title:Undefine C 
keywords:c,linux,asm 

# Undefined C

There is possible to 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.

## 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Most simplest and outdated way to do this is when getting input from command line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
### Recursion  
### Macro
### Signed/Unsigned
### Pointers
### Endianess 
### Static binary
### Dynamic binary
### Styles
### Compiler flags
### Allocate memory
### stdin,stdout,stderr



## 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