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Calling a function with incorrect arguments can result in unexpected or unintended program behavior. Functions that are appropriately declared \[[DCL07-A. Include the appropriate type information in function declarators]\] will 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 only generate compiler warnings. These warnings should be resolved \[[MSC00-A. Compile cleanly at high warning levels]\], but do not prevent program compilation.

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

char *(*fp) ();

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

}

Note that this example also violates recommendation \[[DCL07-A. Include the appropriate type information in function declarators]\].

#include <string.h>

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

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

}

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

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

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 \[[FIO06-A. Create files with appropriate access permissions]\].

/* ... */
int fd = open(file_name, O_CREAT | O_WRONLY); /* access permissions are missing */
if (fd == -1){
  /* Handle Error */
}
/* ... */

/* ... */
int fd = open(file_name, O_CREAT | O_WRONLY, file_access_permissions);
if (fd == -1){
  /* Handle Error */
}
/* ... */

\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Forward, Section 6.9.1, "Function definitions"
\[[Spinellis 06|AA. C References#Spinellis 06]\] Section 2.6.1, "Incorrect Routine or Arguments"