implicit declaration of function 'execle' error

Question

I keep getting

implicit declaration of function 'execle' is invalid in C99

when compiling the code below. What am I missing?

#include <stdio.h>
#include <stdlib.h>

char *my_env[] = {"JUICE=PEACH and apple", NULL};

int main (int argc, char *argv[])
{
  execle ("diner_info", "diner_info", "4", NULL, my_env);
  printf ("Diners: %s\n", argv[1]);
  printf ("Juice: %s\n", getenv("JUICE"));
  return 0;
}

Answer 1

In C99, the implicit declaration of a function is not allowed. That means, the compiler should be aware of the function signature before it encounters a call to that function. This can be achieved two ways: 

1. Define the function before using it.
2. Provide a forward declaration of the function and define it later.

Usually, the function signature is provided as a forward declaration through the header files.

As per the man page of execle(), you need to include unistd.h to get the forward declaration.

Answer 2

You need to include unistd.h to resolve the implicit dec warning

Answer 3

I got it working. That's the order the statements should be as it turns out. Anything after execle won't run.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>


  char *my_env[] = {"JUICE=PEACH and apple", NULL};
int main (int argc, char *argv[]) 
{
  printf ("Diners: %s\n", argv[1]);
  printf ("Juice: %s\n", getenv("JUICE"));
  execle ("diner_info", "diner_info", "4", NULL, my_env);
  return 0;
}

Result:

# :$ gcc diner_info.c -o diner_info && ./diner_info 
Diners: (null)
Juice: (null)
Diners: 4
Juice: PEACH and apple
Diners: 4
Juice: PEACH and apple
Diners: 4
Juice: PEACH and apple
Diners: 4
Juice: PEACH and apple
Diners: 4
Juice: PEACH and apple
Diners: 4
Juice: PEACH and apple

But I still don't understand why the null values on the top, though.