Question

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.

When 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

Answer 1

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