Why this Implicit declaration of function X

Question

I wrote a simple program to find the Sum  average  biggest and smallest number of 3 numbers  It lets the user to input three  integer  numbers and return the sum  average  max and min  It has no errors but a warning  Here is my source code   main c    include  lt stdio h gt   int main         int num1  num2  num3  sum  max  min  avg       printf  Enter Three   Integer   Numbers          scanf   i i i    amp num1   amp num2   amp num3        sum   summation  amp num1   amp num2   amp num3       avg   average  amp sum       max   max val  amp num1   amp num2   amp num3       min   min val  amp num1   amp num2   amp num3        printf  Sum   i Avg   i MAX   i MIN   i   sum  avg  max  min        return 0     int summation int  n1  int  n2  int  n3        int s      s    n1    n2    n3       return s     int average int  s        int a      a    s   3       return a     int max val int  n1  int  n2  int  n3        int MAX       if   n1  gt   n2  MAX    n1      else if   n2  gt   n3  MAX    n2      else MAX    n3       return MAX     int min val int  n1  int  n2  int  n3        int MIN       if   n1  lt   n2  MIN    n1      else if   n2  lt   n3  MIN    n2      else MIN    n3       return MIN      I think there is no need to make a header file because all functions are in type of  int    When I compile this  gcc main c -o test   It says  main c  In function  main   main c 34 5  warning  implicit declaration of function  summation   -Wimplicit-function-declaration    Why this warning  I can t find any wrong in that declaration  What s that

User · Accepted Answer

summation and your other functions are defined after they re used in main  and so the compiler has made a guess about it s signature  in other words  an implicit declaration has been assumed   You should declare the function before it s used and get rid of the warning  In the C99 specification  this is an error   Either move the function bodies before main  or include method signatures before main  e g     include  lt stdio h gt   int summation int    int    int      int main

[c] Why this "Implicit declaration of function 'X'"?

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