youre interested in before setting a breakpoint. arduino correct selected boards sure file specifier