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Wiki MarkupAn application programming interface (API) specifies how a function is intended to be called.    Calling a function with incorrect arguments can result in unexpected or unintended program behavior. Functions that are appropriately declared \[[declared (see DCL07-A. Include the appropriate type information in function declarators]\] ) will typically fail compilation if they are supplied with the wrong number or types of arguments. However, there are cases where supplying the incorrect arguments to a function will at best generate compiler warnings. These warnings should be resolved \[[resolved (see MSC00-A. Compile cleanly at high warning levels]\], ) but do not prevent program compilation.

Non-Compliant Code Example

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Function Pointers)

In this example, the function pointer fp is used to refer to the function strchr(). However, fp is declared without a function prototype.  As a result, there is no type checking performed on the call to fp(12,2);.

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

char *(*fp) ();

int main(void) {
  char *c;
  fp = strchr;
  c = fp(12, 2);
  printf("%s\n", c);

}

Compliant Solution

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Function Pointers)

Declaring fp with a function prototype corrects this example.

#include <string.h>

char *(*fp) (char const *, int);

int main(void) {
  char *c;
  fp = strchr;
  c = fp("Hello",'H');
  printf("%s\n", c);

}

Non-Compliant Code Example

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Variadic Functions)

The POSIX function {{open()}} \[[Open Group 04|AA. C References#Open Group 04]\] is a variadic function with the following prototype:

int open(char const *path, int oflag, ... );

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The {{open()}} function accepts a third argument to determine a newly created file's access mode. If {{open()}} is used to create a new file and the third argument is omitted, the file may be created with unintended access permissions \[[permissions (see FIO06-A. Create files with appropriate access permissions]\]).

In this non-compliant code example from a vulnerability in the useradd() function of the shadow-utils package CVE-2006-1174 , the third argument to open() has been accidentally omitted.

fd = open(ms, O_CREAT|O_EXCL|O_WRONLY|O_TRUNC);

Compliant Solution

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Variadic Functions)

To correct this example, a third argument is specified in the call to open().

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\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Forward, and Section 6.9.1, "Function definitions"
\[[Spinellis 06|AA. C References#Spinellis 06]\] Section 2.6.1, "Incorrect Routine or Arguments"

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