EXPORT_SYMBOL Meaning and Usage

Define a function func1 in module mod1; define a function func2 in another module mod2, where func2 calls func1. In module mod1, use EXPORT_SYMBOL(func1); In module mod2, declare extern int func1(); Then func1 can be called from mod2.

References: http://topic.csdn.net/u/20070910/09/ee2cff13-9179-41e3-9292-4fd73261f709.html http://www.dev-archive.com/msdn-archive/524/kernel-driver-5244619.shtm

mod1.c

#include<linux/init.h>
#include<linux/module.h>
#include<linux/kernel.h>

static int func1(void)
{
        printk("In Func: %s.../n",__func__);
        return 0;
}

EXPORT_SYMBOL(func1);

static int __init hello_init(void)
{
        printk("Module 1,Init!/n");
        return 0;
}

static void __exit hello_exit(void)
{
        printk("Module 1,Exit!/n");
}

module_init(hello_init);
module_exit(hello_exit);

############################################################# mod2.c

#include<linux/init.h>
#include<linux/kernel.h>
#include<linux/module.h>

static int func2(void)
{
        extern int func1(void);
        func1();
        printk("In Func: %s.../n",__func__);
        return 0;
}

static int __init hello_init(void)
{
        printk("Module 2,Init!/n");
        func2();
        return 0;
}

static void __exit hello_exit(void)
{
        printk("Module 2,Exit!/n");
}

module_init(hello_init);
module_exit(hello_exit);

################################################################ Makefile

ifneq ($(KERNELRELEASE),)
obj-m   := XXXX.o
else
KDIR    := /lib/modules/$(shell uname -r)/build
PWD             := $(shell pwd)

default:
        $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

clean:
        rm -rf Module.symvers *.ko *.o *.mod.c .*.cmd .tmp_versions

endif

################################################################

#insmod ./mod1.ko
#insmod ./mod2.ko
#rmmod mod2
#rmmod mod1

Jan 11 11:59:17 wangyao-desktop kernel: [ 9886.801010] Module 2,Exit!
Jan 11 11:59:21 wangyao-desktop kernel: [ 9891.450214] Module 1,Exit!
Jan 11 12:05:29 wangyao-desktop kernel: [10258.385014] Module 1,Init!
Jan 11 12:05:38 wangyao-desktop kernel: [10267.465923] Module 2,Init!
Jan 11 12:05:38 wangyao-desktop kernel: [10267.465928] In Func: func1...
Jan 11 12:05:38 wangyao-desktop kernel: [10267.465930] In Func: func2...
Jan 11 12:05:50 wangyao-desktop kernel: [10280.091755] Module 2,Exit!
Jan 11 12:05:57 wangyao-desktop kernel: [10287.332596] Module 1,Exit!

It can be seen that the func2 function in mod2 successfully called the func1 function in mod1.

Note: When compiling mod2, a WARNING appeared:

root@wangyao-desktop:~/modules/export_symbol/mod2# make
make -C /lib/modules/2.6.22-14-generic/build SUBDIRS=/root/modules/export_symbol/mod2 modules
make[1]: Entering directory `/usr/src/linux-headers-2.6.22-14-generic'
  Building modules, stage 2.
  MODPOST 1 modules
WARNING: "func1" [/root/modules/export_symbol/mod2/mod2.ko] undefined!
make[1]: Leaving directory `/usr/src/linux-headers-2.6.22-14-generic'

This is mainly because the kernel is not involved during the compilation and linking phase, so the symbol cannot be found. However, since you are generating a kernel module, ld does not report an error but rather a warning, hoping that the kernel will link this symbol during insmod.