what is Automatic storage class?
Automatic variables are allocated memory automatically at  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables.
1.The visibility of the  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables variables is limited to the block in which they are defined.
2.The  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables of the automatic variables is limited to the block in which they are defined.
3.The automatic variables are initialized to  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables by default.
4.The  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables assigned to automatic  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables gets freed upon exiting from the block.
5.The  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables used for defining automatic variables is auto.
6.Every local variable is automatic in C by  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables.


what is Static storage class?
1.The variables defined as static specifier can hold their value between the  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables function calls.
2. addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables local variables are visible only to the function or the  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables in which they are defined.
3.A same static variable can be declared many times but can be assigned at only one time.
4.Default initial value of the static  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables variable is 0 otherwise  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables.
5.The visibility of the static  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables variable is limited to the file in which it has declared.
6.The  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables used to define static variable is static.


what is Register storage class?
1.The variables defined as the register is allocated the memory into the CPU registers depending upon the size of the  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables remaining in the CPU.
2.We can not dereference the register variables, i.e., we can not use &operator for the  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables variable.
3.The access time of the register  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables is faster than the  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables variables.
4.The  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables default value of the register local variables is 0.
5.The register keyword is used for the  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables which should be stored in the CPU  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables. However, it is compiler?s choice whether or not; the variables can be stored in the register.
6.We can store  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables into the register, i.e., a register can store the  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables of a variable.
7.Static variables can not be stored into the register since we can not use more than one  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables specifier for the  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables variable.


what is External storage class?
1.The external  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables class is used to tell the compiler that the variable defined as extern is declared with an external linkage elsewhere in the program.
2.The  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables declared as  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables are not allocated any  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables. It is only  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables and intended to specify that the variable is declared elsewhere in the program.
3.The default initial value of external  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables type is 0 otherwise  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables.
4.We can only  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables the extern variable globally, i.e., we can not initialize the external variable within any block or method.
5.An external variable can be declared many times but can be  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables at only once.
6.If a  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables is declared as external then the  addressautomaticblockcompilerdeclarationdefaultexterngarbageglobalinitialinitializeinitializedintegralkeywordmemorymultiplenullpointersregisterruntimesamescopeStaticstoragevariablevariables searches for that variable to be initialized somewhere in the program which may be extern or static. If it is not, then the compiler will show an error.