Storage Classes

In C, a storage class is a specifier that determines the characteristics of a variable or function. These characteristics include its scope (where it can be accessed), lifetime (how long it exists in memory), and storage location (memory or CPU registers). There are four main storage classes: `auto`, `extern`, `static`, and `register`. The `auto` storage class is the default for all local variables declared inside a function or block. The `auto` keyword is rarely used explicitly because it's implied. These variables are created when the block is entered and destroyed when the block is exited. Their scope is limited to the block in which they are defined. The `extern` storage class is used to give a reference to a global variable that is defined in another file. It extends the visibility of the variable, telling the compiler that it exists elsewhere. It's a way to share variables across multiple source files. The `static` storage class has two main uses. When applied to a local variable, it extends the variable's lifetime to the entire duration of the program. The variable retains its value between function calls, but its scope remains local to the function. When applied to a global variable or a function, it limits its visibility to the file in which it is declared, preventing it from being accessed from other files. The `register` storage class is a hint to the compiler to store a variable in a CPU register instead of memory. This is done for faster access, typically for frequently used variables like loop counters. However, the compiler is free to ignore this suggestion. You cannot take the address of a register variable. Understanding storage classes is key to managing data effectively in larger, multi-file projects.