Incompatible pointer type in function call - C

Question

I have a program below

#include<stdio.h>

void int_frac(double a, long *int_part,double *frac_part)
{
  *int_part = (long int)(a);
  *frac_part = a - *int_part;
}
int main()
{
  long int* ip;
  double* fp;
  double i = 1.2345467;

  int_frac(i,&ip,&fp);
  printf("%f = %ld + %f",i,*ip,*fp);
  return 0;
}

I am using gcc compiler. This program gives an error:

expected long int* but argument is of type long int**

and

expected double* but argument is of type double**

Why this error comes up.

@BLUEPIXY Why are you providing links to http://linux.die.net/ which is hopelessly out of date, and also specific to Linux. Here is a much better link: http://en.cppreference.com/w/c/numeric/math/modf — David Heffernan, Mar 03 '14 at 11:33
Nor does it mean that linux limited and nor does old information. It is like have well-written. — BLUEPIXY, Mar 03 '14 at 12:07
@BLUEPIXY Wrong. It's Linux documentation, that may have overlap, and is out of date. Linux docs are found at kernel.org. — David Heffernan, Mar 03 '14 at 13:44
@DavidHeffernan Please tell me if there is an error in the **content**. — BLUEPIXY, Mar 03 '14 at 21:14
@BLUEPIXY It's just irrelevant to the question. The question is tagged C. There is no mention of POSIX. Do you want to learn or not? — David Heffernan, Mar 03 '14 at 21:18
@DavidHeffernan It does not mean POSIX only , also be written as c99. — BLUEPIXY, Mar 03 '14 at 21:23
@BLUEPIXY so you think linux.die is the best documentation source for the C standard library? If so, I give up. — David Heffernan, Mar 03 '14 at 21:24
&BLUEPIXY, I knew about modf already. I was just learning passing pointer variables to a function. — Vinod, Mar 06 '14 at 04:50

David Heffernan · Accepted Answer · 2014-03-03T11:21:21.977

The compiler is telling you that you are passing long int** where it expects long int*. And that you are passing double** where it expects double*.

The compiler is correct. You declared ip to be long int* and then passed &ip which has type long int**. You made the same mistake with fp.

What you have done is declare two pointers, ip and fp, but not initialized the pointers. The fundamental problem is that you have declared pointers but have not allocated any variables. If int_frac is to be able to write to variables, those variables must be allocated somewhere. So, what you need to do is declare two values, and pass the addresses of those values to int_frac.

Your code should be:

int main()
{
    long int ip; // allocates a variable
    double fp; // as does this
    double i = 1.2345467;

    int_frac(i,&ip,&fp); // pass the addresses of ip and fp
    printf("%f = %ld + %f",i,ip,fp);
    return 0;
}

score 4 · Answer 2 · answered Mar 03 '14 at 11:07

The error which you received is the answer in my opinion as you are passing long int** when it expects long int* You need to declare ip as long int ip like this:

int main()
{
    long int ip;
    double fp;
    double i = 1.2345467;

    int_frac(i,&ip,&fp);
    printf("%f = %ld + %f",i,ip,fp);
    return 0;
}

Incompatible pointer type in function call - C

2 Answers2