Question

There are many endearing string functions in the C standard library, such as (in string.h)

char *strcat(char *str1, const char *str2);

or (in stdlib.h)

long int strtol(const char *nptr, char **endptr, int base);

(Ignore the wisdom of calling these functions, for the purposes of this question.)

What will happen if I pass any of these functions a NULL pointer? (I mean (char *) 0, not the empty string.)

I haven’t found any answers in the man pages or on the web.

This leads me to think it’s implementation-defined, but it could just as well mean an automatic segmentation fault; no special error behavior or return values are specified, either.

Could the behavior even vary from function to function, within the same implementation?

Answer 1

The C standard says it in 7.21.1 String Function Conventions, clause 2:

Unless explicitly stated otherwise in the description of a particular
function in this subclause, pointer arguments on such a call shall
still have valid values, as described in 7.1.4.

7.1.4 Use of library functions:

If an argument to a function has an invalid value (such as a value
outside the domain of the function, or a pointer outside the address
space of the program, or a null pointer, or a pointer to
non-modifiable storage when the corresponding parameter is not
const-qualified) or a type (after promotion) not expected by a
function with variable number of arguments, the behavior is undefined.

strcat()‘s description in 7.21.3.1 says nothing about the NULL pointer being a valid input, hence, I conclude, the behavior is officially undefined if any of its input pointers is NULL.