How to create a log file for any program in C

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Hello,

I am a new to C. I had written a code in C and want to generate a log file for all errors and unsupported functionality. How can I create a log file for a program?

My program structure is like:

main()
{ 
  int i = abc();
  if (i != 0)
    //put error in log file

  int j = PQR();
  if ( j != 0)
    //put error in log file
}
abc()
{
 int k = xyz();
 if ( k != 0)
    //put error in log file
}
PQR()
{
 //some code
}
xyz()
{
 //some code
}

Please help me out.
Thanks in advance.