歷經一個月,昨日完成印表機network部分的編寫(c語言),編寫makefile構建動態庫。構建完成後遂進行呼叫測試,出現:
./network: symbol lookup error: /usr/lib64/netPrnctl.so: undefined symbol: cupsGetDests
將解決方法與過程記錄,以便日後查閱。
在編譯時未出現問題,沒有報錯,成功編譯生成動態庫。以下爲編譯的makefile檔案。
###########################################################
# File: netPrnctl.so Makefile
# Author: Neko
###########################################################
CC = gcc
CFLAGS = -Wall -g -fPIC
INCLUDE = -I./inc -I../ -I../cups -I../backend -I/usr/include/libusb-1.0
TARGET = netPrnctl.so
vpath % .h ./inc
OBJS = public.o prnctlAvision.o prnctlNetwork.o prnctl.o
SRCS = ./src/public.c ./src/prnctlAvision.c ./src/prnctlNetwork.c ./src/prnctl.c
$(OBJS):$(SRCS)
$(CC) $(CFLAGS) $(INCLUDE) -c $^
all:$(OBJS)
$(CC) -shared -fPIC -o $(TARGET) $(OBJS) -pthread
cp $(TARGET) /usr/bin
cp $(TARGET) /usr/lib64
clean:
rm -f *.o
rm -f netPrnctl.so
雖成功生成了動態庫,但是在呼叫時卻出現找不到符號的問題,於是使用 ldd -r 檢視
neko@neko:~/ZCPrinterDevice/NetworkSetupTool/filter$ ldd -r /usr/lib64/netPrnctl.so
linux-vdso.so.1 (0x00007ffeef44c000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f9607add000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f96076ec000)
/lib64/ld-linux-x86-64.so.2 (0x00007f9607f0f000)
undefined symbol: libusb_open (/usr/lib64/netPrnctl.so)
undefined symbol: cupsArrayNew (/usr/lib64/netPrnctl.so)
undefined symbol: cupsDirOpen (/usr/lib64/netPrnctl.so)
undefined symbol: ppdFindAttr (/usr/lib64/netPrnctl.so)
undefined symbol: cupsParseOptions (/usr/lib64/netPrnctl.so)
undefined symbol: cupsGetPPD (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_set_interface_alt_setting (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_release_interface (/usr/lib64/netPrnctl.so)
undefined symbol: ppdClose (/usr/lib64/netPrnctl.so)
undefined symbol: cupsGetOption (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_detach_kernel_driver (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_close (/usr/lib64/netPrnctl.so)
undefined symbol: cupsFreeOptions (/usr/lib64/netPrnctl.so)
undefined symbol: cupsDirClose (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_get_string_descriptor_ascii (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_free_config_descriptor (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_get_config_descriptor (/usr/lib64/netPrnctl.so)
undefined symbol: cupsBackChannelWrite (/usr/lib64/netPrnctl.so)
undefined symbol: _cupsGet1284Values (/usr/lib64/netPrnctl.so)
undefined symbol: _ppdNormalizeMakeAndModel (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_attach_kernel_driver (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_get_device_list (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_kernel_driver_active (/usr/lib64/netPrnctl.so)
undefined symbol: _cups_strcasecmp (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_bulk_transfer (/usr/lib64/netPrnctl.so)
undefined symbol: _cupsLangPrintFilter (/usr/lib64/netPrnctl.so)
undefined symbol: cupsSideChannelRead (/usr/lib64/netPrnctl.so)
undefined symbol: cupsArrayCount (/usr/lib64/netPrnctl.so)
undefined symbol: cupsFileClose (/usr/lib64/netPrnctl.so)
undefined symbol: cupsDirRead (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_set_configuration (/usr/lib64/netPrnctl.so)
undefined symbol: cupsFreeDests (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_open_device_with_vid_pid (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_reset_device (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_exit (/usr/lib64/netPrnctl.so)
undefined symbol: ppdOpenFile (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_init (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_get_device_descriptor (/usr/lib64/netPrnctl.so)
undefined symbol: cupsBackendReport (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_free_device_list (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_claim_interface (/usr/lib64/netPrnctl.so)
undefined symbol: cupsFileOpen (/usr/lib64/netPrnctl.so)
undefined symbol: libusb_control_transfer (/usr/lib64/netPrnctl.so)
undefined symbol: cupsFileGets (/usr/lib64/netPrnctl.so)
undefined symbol: cupsArrayAdd (/usr/lib64/netPrnctl.so)
undefined symbol: backendGetMakeModel (/usr/lib64/netPrnctl.so)
undefined symbol: cupsArrayFind (/usr/lib64/netPrnctl.so)
undefined symbol: _cups_strlcpy (/usr/lib64/netPrnctl.so)
undefined symbol: cupsGetDests (/usr/lib64/netPrnctl.so)
undefined symbol: cupsSideChannelWrite (/usr/lib64/netPrnctl.so)
undefined symbol: httpAssembleURIf (/usr/lib64/netPrnctl.so)
發現動態庫沒有鏈接libusb 與 libcups 兩個庫,所以netPrnctl.so用到的這兩個的函數都沒有符號。
雖然構建netPrnctl.so的c檔案已包含libusb與cups函數的標頭檔案,編譯器做的工作僅是將原始檔中以「文字形式」存在的原始碼翻譯成機器語言(二進制)的形式,並生成目標檔案(原始碼全部變成"二進制"的形式),在此期間對原始碼中的語法進行檢查,有語法錯誤則報錯,程式無法通過編譯。故編譯沒有錯誤僅是說明沒有語法上的問題。在實際呼叫時也可能出現其他情況。
通過上面的makefile看到,編譯生成了
OBJS = public.o prnctlAvision.o prnctlNetwork.o prnctl.o
這4個目標檔案。
而當鏈接器進行鏈接的時候,會先把各個目標檔案在可執行檔案(ELF檔案的一種,共用庫也是屬於其中)裏面的位置確定下來,然後經過一系列處理,直到把所有目標檔案的內容都寫在可執行檔案中對應的位置上,再經過一些工作,就會生成一個可執行程式。
由此判斷,netPrnctl.so依賴的libusb 與 libcups 兩個庫沒有鏈接上。
既然是因爲沒有鏈接libusb 與 libcups,那麼將這兩個庫鏈接上是不是就能解決問題?
首先檢視這兩個庫的路徑。一般gcc裡會有。
neko@neko:~/ZCPrinterDevice/NetworkSetupTool/filter$ ls /usr/lib/x86_64-linux-gnu/ | grep libcups
libcupscgi.so.1
libcupsfilters.so.1
libcupsfilters.so.1.0.0
libcupsimage.so.2
libcupsmime.so.1
libcupsppdc.so.1
libcups.so.2
neko@neko:~/ZCPrinterDevice/NetworkSetupTool/filter$ ls /usr/lib/x86_64-linux-gnu/ | grep libusb
libusb-1.0.a
libusb-1.0.so
libusbmuxd.so.4
libusbmuxd.so.4.0.0
於是將makefile進行調整,新增鏈接libusb 與 libcups的語句。
###########################################################
# File: netPrnctl.so Makefile
# Author: Neko
###########################################################
CC = gcc
CFLAGS = -Wall -g -fPIC
INCLUDE = -I./inc -I../ -I../cups -I../backend -I/usr/include/libusb-1.0
TARGET = netPrnctl.so
LIBVAR = -lusb-1.0 -lcups
LIBPATH = -L/usr/lib64/x86_64-linux-gnu
vpath % .h ./inc
OBJS = public.o prnctlAvision.o prnctlNetwork.o prnctl.o
SRCS = ./src/public.c ./src/prnctlAvision.c ./src/prnctlNetwork.c ./src/prnctl.c
$(OBJS):$(SRCS)
$(CC) $(CFLAGS) $(INCLUDE) -c $^
all:$(OBJS)
$(CC) -shared -fPIC -o $(TARGET) $(OBJS) $(LIBPATH) $(LIBVAR) -pthread
cp $(TARGET) /usr/bin
cp $(TARGET) /usr/lib64
clean:
rm -f *.o
rm -f netPrnctl.so
編譯時卻報錯
/usr/bin/ld: 找不到 -lcups
這裏說一下:
出現 /usr/bin/ld: cannot find(找不到) -lxxx
xxx表示函式庫檔名稱,如:libc.so、libltdl.so、libXtst.so。
命名規則是:lib+庫名(即xxx)+.so。
會發生這樣的原因有以下三種情形:
1 系統沒有安裝相對應的lib
2 相對應的lib版本不對
3 lib(.so檔)的symbolic link不正確,沒有連結到正確的函式庫檔案(.so)
而libcups.so.2不符合命名規則。故報錯找不到。
解決辦法:
到libcups所在路徑下,鏈接新的符合規則的libcups.so
neko@neko:~/ZCPrinterDevice/NetworkSetupTool/filter$ cd /usr/lib/x86_64-linux-gnu
neko@neko:/usr/lib/x86_64-linux-gnu$ sudo ln -s libcups.so.2 libcups.so
neko@neko:/usr/lib/x86_64-linux-gnu$ cd -
/home/neko/ZCPrinterDevice/NetworkSetupTool/filter
neko@neko:~/ZCPrinterDevice/NetworkSetupTool/filter$ ls /usr/lib/x86_64-linux-gnu/ | grep libcups
libcupscgi.so.1
libcupsfilters.so.1
libcupsfilters.so.1.0.0
libcupsimage.so.2
libcupsmime.so.1
libcupsppdc.so.1
libcups.so
libcups.so.2
重新編譯成功,沒有報錯。再次呼叫測試,也沒有出現找不到符號的問題。
neko@neko:~/ZCPrinterDevice/NetworkSetupTool/filter$ ldd -r /usr/lib64/netPrnctl.so
linux-vdso.so.1 (0x00007fff43f66000)
libusb-1.0.so.0 => /lib/x86_64-linux-gnu/libusb-1.0.so.0 (0x00007f4087f39000)
libcups.so.2 => /usr/lib/x86_64-linux-gnu/libcups.so.2 (0x00007f4087cad000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f4087a8e000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f408769d000)
libudev.so.1 => /lib/x86_64-linux-gnu/libudev.so.1 (0x00007f408747f000)
libgssapi_krb5.so.2 => /usr/lib/x86_64-linux-gnu/libgssapi_krb5.so.2 (0x00007f4087234000)
libgnutls.so.30 => /usr/lib/x86_64-linux-gnu/libgnutls.so.30 (0x00007f4086ecf000)
libavahi-common.so.3 => /usr/lib/x86_64-linux-gnu/libavahi-common.so.3 (0x00007f4086cc3000)
libavahi-client.so.3 => /usr/lib/x86_64-linux-gnu/libavahi-client.so.3 (0x00007f4086ab2000)
libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f4086895000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f40864f7000)
/lib64/ld-linux-x86-64.so.2 (0x00007f4088364000)
librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f40862ef000)
libkrb5.so.3 => /usr/lib/x86_64-linux-gnu/libkrb5.so.3 (0x00007f4086019000)
libk5crypto.so.3 => /usr/lib/x86_64-linux-gnu/libk5crypto.so.3 (0x00007f4085de7000)
libcom_err.so.2 => /lib/x86_64-linux-gnu/libcom_err.so.2 (0x00007f4085be3000)
libkrb5support.so.0 => /usr/lib/x86_64-linux-gnu/libkrb5support.so.0 (0x00007f40859d8000)
libp11-kit.so.0 => /usr/lib/x86_64-linux-gnu/libp11-kit.so.0 (0x00007f40856a9000)
libidn2.so.0 => /usr/lib/x86_64-linux-gnu/libidn2.so.0 (0x00007f408548c000)
libunistring.so.2 => /usr/lib/x86_64-linux-gnu/libunistring.so.2 (0x00007f408510e000)
libtasn1.so.6 => /usr/lib/x86_64-linux-gnu/libtasn1.so.6 (0x00007f4084efb000)
libnettle.so.6 => /usr/lib/x86_64-linux-gnu/libnettle.so.6 (0x00007f4084cc5000)
libhogweed.so.4 => /usr/lib/x86_64-linux-gnu/libhogweed.so.4 (0x00007f4084a91000)
libgmp.so.10 => /usr/lib/x86_64-linux-gnu/libgmp.so.10 (0x00007f4084810000)
libdbus-1.so.3 => /lib/x86_64-linux-gnu/libdbus-1.so.3 (0x00007f40845c3000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f40843bf000)
libkeyutils.so.1 => /lib/x86_64-linux-gnu/libkeyutils.so.1 (0x00007f40841bb000)
libresolv.so.2 => /lib/x86_64-linux-gnu/libresolv.so.2 (0x00007f4083fa0000)
libffi.so.6 => /usr/lib/x86_64-linux-gnu/libffi.so.6 (0x00007f4083d98000)
libsystemd.so.0 => /lib/x86_64-linux-gnu/libsystemd.so.0 (0x00007f4083b14000)
liblzma.so.5 => /lib/x86_64-linux-gnu/liblzma.so.5 (0x00007f40838ee000)
liblz4.so.1 => /usr/lib/x86_64-linux-gnu/liblz4.so.1 (0x00007f40836d2000)
libgcrypt.so.20 => /lib/x86_64-linux-gnu/libgcrypt.so.20 (0x00007f40833b7000)
libgpg-error.so.0 => /lib/x86_64-linux-gnu/libgpg-error.so.0 (0x00007f40831a2000)
可以看到,依賴的庫基本都鏈接上了。
所有錯誤都有原因,有跡可循。遇到問題不要急躁,靜心思考,理出種種可能導致問題的原因,再排除,找到根源去解決。搜尋引擎和書籍是很好的工具,凡事先儘量自己想辦法,獨立解決問題也是一種能力。
參加工作半年,此時的我也只是一個剛進門的初級工程師。這些話是對我自己的告誡,也是對與我一樣在門內探索的同學的建議,希望有所幫助。共勉。