InterviewSkill
C fundamentals for memory, pointers, compilation, and low-level programming interview rounds.
A pointer is a variable that stores the memory address of another value.
Pointers let C programs access and modify memory directly. They are used for dynamic memory, arrays, strings, pass-by-reference style APIs, and low-level system programming.
In many expressions, an array decays into a pointer to its first element, but an array and a pointer are not the same type.
An array owns a fixed block of elements, while a pointer stores an address. Array decay explains why arr[i] and *(arr + i) work similarly, but sizeof(arr) and sizeof(pointer) behave differently.
C strings are arrays of char terminated by a null character, '\0'.
C does not have a built-in string object. String functions scan memory until they find the null terminator, so missing terminators can cause buffer overreads and undefined behavior.
malloc allocates memory on the heap, and free releases heap memory that is no longer needed.
malloc returns a pointer to a requested number of bytes. The programmer must check for NULL, use the memory within bounds, and free it exactly once to avoid leaks, crashes, or double-free bugs.
Stack memory is automatically managed for function calls, while heap memory is manually managed for dynamic allocation.
Local variables usually live on the stack and disappear when the function returns. Heap memory can outlive a function call, but the program must free it to avoid leaks.
A struct groups multiple fields under one user-defined type.
Structs are used to model records such as nodes, coordinates, records, and configuration objects. Their memory layout may include padding for alignment.
Header files share declarations such as function prototypes, macros, and type definitions across source files.
A .h file describes an interface, while .c files usually contain implementations. Include guards or #pragma once prevent duplicate declarations during preprocessing.
Undefined behavior means the C standard gives no guarantee about what the program will do.
Examples include out-of-bounds access, signed integer overflow, use-after-free, and dereferencing invalid pointers. Compilers may optimize assuming undefined behavior never happens.