[Solved] igb Compile Error: igb_main.c:10044:7: error: implicit declaration of function ‘isdigit’

Recently, the IGB compiler driver found this problem:

zacha@Superman:~/igb/igb-5.7.2/src$ make
#@+ echo "*** The target kernel has CONFIG_MODULE_SIG_ALL enabled, but" ; echo "*** the signing key cannot be found. Module signing has been" ; echo "*** disabled for this build." ; make  ccflags-y="" -C "/lib/modules/4.15.0-142-generic/build" CONFIG_IGB=m CONFIG_MODULE_SIG=n CONFIG_MODULE_SIG_ALL= M="/home/zacha/igb/igb-5.7.2/src"   modules  
make -C /home/zacha/yulong810/kernel M=/home/zacha/igb/igb-5.7.2/src CROSS_COMPILE=arm-linux-gnueabihf- ARCH=arm
make[1]: Entering directory '/home/zacha/yulong810/kernel'
  CC [M]  /home/zacha/igb/igb-5.7.2/src/igb_main.o
/home/zacha/igb/igb-5.7.2/src/igb_main.c: In function ‘igb_get_os_driver_version’:
/home/zacha/igb/igb-5.7.2/src/igb_main.c:10044:7: error: implicit declaration of function ‘isdigit’ [-Werror=implicit-function-declaration]
   if(!isdigit(*c) && *c != '.')
       ^~~~~~~
cc1: some warnings being treated as errors
scripts/Makefile.build:303: recipe for target '/home/zacha/igb/igb-5.7.2/src/igb_main.o' failed
make[2]: *** [/home/zacha/igb/igb-5.7.2/src/igb_main.o] Error 1
Makefile:1519: recipe for target '_module_/home/zacha/igb/igb-5.7.2/src' failed
make[1]: *** [_module_/home/zacha/igb/igb-5.7.2/src] Error 2
make[1]: Leaving directory '/home/zacha/yulong810/kernel'
Makefile:88: recipe for target 'default' failed
make: *** [default] Error 2

There are also solutions online:
https://github.com/geerlingguy/raspberry-pi-pcie-devices/issues/3

Solution:

Add #include <linux/ctype.h> inigb)main.c
then compile and generate IGB.ko

zacha@Superman:~/igb/igb-5.7.2/src$ make
#@+ echo "*** The target kernel has CONFIG_MODULE_SIG_ALL enabled, but" ; echo "*** the signing key cannot be found. Module signing has been" ; echo "*** disabled for this build." ; make  ccflags-y="" -C "/lib/modules/4.15.0-142-generic/build" CONFIG_IGB=m CONFIG_MODULE_SIG=n CONFIG_MODULE_SIG_ALL= M="/home/zacha/igb/igb-5.7.2/src"   modules  
make -C /home/zacha/yulong810/kernel M=/home/zacha/igb/igb-5.7.2/src CROSS_COMPILE=arm-linux-gnueabihf- ARCH=arm
make[1]: Entering directory '/home/zacha/yulong810/kernel'
  CC [M]  /home/zacha/igb/igb-5.7.2/src/igb_main.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/e1000_api.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/igb_ethtool.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/igb_hwmon.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/e1000_mbx.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/e1000_mac.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/e1000_manage.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/e1000_nvm.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/igb_param.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/e1000_phy.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/igb_procfs.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/igb_vmdq.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/e1000_82575.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/e1000_i210.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/igb_debugfs.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/igb_ptp.o
  CC [M]  /home/zacha/igb/igb-5.7.2/src/kcompat.o
  LD [M]  /home/zacha/igb/igb-5.7.2/src/igb.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /home/zacha/igb/igb-5.7.2/src/igb.mod.o
  LD [M]  /home/zacha/igb/igb-5.7.2/src/igb.ko
make[1]: Leaving directory '/home/zacha/yulong810/kernel'