"Undefined reference " to a function defined in static library

Question

I am trying to build a Library to use in an application. I built the library as below, and when i compile the application i get the below error:

I have done the beolw things.

I use:

gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) 

In Library to be called from application:

Here i have lot of modules, but entry point to this library is func() (i.e., main () is replaced with func() so that i can call the module, also func () is not declared as 'static'.)

In one of files:

int func ();
...

int func () 
{ ... } 

Then built the Library as:

gcc -Wall file.c -o file.o
...
...

ar rvs libfun.a $(OBJS)

Also used ranlib and nm -s on libfun.a seperately to build symbol table, but the total size of archive did not change after using these commands and still got the linking error. Here $(OBJS) contains all the object files

In Application:

extern int func ();

Compile with:

gcc -Wall -L./path-to-lib  -lfun  -o appl

Then i get the below error:

In function `main':
undefined reference to `func()'
collect2: ld returned 1 exit status

I tried to build symbol table with "ar s" and "ranlib" but the results is same.

One thing i observed is there is a difference in contents of "ar" which i built and the archives already present in project for other modules.

The archive built by me contains (ouput with "nm -s libfun.a" ):

Archive index:
Cfg1 in f1.o
mCfg1 in f1.o
dpCfg in f1.o

But the other archives which i am using without any changes contain below strange pattern:

Archive index:
_Z29platformSetjP38tagTCPIP_INTERFACE_INSTANCE_ATTRIBUTES in platform.o
_Z27platformTestSetTcpjP20tagTCPIP_CONFIG_DATAPh in platform.o
_Z23platformSetTcpIpjP20tagTCPIP_CONFIG_DATA in platform.o

I am not sure what is the difference above. Is it a shared Library or a Static library ?

I am trying to compile with GCC and build archive with 'ar', but the other library files may be using g++ compiler. I am not sure. Just in case it matters.

What am i doing wrong here in building my library ? Please help?

Regards.

Answer 1

I found out the reason for the error. I was mixing the static library with other libraries compiled with position independent code (PIC) and some other flags. Adding "-fPIC" flag solved the issue.