Lei*_*sen 5 c linux shared-libraries dynamic-linking racket
如果我有用C编写的以下程序(在Debian 8.7上使用GCC编译),我可以atexit()按照您的预期调用:
#include <stdlib.h>
void exit_handler(void) {
return;
}
int main () {
atexit(exit_handler);
return 0;
}
当我编译并运行它时:
$ gcc test.c
$ ./a.out
输出什么都没有,就像你期望的那样.事实上,当我跑步时ldd,我得到:
$ ldd a.out
linux-vdso.so.1 (0x00007fffbe592000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fe07d3a8000)
/lib64/ld-linux-x86-64.so.2 (0x00007fe07d753000)
但是,libc似乎没有任何符号atexit,amd只有__cxa_atexit和__cxa_threaded_atexit_impl:
$ nm --dynamic /lib/x86_64-linux-gnu/libc.so.6 | grep 'atexit'
0000000000037d90 T __cxa_atexit
0000000000037fa0 T __cxa_thread_atexit_impl
正如您所期望的那样,如果我尝试libc动态链接,我实际上无法调用atexit(),例如在下面的Racket程序中链接libc并尝试查找atexit:
#lang racket
(require ffi/unsafe)
(get-ffi-obj 'atexit (ffi-lib "libc" '("6")) (_fun (_fun -> _void) -> _int))
给出输出:
$ racket findatexit.rkt
ffi-obj: couldn't get "atexit" from "libc.so.6" (/lib/x86_64-linux-gnu/libc.so.6: undefined symbol: atexit)
我想知道的是:
libc上没有任何符号atexit,为什么我仍然可以从C程序中调用它？atexit在Linux上动态调用或类似的功能吗？(我应该注意,atexit它似乎确实是OS X上的一个符号,所以它只是Linux在这里看起来很不寻常.)
编辑:
在@Jonathan的建议下,我也跑了:
$ gcc -c test.c
$ nm test.o
U atexit
0000000000000000 T exit_handler
0000000000000007 T main
这似乎表明atexit符号存在于某处,但它并没有出现在任何库ldd中.
我在Centos 7虚拟机上做了一些探测,我想我找到了它 - 但它不是显而易见的!
在/usr/lib64/libc_nonshared.a:
$ nm /usr/lib64/libc_nonshared.a | grep -i atexit
atexit.oS:
0000000000000000 T atexit
U __cxa_atexit
$
为什么要看那个图书馆？好问题 - 长话题.你坐得舒服吗？然后我会开始......
test.c代码.编译它gcc -v test.c:
$ gcc -v test.c
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-linker-hash-style=gnu --enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin --enable-initfini-array --disable-libgcj --with-isl=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-linux/isl-install --with-cloog=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-linux/cloog-install --enable-gnu-indirect-function --with-tune=generic --with-arch_32=x86-64 --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.8.5 20150623 (Red Hat 4.8.5-11) (GCC)
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/cc1 -quiet -v test.c -quiet -dumpbase test.c -mtune=generic -march=x86-64 -auxbase test -version -o /tmp/ccPHTer7.s
GNU C (GCC) version 4.8.5 20150623 (Red Hat 4.8.5-11) (x86_64-redhat-linux)
compiled by GNU C version 4.8.5 20150623 (Red Hat 4.8.5-11), GMP version 6.0.0, MPFR version 3.1.1, MPC version 1.0.1
GGC heuristics: --param ggc-min-expand=96 --param ggc-min-heapsize=124992
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../x86_64-redhat-linux/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/include
/usr/local/include
/usr/include
End of search list.
GNU C (GCC) version 4.8.5 20150623 (Red Hat 4.8.5-11) (x86_64-redhat-linux)
compiled by GNU C version 4.8.5 20150623 (Red Hat 4.8.5-11), GMP version 6.0.0, MPFR version 3.1.1, MPC version 1.0.1
GGC heuristics: --param ggc-min-expand=96 --param ggc-min-heapsize=124992
Compiler executable checksum: 356f86e67978d665416e07d560c8ba0d
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
as -v --64 -o /tmp/cc5WHEA4.o /tmp/ccPHTer7.s
GNU assembler version 2.25.1 (x86_64-redhat-linux) using BFD version version 2.25.1-22.base.el7
COMPILER_PATH=/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/:/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/:/usr/libexec/gcc/x86_64-redhat-linux/:/usr/lib/gcc/x86_64-redhat-linux/4.8.5/:/usr/lib/gcc/x86_64-redhat-linux/
LIBRARY_PATH=/usr/lib/gcc/x86_64-redhat-linux/4.8.5/:/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/collect2 --build-id --no-add-needed --eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crt1.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crti.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtbegin.o -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5 -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64 -L/lib/../lib64 -L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../.. /tmp/cc5WHEA4.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtend.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crtn.o
$
有趣的是最后的collect2命令行.每行写一个参数,即:
/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/collect2
--build-id
--no-add-needed
--eh-frame-hdr
--hash-style=gnu
-m
elf_x86_64
-dynamic-linker
/lib64/ld-linux-x86-64.so.2
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crt1.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crti.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtbegin.o
-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5
-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64
-L/lib/../lib64
-L/usr/lib/../lib64
-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../..
/tmp/cc5WHEA4.o
-lgcc
--as-needed
-lgcc_s
--no-as-needed
-lc
-lgcc
--as-needed
-lgcc_s
--no-as-needed
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtend.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crtn.o
因此,有一堆的cr*.o文件,再加上三个库:-lc,-lgcc并-lgcc_s寻找和一堆目录看在:
-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5,-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64,-L/lib/../lib64,-L/usr/lib/../lib64,-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../...这/tmp/cc5WHEA4.o是从中创建的目标文件test.c.
将一些清理代码应用于路径名,然后使用ls以帮助查找库会生成要进一步检查的文件列表:
/lib64/ld-linux-x86-64.so.2
/usr/lib64/crt1.o
/usr/lib64/crti.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtbegin.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtend.o
/usr/lib64/crtn.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/libgcc.a
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/libgcc_s.so
/usr/lib64/libgcc_s.so.1
/lib64/libgcc_s.so.1
/usr/lib64/libgcc_s.so.1
/usr/lib64/libc.so
/usr/lib64/libc.so.6
/lib64/libc.so
/lib64/libc.so.6
/usr/lib64/libc.so
/usr/lib64/libc.so.6
该文件列表保存在一个文件yy(缺乏想象力的名称)中,然后用于:
$ nm -o $(<yy) | tee nm.log | grep -i atexit
nm: _trampoline.o: no symbols
nm: __main.o: no symbols
nm: _ctors.o: no symbols
nm: /usr/lib/gcc/x86_64-redhat-linux/4.8.5/libgcc_s.so: no symbols
nm: /usr/lib64/libgcc_s.so.1: no symbols
nm: /lib64/libgcc_s.so.1: no symbols
nm: /usr/lib64/libgcc_s.so.1: no symbols
nm: /usr/lib64/libc.so: File format not recognized
/usr/lib64/libc.so.6:00000000003bcc00 b added_atexit_handler.9157
/usr/lib64/libc.so.6:0000000000038c90 T __cxa_atexit
/usr/lib64/libc.so.6:0000000000038c90 t __cxa_atexit_internal
/usr/lib64/libc.so.6:00000000003b6838 d __elf_set___libc_atexit_element__IO_cleanup__
/usr/lib64/libc.so.6:0000000000038c40 t __internal_atexit
/usr/lib64/libc.so.6:00000000003b6838 d __start___libc_atexit
/usr/lib64/libc.so.6:00000000003b6840 d __stop___libc_atexit
nm: /lib64/libc.so: File format not recognized
/lib64/libc.so.6:00000000003bcc00 b added_atexit_handler.9157
/lib64/libc.so.6:0000000000038c90 T __cxa_atexit
/lib64/libc.so.6:0000000000038c90 t __cxa_atexit_internal
/lib64/libc.so.6:00000000003b6838 d __elf_set___libc_atexit_element__IO_cleanup__
/lib64/libc.so.6:0000000000038c40 t __internal_atexit
nm: /usr/lib64/libc.so: File format not recognized
/lib64/libc.so.6:00000000003b6838 d __start___libc_atexit
/lib64/libc.so.6:00000000003b6840 d __stop___libc_atexit
/usr/lib64/libc.so.6:00000000003bcc00 b added_atexit_handler.9157
/usr/lib64/libc.so.6:0000000000038c90 T __cxa_atexit
/usr/lib64/libc.so.6:0000000000038c90 t __cxa_atexit_internal
/usr/lib64/libc.so.6:00000000003b6838 d __elf_set___libc_atexit_element__IO_cleanup__
/usr/lib64/libc.so.6:0000000000038c40 t __internal_atexit
/usr/lib64/libc.so.6:00000000003b6838 d __start___libc_atexit
/usr/lib64/libc.so.6:00000000003b6840 d __stop___libc_atexit
$
那里没有明显的atexit功能证据.隐藏在哪里,以及那些"文件格式无法识别"的消息是什么？
$ file /usr/lib64/libc.so
/usr/lib64/libc.so: ASCII text
$
ASCII文字？什么？
$ cat /usr/lib64/libc.so
/* GNU ld script
Use the shared library, but some functions are only in
the static library, so try that secondarily. */
OUTPUT_FORMAT(elf64-x86-64)
GROUP ( /lib64/libc.so.6 /usr/lib64/libc_nonshared.a AS_NEEDED ( /lib64/ld-linux-x86-64.so.2 ) )
$
好; 什么在/usr/lib64/libc_nonshared.a？
$ nm /usr/lib64/libc_nonshared.a | grep -i atexit
atexit.oS:
0000000000000000 T atexit
U __cxa_atexit
$
答对了!找到了!
因此,似乎collect2GCC使用的链接器能够加载未在命令行中列出的文件,并且其中一个文件是/usr/lib64/libc_nonshared.a,并且该库中包含atexit()该文件.因此,你应该可以调用atexit()它,因为它静态地链接到可执行文件中...除非在这里隐藏了一些我没有怀疑过的黑魔法.