chqrlie chqrlie - 1 year ago 104
C Question

Does calling printf without a proper prototype invoke undefined behavior?

Does this innocent looking program invoke undefined behavior:

int main(void) {
printf("%d\n", 1);
return 0;

Answer Source

Yes invoking printf() without a proper prototype (from the standard header <stdio.h> or from a properly written declaration) invokes undefined behavior.

As documented in C11 Annex J (informative only)

J2 Undefined Behavior

  • For 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 (

This annex is not normative, but clearly documents the above code as an example of undefined behavior.

In more pragmatic words, in the absence of a prototype for printf, the compiler generates the calling sequence as if printf was defined as int printf(const char*, int) which may be quite different and incompatible with the actual implementation of printf in the standard library, defined as int printf(const char restrict *format, ...).

Ancient ABIs were regular enough that this would not cause a problem, but modern (eg 64-bit) ABIs use more efficient calling sequences that make the above code definitely incorrect.

As a consequence, this famous classic C program would fail too, without the #include <stdio.h> or at least a proper prototype for printf:

int main(void) {
    printf("Hello world\n");  // undefined behavior
    return 0;
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