[Book] Head First C (Part1)

· 5 min read · (888 words)
C programming Coding Books

  • 0.Intro.
  • 1.Getting Started with C
  • 2.Memory & Pointers

0. Intro

Q: How to learn (anything) efficiently?

  • Be serious about it. –> This MATTERS !!!
  • Understand –> Remember
  • Multiple learning types
    • Pictures
    • Conversation
    • Read out loud
    • …
    • WRITE A LOT OF CODE!!! –> Practice, Practice, Practice.

GCC (Gnu Compiler Collection)

1. Getting Started with C

Features of C

  • Low level

  • Advantages

    • Speed
    • Space
    • Portability (Cross-platform, WOCA )

  • Three C standards

    • ANSI C (=C89, 1989)
    • @ C89 –> C99
      • semi-dynamic allocate. (eg. int array[int_var];) ref
    • C99 (1999)
    • C11 (2011)
    • @ C99 –> C11 Changes:
      • Don’t need return 0; at the end of main()
      • gets() –> gets_s()
      • …

C standard library

  • stdio.h: Allow read/write data from/to terminal.

    • puts(): Display a string on cmd prompt/terminal.

  • stdlib.h: eg. NULL, malloc(), rand(), atoi(), abs(), exit(), system() …

    • atoi(): Convert string to int.
    • Eg. intVar = atoi(strVar);
    • sprintf(): Write formatted data to string.
    • Eg. charNum = sprintf(outStr, "Format String: %d, %s\n", intVar, inStr);
    • @[C] printf format

  • @Ref:
    assert.h ctype.h errno.h float.h limits.h
    locale.h math.h setjmp.h signal.h stdarg.h
    stddef.h stdio.h stdlib.h string.h time.h

💥 Processes of C programs

  • Program start from main, and:

    • return 0: success
    • return others: false
    • C99: -std=c99, auto return
      • Eg. gcc -std=c99 test.c -o test_program
      • @ -std=c99 -pedantic ??

  • Cmd to check newest exit status:

    • Linux/ Mac: echo $?
    • Windows: echo %ErrorLevel%

  • Compile and run: gcc test.c -o test && ./test
    On Unix-style operating systems, programs are run only if you:

    1. specify the directory(path)
    2. the directory is listed in the PATH environment variable.
      • .: current dir
      • ..: parent dir
      • ./ specify the path of current directory

Control Statements

  • switch v.s. if-else

    • switch: multi-check in same variable.

  • break; v.s. continue;

2. Memory & Pointers

  • Pointers are just variables that store memory addresses.

💥 Memory Layout

alt text


  • Variable location
    1. Stack: local var. //address: high –> low; pop!
    2. Heap: malloc() //address: low –> high
    3. Globals: global var.
    4. Constants: const var. //string literal char *card="JQK"; (Read-only!!)
    5. Code

⭐️ Array v.s. Pointer

  • Pointer var.

    • & where: find address of a variable.
    • * dereference: read/set contents of a memory address.

  • *array == array[0] , *(array+2) == array[2]

    • No matter what data types they have!
    • **Pointer variables have types so they can adjust pointer arithmetic.

  • Array variables can be used as pointers, but array variables are not quite the same…

    Array Pointer
    Eg. char a[] = "How big is it?"; char *p = a;
    1. sizeof() string/char array size
    sizeof(a) = 15;    		 4 or 8 bytes (32- or 64-bit)
    sizeof(p) = 4 or 8;
    2. address & &a == a; &p: address of p variable
    3. re-point
    (when create, …)    		 ✖️
    allocate space to store the array a[];
    but no space for array variable a    		 ✔️
    allocate 4 or 8 bytes to store the ptr.
    • pointer decay: assign an array to a ptr (when passing to a function), ptr only contain address without the size. (p59.)

string literals v.s. char (Screenshot)

alt text

*string literals

  • ptr (address) to string literals (constants).
  • Can NOT be update.

alt text

char array[]

  • no space for the array variable, only allocate space to store the contents.
  • copy string literals (const) to a new array to modify.
  • array content can be modified.

alt text

Tips

  • String literals are stored in read-only memory.
  • If you want to modify a string, you need to make a copy in a new array.
  • You can declare a char pointer as const char * to prevent the code from using it to modify a string

alt text


⭐️ scanf() v.s. fgets()

(p68.) scanf() fgets()
Usage int scanf ( const char * format, ... ); char * fgets ( char * str, int num, FILE * stream );
Eg. char food[5]; scanf("%4s", food); //return value=1 fgets(food, sizeof(food), stdin); //sizeof(food)=5
param. %[*][width][length]specifier
similar to @printf format		 [str]: ptr of char array
[num]: max num
[stream]: ptr of input FILE
return [int]: #num of arguments successfully filled [char*]: returns str.
1. Safty(limit) remember! mandatory
2. Multiple fields ✔️ structured data, use [format] ✖️ string buffer*1
3. Space in string hit space, %s stops
@Regular expression		 read whole string (terminate: newline/EOF)
❗️(and add newline, if it exist)
When to use which? structured data single unstructured string
  • Limit string’s length to prevent buffer overflows. (segmentation fault/ abort trap –> cause bugs. –> crash) (p66.)
  • gets(): Danger, don’t use.



@TODO


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