How to check if a variable has been initialized in C?

C does not have this ability. You have two main options:

A sentinel value

For example, if you know that the value of c will never be negative, then initialize it to -1, and test that for that.

Add another variable

Create another variable bool we_set_c_to_something = false; and then set it to true when you write to c.

C is a compiled language which doesn't support runtime variable binding, while Perl is a interpreted language which support dynamic typing. So you can check the definition of a variable in Perl, but not in C.

When you declare a variable in C int c;, this variable c is defined but without initialization. The declaration and definition are in one statement.

The definition of a variable in C is not checked by code writer. The compilers do it for you. When compile and link your C code, the compiler will check all variable's definitions. An error will be invoked and the compiling or linking process will stop if there are undefined variables found in your code.

Hope this will make you distinguish the differences.

Wrong question. You're not asking whether the variable is defined. If the variable is not defined then compilation fails. Look up the difference between "declaration" and "definition". In the case of those local variables, you have defined the variable c.

What you're looking for is initialisation. Many compilers will warn you about using variables before they're initialised, but if you persist in running that code then the assumption is that you know better than the compiler. And at that point it's your problem. :) Some languages (e.g. Perl) have an extra flag that travels along with a variable to say whether it's been initialised or not, and they hide from you that there's this extra flag hanging around which you may or may not need. If you want this in C, you need to code it yourself.

Since C++ allows operator overloading, it's relatively easily to implement this in C++. Boost provides an "optional" template which does it, or you could roll your own if you want a coding exercise. C doesn't have the concept of operator overloading though (hell, the concept didn't really exist, and the compilers of the day probably couldn't have supported it anyway) so you get what you get.

Perl is a special case because it rolls the two together, but C doesn't. It's entirely possible in C to have variables which are defined but not initialised. Indeed there are a lot of cases where we want that to be the case, particularly when you start doing low-level access to memory for drivers and stuff like that.