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Functions should always be declared with the appropriate function prototype. Failure to specify function prototypes results in a function being implicitly defined. Without a function prototype, the compiler will assume the the correct number and type of parameters have been supplied to a function. This can result in unintended and undefined , and perhaps unintended behavior. Given this, functions should always be declared with the appropriate function prototype.

C99 eliminated implicit function declarations from the C language \[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\]. However, many compilers allow compilation of programs containing implicitly defined functions, although they may issue a warning message. These warnings should be resolved \[[MSC00-A|MSC00-A. Compile cleanly at high warning levels]\], but do not prevent program compilation.

Non-Compliant Code Example

This non-compliant program makes use of function declarators with empty parentheses. Consequently, the program compiles cleanly at high warning levels but contains serious errors.

#include <stdio.h>
extern char *strchr();

int main(void) {
  char *c = strchr(12, 5);
  printf("Hello %c!\n", *c);
}

Section 6.11 of the C99 standards, "Future language directions", states that "The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature." The use of these declarations prevents the compiler from performing type checking.

Compliant Solution

The following compliant solution includes the header file containing the appropriate library function declaration.

#include <stdio.h>
#include <string.h>

int main(void) {
  char *c = strchr("world", 'w');
  printf("Hello %c!\n", *c);
}

6.11.7 Function definitions
1 The use of function definitions with separate parameter identifier and declaration lists
(not prototype-format parameter type and identifier declarators) is an obsolescent feature.

Non-Compliant Code Example 1

In this example, the definition of func() expects three parameters but is supplied only two. However, because there is no prototype for func(), the compiler assumes that the correct number of arguments has been supplied, and uses the next value on the program stack as the missing third argument.

functionfunc(1, 2);
...
int func(int one, int two, int three){
  printf("%d %d %d", one, two, three);
  return 1;
}

...

int functionfunc(int, int, int);
...

functionfunc(1,2);
...
int func(int one, int two, int three){
  printf("%d %d %d", one, two, three);
  return 1;
}

...

• Wiki Markup
  \[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Forward, Section 6.9.1, "Function definitions"