C/C++ std libs
Recently, I was revisiting the C++ std::time function and suddenly get triggered by the std::time_t type. Quoted from cppreference,
Although not defined, this is almost always an integral value holding the number of seconds (not counting leap seconds) since 00:00, Jan 1 1970 UTC, corresponding to POSIX time
I have visited this page many times in my career life, but this is the first time I go deeper into it. This type is only declared by C++ standard, but not defined by C++ standard. Then where is it defined?
libc++ and libstdc++
We know there are three main C++ compilers: GCC, LLVM and Microsoft Visual C++ compiler. Each of them has their own implementation of the C++ standard library. Gcc’s implementation is called libstdc++ and the source code is here. LLVM’s implementation is called libc++ and the source code is here.
For example, std::vector implementation is here in libc++. The libstdc++ version is more complicated. It is scattered in a few places: here, here and here. As you can see that files under the bits subfolder make up the implementation.
Libc and glibc
Before we answer the question where std::time_t is defined, let’s take a look at the function std::time_t std::time(std::time_t *) in ctime header.
First In libc++, the declaration is here. If I click go to definition, my IDE jumps to file /Library/Developer/CommandLineTools/SDKs/MacOSX14.0.sdk/usr/include/time.h, which is here. This repo Libc is the standard C library provided by MacOS.
In libstdc++, the std::time function is declared here
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#pragma GCC system_header
#include_next <time.h>
Actually, it is not defined at all. It is imported from somewhere. include_next means please use the second time.h file in the search path. Each C++ program will link with glibc as well. And time.h header is defined here.
To sum up, under libc++ and libstdc++ are Libc and glibc. They implement the C standard.
Operating systems
We still haven’t answered the question where std::time_t is defined.
In my Macbook with Apple chip, the definition of std::time_t is in /Library/Developer/CommandLineTools/SDKs/MacOSX14.0.sdk/usr/include/sys/_types/_time_t.h.
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typedef __darwin_time_t time_t;
Also, __darwin_time_t is defined in another file /Library/Developer/CommandLineTools/SDKs/MacOSX14.0.sdk/usr/include/arm/_types.h,
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typedef long __darwin_time_t; /* time() */
Ah! std::time_t is just long. This type may be different in your laptop because your laptop may have a different architecture. So we know std::time_t is defined by the operating system.
So I keep showing the header files in my local laptop, but where are the source code? The answer is “XNU”, i.e., “X is Not Unix.”, namely, the MacOS operating system kernel. For example, the above definition of __darwin_time_t is here.
How about Linux? Below is what I found in a Linux docker container.
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$ cat /usr/include/x86_64-linux-gnu/sys/time.h | grep time_t
#include <bits/types/time_t.h>
$ cat /usr/include/x86_64-linux-gnu/bits/types/time_t.h
typedef __time_t time_t;
$ cat /usr/include/x86_64-linux-gnu/bits/types.h | grep __time_t
__STD_TYPE __TIME_T_TYPE __time_t; /* Seconds since the Epoch. */
$ cat /usr/include/x86_64-linux-gnu/bits/typesizes.h | grep __TIME_T_TYPE
#define __TIME_T_TYPE __SYSCALL_SLONG_TYPE
$ cat /usr/include/x86_64-linux-gnu/bits/typesizes.h | grep __SLONGWORD_TYPE
# define __SYSCALL_SLONG_TYPE __SLONGWORD_TYPE
$ cat /usr/include/x86_64-linux-gnu/bits/types.h | grep __SLONGWORD_TYPE
#define __SLONGWORD_TYPE long int
Ah! OK. std::time_t is defined as long int in Linux. You can find this file in the Linux github repo as well.
We have gone so far in finding the definition of std::time_t!
glibc
Glibc is the standard C library for the GNU operating system and other systems that use the Linux kernel. For a long time, I am confused by what exactly glibc provides because I saw lots of code that include headers such as <sys/epoll.h> and <unistd.h>. I definitely did not see these headers when I was learning C when I was a undergraduate. Cited from glibc’s home page
The GNU C Library - The project provides the core libraries for the GNU system and GNU/Linux systems, as well as many other systems that use Linux as the kernel. These libraries provide critical APIs including ISO C11, POSIX.1-2008, BSD, OS-specific APIs and more. These APIs include such foundational facilities as open, read, write, malloc, printf, getaddrinfo, dlopen, pthread_create, crypt, login, exit and more.
So basically, it not only supports ISO C, but also POSIX.1 standard. And there are quite a few Linux specific system calls. The C standard header files are here. POSIX.1 header files are here. The headers provided by glibc is under the include directory. There are much more headers inside include/sys than specified by POSIX.1. Most of them are wrappers to Linux system calls.
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Header POSIX header
-----------------------------------------------------------
include/sys/ipc.h Y
include/sys/mman.h Y
include/sys/msg.h Y
include/sys/resource.h Y
include/sys/select.h Y
include/sys/sem.h Y
include/sys/shm.h Y
include/sys/socket.h Y
include/sys/statvfs.h Y
include/sys/stat.h Y
include/sys/times.h Y
include/sys/time.h Y
include/sys/types.h Y
include/sys/uio.h Y
include/sys/un.h Y
include/sys/utsname.h Y
include/sys/wait.h Y
-----
include/sys/unistd.h N
include/sys/ttychars.h N
include/sys/timeb.h N
include/sys/termios.h N
include/sys/sysmacros.h N
include/sys/syslog.h N
include/sys/sysinfo.h N
include/sys/statfs.h N
include/sys/single_threaded.h N
include/sys/signal.h N
include/sys/sendfile.h N
include/sys/resource.h N
include/sys/random.h N
include/sys/queue.h N
include/sys/profil.h N
include/sys/prctl.h N
include/sys/poll.h N
include/sys/param.h N
include/sys/ioctl.h N
include/sys/gmon_out.h N
include/sys/gmon.h N
include/sys/file.h N
include/sys/fcntl.h N
include/sys/errno.h N
include/sys/epoll.h N
include/sys/dir.h N
include/sys/cdefs.h N
include/sys/bitypes.h N
include/sys/auxv.h N
include/sys/xattr.h N
include/sys/vlimit.h N
include/sys/vfs.h N
build
Building glibc in Linux is straightforward.
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git clone git://sourceware.org/git/glibc.git --depth=1
cd glibc
mkdir -p build
cd build
../configure --prefix /home/admin/code/glibc/build
bear -- make -j
cd ..
ln -s build/compile_commands.json compile_commands.json
The bear make step takes a long time. Be patient!
I failed to cross compile it in Macbook M1.
backtrace in glibc
There are multiple ways to get stacktraces in C/C++. This is great blog on this topic. It claims there are three ways to obtain a stacktrace. First is the gcc’s builtin function __builtin_return_address. I tried to read relevant code in gcc, but quickly got lost as the implementation of this builtin function finally resorts to register manipulation.
Let’s talk about the third option libunwind then. It claims to be portable and efficient to unwind stack traces. Let’s focus on the backtrace functionality. The core code is in file Backtrace.c which defines function _Unwind_Backtrace.
The second’s option is glibc’s backtrace function. Note, the backtrace symbol is exposed by weak_alias (__backtrace, backtrace). This function is simple. It calls __libc_unwind_link_get() to get the real loaded shared library which contains a symbol _Unwind_Backtrace. So basically backtrace function delegates all work to this shared library. The shared library is called LIBGCC_S_SO. I searched the whole repo but cannot find the definition of this macro. After compiling glibc, I find a file called lib-names-64.h with below content
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/* This file is automatically generated. */
#ifndef __GNU_LIB_NAMES_H
# error "Never use <gnu/lib-names-64.h> directly; include <gnu/lib-names.h> instead."
#endif
#define LD_LINUX_X86_64_SO "ld-linux-x86-64.so.2"
#define LD_SO "ld-linux-x86-64.so.2"
#define LIBANL_SO "libanl.so.1"
#define LIBBROKENLOCALE_SO "libBrokenLocale.so.1"
#define LIBC_MALLOC_DEBUG_SO "libc_malloc_debug.so.0"
#define LIBC_SO "libc.so.6"
#define LIBDL_SO "libdl.so.2"
#define LIBGCC_S_SO "libgcc_s.so.1"
#define LIBMVEC_SO "libmvec.so.1"
#define LIBM_SO "libm.so.6"
#define LIBNSL_SO "libnsl.so.1"
#define LIBNSS_COMPAT_SO "libnss_compat.so.2"
#define LIBNSS_DB_SO "libnss_db.so.2"
#define LIBNSS_DNS_SO "libnss_dns.so.2"
#define LIBNSS_FILES_SO "libnss_files.so.2"
#define LIBNSS_HESIOD_SO "libnss_hesiod.so.2"
#define LIBNSS_LDAP_SO "libnss_ldap.so.2"
#define LIBPTHREAD_SO "libpthread.so.0"
#define LIBRESOLV_SO "libresolv.so.2"
#define LIBRT_SO "librt.so.1"
#define LIBTHREAD_DB_SO "libthread_db.so.1"
#define LIBUTIL_SO "libutil.so.1"
So this shared library is called libgcc_s.so.1. It is the GCC low-level runtime function. OK, so let’s see what is inside this .so file.
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> gcc -print-file-name=libgcc_s.so.1
/usr/lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/libgcc_s.so.1
> readlink -f /usr/lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/libgcc_s.so.1
/usr/lib/x86_64-linux-gnu/libgcc_s.so.1
> nm -D /usr/lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/libgcc_s.so.1
...
0000000000011250 T _Unwind_Backtrace@@GCC_3.3
0000000000011230 T _Unwind_DeleteException@@GCC_3.0
000000000000e8b0 T _Unwind_FindEnclosingFunction@@GCC_3.3
0000000000013180 T _Unwind_Find_FDE@@GCC_3.0
0000000000010e20 T _Unwind_ForcedUnwind@@GCC_3.0
000000000000e7e0 T _Unwind_GetCFA@@GCC_3.3
000000000000e8d0 T _Unwind_GetDataRelBase@@GCC_3.0
000000000000e7a0 T _Unwind_GetGR@@GCC_3.0
000000000000e850 T _Unwind_GetIP@@GCC_3.0
000000000000e860 T _Unwind_GetIPInfo@@GCC_4.2.0
000000000000e890 T _Unwind_GetLanguageSpecificData@@GCC_3.0
000000000000e8a0 T _Unwind_GetRegionStart@@GCC_3.0
000000000000e8e0 T _Unwind_GetTextRelBase@@GCC_3.0
0000000000010a90 T _Unwind_RaiseException@@GCC_3.0
0000000000010fc0 T _Unwind_Resume@@GCC_3.0
0000000000011150 T _Unwind_Resume_or_Rethrow@@GCC_3.3
000000000000e7f0 T _Unwind_SetGR@@GCC_3.0
000000000000e880 T _Unwind_SetIP@@GCC_3.0
First gcc -print-file-name command is so useful! Second, the file path is so wired. It is inside /urs/lib/x86_64-linux-gnu, but not /usr/lib/gcc. I suspect it is shipped with Linux instead of installed together with gcc. So I did a quick experiment. I launched a docker container docker run -i -t ubuntu /bin/bash and inside this fresh ubuntu image
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root@6adc2dc89d0b:/# ll /usr/lib/aarch64-linux-gnu/libgcc_s.so.1
-rw-r--r-- 1 root root 84296 May 13 2023 /usr/lib/aarch64-linux-gnu/libgcc_s.so.1
This .so file exits without installing gcc!
Third, this shared library has quite a few function starting with _Unwind_. And you notice that all these functions show up in libunwind library as well. So basically, this is glibc’s own implementation of libunwind? Actually, it depends. glibc has its own implementation such as this one. But it is not always used. It depends on the flag USE_LIBUNWIND_EXCEPTIONS. If this flag is defined, GCC will link with -lunwind and all its internal _Unwind_ implementations delegate to corresponding ones in libunwind. glics’s own doc has clear statement about this well. See doc.