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09_pointers_make.md

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name: main

.aim[Systems: Get to the Point]

<style> .aim { font-size: .75em; border-bottom: 1px solid lightgray; margin: 1px; } .remark-inline-code { background-color: lightgray; border-radius: 3px; padding-left: 2px; padding-right: 2px; } h4 { font-size: 1.5em; margin: 1px; } </style>

template: main

Makefiles Revisited

  • You can run the rules for a specific target with $ make TARGET

--

  • rules do not need to always involve compilation. Common non-compiling targets include:

--

  • run: run the created program.

--

  • clean: remove unnecessary files (.o, ~, etc)

??? show euler

add euler.h to euler.c, get into #ifndef thing.

template: main

Pointers

  • Even though all pointers are the same size, we declare them using the type of the value pointed to.

--

  • * is used to declare a pointer variable.
    • int *p = &x;
    • p is a pointer variable that stores the address of the variable x.

--

  • In Java, object variables, or references, are essentially pointers, called references instead.

--

  • The exception you get when you try to use an uninitialized object variable in Java is null pointer, meaning the reference stored is 0 (null), which is an invlaid memory address.

template: main

Pointers

  • Consider the following C snippet:
unsigned int i = 2151686160;
int *ip = &i;
char *cp = &i;
  • ip and cp will store the address of the first byte used to store i. Depending on the endianness of the system, that byte will either be 10000000 (big) or 00010000 (little).

--

  • Let's just say that the first byte is located at memory address 3000 (using small number for ease of discussion)

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  • If you perform ip++ and cp++, each pointer will be incremented by 1, but due to pointer arithmetic, ip will increase to 3004 and cp will increase to 3001. In essence, ip would move one int forward in memory, while cp only moves one byte forward.

template: main

Pointers

  • * is also used as the de-reference operator. This will return the value stored at the memory address pointed to by the pointer.

--

  • Given the definitions of x and p above:

--

  • int y = *p + 10; would set y to the value 15.

--

  • *p = y; would set the value at the memory address stored in p (in the example is 2000), to whatever the value stored in y is.