C Language

Introduction to C

How C works?

• Excellent run-time performance: generally much faster than Pythonor Java for comparable code (because it optimizes for a given architecture)

• Reasonable compilation time: enhancements in compilation procedure (Makefiles) allow only modified files to be recompiled

• Mainstream C compiler in Linux:

  • GNU Complier Collection (gcc, not only for C family)

  • clang/LLVM (for C language family)

  • In terminal/command line tool/shell, “man clang/gcc”

C Pre-Processing (CPP)

• C source files first pass through CPP, before compiler sees code (mainly text editing)

• CPP replaces comments with a single space.

• CPP commands begin with #.

Macro

• #include “file.h” /* Inserts file.h into output */
• #include <stdio.h> /* Looks for file in standard location */
• #define M_PI (3.14159) /* Define constant */

• #if/#endif /* Conditional inclusion of text */

• Text replacement.

• Avoid using macros whenever possible.
• NO or very tiny speedup.
• Instead use C function.

C Compilation

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C Memory Management

Pointer

Array

Pointer Arithmetic

• Hardware’s memory composed of 8-bit storage cells, each has a unique address.
• A C pointer is just abstracte memory address.
• Type declaration tells compiler how many bytes to fetch on eacch access through pointer.
• Alignment: Store the data in lines that can be divided by its size.
• pointer + 1 equals to adding 1 * sizeof(type) to the memory address.

sizeof() Operator

• sizeof(type) returns¹ number of bytes in object.

Memory Management

• A program's address space contains 4 regions:

  • stack: local variables inside functions, grows downward.
  • heap: space requested for dynamic data via malloc(); resizes dynamically, grows upward.
  • static data: variables declared outside functions, does not grow or shrink. Loaded when program starts, can be modified.
  • code (a.k.a. text): loaded when program starts, does not change.
  • 0x0 unwritable/unreadable (NULL pointer).

The Stack

• Every time a function is called, a new "stack frame" (contiguous blocks of memory; stack pointer indicates start of stack frame) is allocated on the stack, includes:
  • Return address
  • Arguments
  • Space for local variables

Managing the Heap

• The heap is dynamic memory – memory that can be allocated, resized, and freed during program runtime.

• C supports four functions for heap management:

  • malloc(): allocate a block of uninitialized memory
  • calloc(): allocate a block of zeroed memory
  • free(): free previously allocated block of memory
  • realloc(): change size of previously allocated block (might move)

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1. sizeof() is a operator, not a function. ↩

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