What do we mean by if (!string) in C

Question

I am trying to modify a code to adjust it to my liking. In order to do so I needed to understand what do we mean by if (!String) in C language if someone has any ideas ?

Example :

const char *String;
if (!String) {
   //do something
}

`const char*` can be interpreted as a boolean when the value of the pointer > 0. The `!` operator inverses that result. So that check returns true when `String == 0` — Neijwiert, Jun 13 '19 at 12:43
Note that in your snippet (taken as a whole program) `String` is not initialized or otherwise assigned a value. Testing with `if (!String)` (or otherwise inspecting its value) invokes UB. — pmg, Jun 13 '19 at 12:44

Vlad from Moscow · Answer 1 · 2019-06-13T13:02:36.727

From the C Standard (6.5.3.3 Unary arithmetic operators)

5 The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int. The expression !E is equivalent to (0==E).

And (6.3.2.3 Pointers)

3 An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. 66) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

So this if statement

if (!String) {
   //do something
}

checks whether the pointer String is equal to null-pointer provided that the pointer String was initialized.

While for example this statement

if (!*String) {
   //do something
}

checks whether the character pointed to by the pointer is equal to the null-terminating character '\0'.

If you'll include standard header <iso646.h> in your C-program then you can substitute the operator ! for the word not.

For example

#include <iso646.h>

//...

if ( not String ) {
   //do something
}

score 0 · Answer 2 · answered Jun 13 '19 at 12:59

It is a short way of writing if(String == NULL).

Often we instead write if(String) which is short for if(String != NULL). Note that this does NOT check if it points at a valid location. This code compiles but will (or may) fail:

char * str = strdup("Hello World!");
free(str);
if(str) { // If str is not a null pointer'
    puts(str); // str is pointing to memory that we have freed.
}

Trying to print str when it's no longer pointing at a valid location is undefined behavior. The program may print "Hello World!", crash or something else.

What do we mean by if (!string) in C

2 Answers2