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uint32_t offset; /* file offset to this object file */
uint32_t size; /* size of this object file */
uint32_t align; /* alignment as a power of 2 */
};
</code></pre>
The header has the **magic** bytes followed by the **number** of **archs** the file **contains** (`nfat_arch`) and each arch will have a `fat_arch` struct.
The header contains basic information about the file, such as magic bytes to identify it as a Mach-O file and information about the target architecture. You can find it in: `mdfind loader.h | grep -i mach-o | grep -E "loader.h$"`
```c
#define MH_MAGIC 0xfeedface /* the mach magic number */
The **file's layout in memory** is specified here, detailing the **symbol table's location**, the context of the main thread at execution start, and the required **shared libraries**. Instructions are provided to the dynamic loader **(dyld)** on the binary's loading process into memory.
uint32_t cmdsize; /* total size of command in bytes */
};
```
There are about **50 different types of load commands** that the system handles differently. The most common ones are: `LC_SEGMENT_64`, `LC_LOAD_DYLINKER`, `LC_MAIN`, `LC_LOAD_DYLIB`, and `LC_CODE_SIGNATURE`.
Basically, this type of Load Command define **how to load the \_\_TEXT** (executable code) **and \_\_DATA** (data for the process) **segments** according to the **offsets indicated in the Data section** when the binary is executed.
These commands **define segments** that are **mapped** into the **virtual memory space** of a process when it is executed.
There are **different types** of segments, such as the **\_\_TEXT** segment, which holds the executable code of a program, and the **\_\_DATA** segment, which contains data used by the process. These **segments are located in the data section** of the Mach-O file.
**Each segment** can be further **divided** into multiple **sections**. The **load command structure** contains **information** about **these sections** within the respective segment.
In the header first you find the **segment header**:
<preclass="language-c"><codeclass="lang-c">struct segment_command_64 { /* for 64-bit architectures */
uint32_t cmd; /* LC_SEGMENT_64 */
uint32_t cmdsize; /* includes sizeof section_64 structs */
char segname[16]; /* segment name */
uint64_t vmaddr; /* memory address of this segment */
uint64_t vmsize; /* memory size of this segment */
uint64_t fileoff; /* file offset of this segment */
uint64_t filesize; /* amount to map from the file */
int32_t maxprot; /* maximum VM protection */
int32_t initprot; /* initial VM protection */
<strong> uint32_t nsects; /* number of sections in segment */
* **`__PAGEZERO`:** It instructs the kernel to **map** the **address zero** so it **cannot be read from, written to, or executed**. The maxprot and minprot variables in the structure are set to zero to indicate there are **no read-write-execute rights on this page**.
*`__data`: Global variables (that have been initialized)
*`__bss`: Static variables (that have not been initialized)
*`__objc_*` (\_\_objc\_classlist, \_\_objc\_protolist, etc): Information used by the Objective-C runtime
* **`__LINKEDIT`**: Contains information for the linker (dyld) such as, "symbol, string, and relocation table entries."
* **`__OBJC`**: Contains information used by the Objective-C runtime. Though this information might also be found in the \_\_DATA segment, within various in \_\_objc\_\* sections.
### **`LC_MAIN`**
Contains the entrypoint in the **entryoff attribute.** At load time, **dyld** simply **adds** this value to the (in-memory) **base of the binary**, then **jumps** to this instruction to start execution of the binary’s code.
Contains information about the **code signature of the Macho-O file**. It only contains an **offset** that **points** to the **signature blob**. This is typically at the very end of the file.\
However, you can find some information about this section in [**this blog post**](https://davedelong.com/blog/2018/01/10/reading-your-own-entitlements/) and this [**gists**](https://gist.github.com/carlospolop/ef26f8eb9fafd4bc22e69e1a32b81da4).
Contains the **path to the dynamic linker executable** that maps shared libraries into the process address space. The **value is always set to `/usr/lib/dyld`**. It’s important to note that in macOS, dylib mapping happens in **user mode**, not in kernel mode.
### **`LC_LOAD_DYLIB`**
This load command describes a **dynamic****library** dependency which **instructs** the **loader** (dyld) to **load and link said library**. There is a LC\_LOAD\_DYLIB load command **for each library** that the Mach-O binary requires.
* This load command is a structure of type **`dylib_command`** (which contains a struct dylib, describing the actual dependent dynamic library):
```objectivec
struct dylib_command {
uint32_t cmd; /* LC_LOAD_{,WEAK_}DYLIB */
uint32_t cmdsize; /* includes pathname string */
struct dylib dylib; /* the library identification */
};
struct dylib {
union lc_str name; /* library's path name */
uint32_t timestamp; /* library's build time stamp */
uint32_t current_version; /* library's current version number */
uint32_t compatibility_version; /* library's compatibility vers number*/
At the core of the file lies the data region, which is composed of several segments as defined in the load-commands region. **A variety of data sections can be housed within each segment**, with each section **holding code or data** specific to a type.
<summary><strong>Learn AWS hacking from zero to hero with</strong><ahref="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
* If you want to see your **company advertised in HackTricks** or **download HackTricks in PDF** Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
