title:Undefine C 
keywords:c,linux,asm 

# Undefined C

There is possible to run piece of code inside online c compiler like https://www.onlinegdb.com/online_c_compiler
Or run locally. With base check is done with gcc compiler. There are many small tricks around running C code
in practice that aren't covered in any generic tutorials.

## Compile


__hello_world.c__
```c
int main() {
	printf("Hello world\n");
}
```

```bash
gcc hello_world.c -o hello_world
gcc -m32 hello_world.c -o hello_world_32 #for 32bit target
```

## Syntax

### Variables

Standard list of available types

#### Check type size

All types have size that are declared in bytes. Some of the types are machine dependents.
like int/long, if there is needed machine independent types then there are int32_t/uint32_t/int64_t/uint64_t

Each architecture 8bit/16bit/32bit/64bit will have different size for those types

Use __sizeof()__

Running on x86 machine
```c
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
int main() {
    printf("Sizeof int %lu\n",sizeof(int));
    printf("Sizeof int32_t %lu\n",sizeof(int32_t));
    printf("Sizeof int64_t %lu\n",sizeof(int64_t));
    printf("Sizeof long %lu\n",sizeof(long));
    printf("Sizeof long long %lu\n",sizeof(long long));
}
```

Most safest/portable way is to use [u]int[8/16/32/64]_t types. 

Defined macros'es to get type max and min values are

https://en.cppreference.com/w/c/types/limits

```c
#include <limits.h>
int main() {
    printf("INT_MIN %d\n",INT_MIN);
    printf("INT_MAX %d\n", INT_MAX);
    printf("LONG_MIN %ld\n",LONG_MIN);
}
```

Example from AVR __stdint.h__   
https://github.com/avrdudes/avr-libc/blob/main/include/stdint.h  
Example from Libc  
https://sourceware.org/git/?p=glibc.git;a=blob;f=stdlib/stdint.h  



#### How to shoot the leg

When code suppose to run on 32bit and 64bit platform the size of type may vary.
Need to take in account this case.





### Functions

Function syntax, there is nothing interesting on functions

```
<RETURN_TYPE> <NAME>(<TYPE> <NAME>,..) {
    <EXPR>
}
```

Write simple function

```c
int fun1() {
    return -1;
}
```

Function can have multiple return statements.
Here is example whne function have 3 return values.
```c
int fun2(int i) {
    if (i<0) return -1;
    if (i>0) return 1;
    return 0;
}
```

Get address of function 

```c
printf("fun1 address %016x",&fun1);//64bit platform
```

### If statement

```c
if () ;
if () {}
```

One of the way to check error of returned functions is 

```c
if ((c = getfun()) == 0) {
}
```

Most simplest and outdated way to do this is when getting input from command line
```c
#include <stdio.h>
int main() {
    int c;
    char ch;
    while ((c = getchar()) != EOF ) {
        ch = c;
        printf("Typed character %c\n",c);
    }
}
```

### For cycle

For loop is one that may involve some trickery, its 
as simple as 

```c
for (<INITIAL>;<TERMINATE CONDITION>;<AFTER CYCLE>) {
}
```

Go over values from 1 till 10

```c
int i=0;
for (i=1;i<=10;i++) {
    printf("%d\n",i)
}
```

Now lets do it from 10 till 1

```c
int i=0;
for (i=10;i>0;i--) {
    printf("%d\n",i)
}
```

Now lets make one liner

```c
for (i=0;i<10;i++,printf("%d\n",i));
```

Yes there is possible to write as many expressions as needed.


### Structure

Structure allows to combine types under one new type. Structure is convenient way how to combine set 
of types and reuse them as one. 

```c
struct struct1 {
    uint8_t a;
    uint16_t b;
    uint32_t c;
    uint64_t d;
};
```

Total intuitive size of structure would be 
```c
int total_szie = sizeof(uint8_t) + sizeof(uint16_t) + sizeof(uint32_t) + sizeof(uint64_t);
int real_size = sizeof(struct1);
```

Types are placed inside structure to make fast access to them. Some instructions of CPU may require
to access aligned memory addresses to not have penalty on accessing types inside structure.

To directly mess with alignment of types use attribute
```c
__attribute__ ((aligned (8)))
```


Use attributes to pack structure and be not architecture dependent.

```c
struct struct2 {
    uint8_t a;
    uint16_t b;
    uint32_t c;
    uint64_t d;
} __attribute__((packed));
```

Now let check size of structure after it packed

```c
int new_size = sizeof(struct2);
```

Also there is possible to add aligmnet to each time in structure
```c
struct struct3 {
    uint8_t a __attribute__((aligned (8)));
    uint16_t b __attribute__((aligned (8)));
    uint32_t c __attribute__((aligned (8)));
    uint64_t d __attribute__((aligned (8)));
} __attribute__((aligned (8)));
```

Now size of structure will be 32. 

All results on amd64, other arch may differ.

### How to shoot leg
Forget that struct size is not consistent. 

### Recursion

Recursion is technique that could be useful to write shorter code 
and deal with cycles. One thing that recursion suffer is that it consumes
stack memory and its have default limit on platform.

```c
#include <stdio.h>
#include <stdlib.h>

int fun_r(int i) {
	printf("val %d\n",i);
	fun_r(i+1);
	return 0;
}

int main() 
{
	fun_r(0);
}
```

Program will fail after its reach out of stack range.
When increase the default stack limit it go more further.


Check default stack size

```
ulimit -s
```

Set stack size

```
ulimit -s 16384
```

### Macro

There is many things useful as macros. There is many tricks in macros to emit
useful parts of code. 

Define values, as its enum. 
```c
#define VAL_0 0
#define VAL_1 1
#define VAL_LAST VAL_1
```

Multiline macro
```c
#define INC_FUN(TYPE) TYPE inc_##TYPE(a TYPE){\
    TYPE c=1\
    return a + c\
}

INC_FUN(int)
INC_FUN(char)
INC_FUN(double)
INC_FUN(notype)
```

to check code expansion of macro run

```
gcc -E <SOURCE_FILE>
```



http://main.lv/writeup/c_macro_tricks.md


https://jadlevesque.github.io/PPMP-Iceberg/


### Pointers

One the C most loved feature is pointers, they allow to access addresses without any sanity check
and they dont have any lifetime, so anything is possible with those.

Pointer contains address which is interpreted according of pointer type

```c
int c;
int ptr=&c;
```

Go over array of chars
```c
#include <stdio.h>
#include <stdlib.h>

int main() {
	char s[]="asd";
	char *c=&s;
	while (*c != 0) {
	    printf("NExt char %c addr %016x\n",*c,c);
	    c++;
	}
}
```
Go over array of ints
```c
    int i=0;
    int arr[] = {9,7,5,3,1};
	int *ptr = arr;
	while (i<5) {
	    printf("Number value %d addr %016x\n",*ptr, ptr);
	    ptr++;
	    i++;
	} 
```

Pointer arithmetics like +1 will move to next address that is offset of type size.
As example below structure size is 12, and increment of pointer to that structure
increment address to sizeof structure. And yes address is pointing to not mapped memory, so it 
will segfault if accessed. 

```c
struct size12 {
    int a,b,c;
}

int main() {
    struct size12 *s=0;
    s++;
    printf("%016x\n",s);
    s++;
    printf("%016x\n",s);
}
```

Double pointers are pointers to pointers

```c
#include <stdio.h>

int main(int argc, char **argv) {
	char *arg = argv[0];
    printf("Program name %s\n",arg);
}
```

#### How to shoot the leg
Run pointer in while loop incrementing pointer. It will stop only when segfaults.

Dont initialize pointer and it will have random value.

### Endianess 
### Static binary
### Dynamic binary
### Styles
### Compiler flags
### Allocate memory
### stdin,stdout,stderr
### Signed/Unsigned



## Basic usage

### File manipulation
### File manipulation with syscalls

## Base usage

### Kernel module
### Write plugins


## Advanced cases

### Linking
### Extern
### Attributes
### Creating shared library
### Create static libraries
### Join all objects together
### Compile with musl
### Inspect elf files
### No standard library
### Memory leaks
### Code coverage
### Profiling
### Canary
### Atomic
### Multithreading


## Embedding  

### Embed in C++
### Embed in Go
### Embed in Swift
### Embed in JS

### Lua in C
### Python in C


## Multiplatform

### Cross compile
### Different flags
### Check architecture
### ARMv8
### AVR8
### Emscripten


## Graphics

### SDL2
### GTK
### OpenGL
### Generate image