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Sahithyan's S2
Sahithyan's S2 — Program Construction

C Basics

Memory management

Memory is allocated either implicitly (variable declaration) or explicitly (malloc()) and deallocated either implicitly (by the compiler) or explicitly (free())

Memory layout in C is similar to the image below.

Memory layout in C

  • text - readonly. stores compiled machine code. fixed size, determined at compile time.
  • data - initialized variables. stores global and static variables. readable & writable. fixed size, determined at compile time.
  • bss - short for block started by symbol. uninitialized variables. stores global and static variables that are uninitialized or initialized to 0. fixed size, determined at compile time. automatically initialized to 0 by OS.
  • heap and stack grows in the opposite directions
  • heap can have gaps but stack cannot

Undefined Behavior

A behavior which is specified as unpredictable in the language specification in which a program is written.

There are a lot of cases for undefined behavior in C.

  • Usage of a deallocated (using free or realloc) pointer
  • The pointer argument to free or realloc doesn’t match a pointer earlier returned by calloc, malloc or realloc.
  • The pointer argument to free or realloc which is already deallocated using free or realloc
  • A non-null pointer pointing to zero requested size is used to access an object

String

A string is an array of char. They are null terminated using the null character (or \0 or NUL). The null character is added automatically to the end of the array by the compiler.