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The C Standard identifies five two distinct situations in which undefined behavior (UB) may arise as a result of invoking a function using a declaration that is incompatible with its definition or by supplying incorrect types or numbers of arguments:
| UB | Description |
A pointer is used to call a function whose type is not compatible with the referenced type (6.3.2.3). | |
For a call to a function without a function prototype in scope, the number of arguments does not equal the number of parameters (6.5.2.2). | |
For a call to a function without a function prototype in scope where the function is defined with a function prototype, either the prototype ends with an ellipsis or the types of the arguments after promotion are not compatible with the types of the parameters (6.5.2.2). | |
| For a call to a function without a function prototype in scope where the function is not defined with a function prototype, the types of the arguments after promotion are not compatible with those of the parameters after promotion (with certain exceptions) (6.5.2.2). | |
A pointer is used to call a function whose type is not compatible with the referenced type (6.3.2.3). | |
A function is defined with a type that is not compatible with the type (of the expression) pointed to by the expression that denotes the called function (6.5.2.2). |
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A pointer to a function of one type may be converted to a pointer to a function of another type and back again; the result shall compare equal to the original pointer. If a converted pointer is used to call a function whose type is not compatible with the referenced type, the behavior is undefined.
| Code Block | ||||
|---|---|---|---|---|
| ||||
#include <stdio.h>
#include <string.h>
char *(*fp)();
int main(void) {
const char *c;
fp = strchr;
c = fp('e', "Hello");
printf("%s\n", c);
return 0;
} |
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