GCC Dynamic library building problem

I am new to linux, while compiling with dynamic library I am getting the segmentationfault error.

I have two files

ctest1.c

void ctest1(int *i)
{ 
   *i =10;
}

ctest2.c

void ctest2(int *i)
{ 
   *i =20;
}

I have compiled both files to a shared library named libtest.so using following command

  gcc -shared -W1,-soname,libtest.so.1 -o libtest.so.1.0.1 ctest1.o ctest2.o -lc

And I have wrote another program prog.c which uses functions exported by this library

prog.c

#include <stdio.h>

void (*ctest1)(int*);
void (ctest2)(int*);


int main()
{
  int a;

  ctest1(&a);

  printf("%d",a);

  return 0;

}

And when I have built the executable with following command

gcc -Wall prog.c -L. -o prog

But when I run the generated executable I get the SegmentationFault error.

Whe开发者_如何学编程n I checked the header of prog with ldd it shows

linux-vdso.so.1 => (0x00007f99dff000) libc.so.6 => /lib64/libc.so.6 (0x0007feeaa8c1000) /lib64/ld-linux-x86-64.so.2 (0x00007feeaac1c000)

Can somebody tell what is the problem

---

You aren't calling into ctest1.c or ctest2.c. Instead, you're creating ctest1 and ctest2 function pointers in prog.c, which you are not initializing, so it is causing a segmentation fault when you try to call them.

You need to declare your functions so prog.c can see them, and then link prog.c to the libraries (probably using the -l option to gcc).

#include <stdio.h>

extern void ctest1(int*);
extern void ctest2(int*);


int main()
{
  int a;

  ctest1(&a);

  printf("%d",a);

  return 0;

}

And something like:

gcc -Wall -L. -ltest prog.c -o prog

---

Try this after using the information WhirlWind gave you (the lines beginning with '#' are comments; you don't need to type them):

# Ensure that any shared objects you use are available in the current directory.
export LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH

# Compile the library with a real name of "libctest.so.1.0.1"
# and a soname of "libctest.so.1".
gcc -shared -Wl,-soname,libctest.so.1 -o libctest.so.1.0.1 ctest1.o ctest2.o

# Create a symbolic link with soname as the name that points to the library.
# (libctest.so.1 -> libctest.so.1.0.1)
/sbin/ldconfig -v -n .

# Create a symbolic link using the "linker name" that points to the newly
# created library.
ln -sf libctest.so.1 libctest.so

# Compile your program.
gcc -Wall -L. prog.c -o prog -l ctest

# Run your program (it won't work without setting LD_LIBRARY_PATH because
# it won't be able to find your library).
./prog

That worked for me. It's a lot of work seemingly, but after a few trial-and-error cases, I think it becomes almost routine.

You can find some more information at http://www.ibm.com/developerworks/library/l-shobj/. :)

Edit: I almost forgot to mention that it seems a lot of tutorials suggest using the -fPIC option to generate position-independent code (don't confuse it with -fpic since that can make your resulting library less portable). It couldn't hurt to have it, but for simplicity I omitted it from the lines above.

---

.so libraries in linux are dynamically linked. You will need to open the .so file using dlopen() in prog.c, find the symbols and then call ctest1() and ctest2() via function pointers.