struct-offset-analyzer

Statically analyze C struct member offsets through code reading to calculate memory layouts

Statically analyze the memory offsets of C language struct members without needing to run code.

Use Cases

• Locating struct members during reverse engineering
• Confirming memory layouts during debugging
• Analyzing data structures in security research
• Understanding struct field positions in binary analysis

Workflow

1. Locate Struct Definition

# Search for struct definition
grep -n "struct xxx_st {" **/*.h
grep -n "typedef struct" **/*.h

2. Collect Type Information

Find definitions for all member types:

• Nested structs
• Enum types
• typedef aliases
• Constant definitions (e.g., #define EVP_MAX_MD_SIZE 64)

3. Calculate Alignment Rules

Type	Size (64-bit)	Alignment Requirement
char/unsigned char	1	1
short	2	2
int/uint32_t	4	4
long/size_t/pointer	8	8
unsigned char[N]	N	1 (no padding needed)
enum	usually 4	4
struct	depends on members	aligned to largest member

Key Rules:

• Member offset must be a multiple of its size
• unsigned char arrays are 1-byte aligned, no padding required
• Overall struct size is aligned to the size of its largest member
• Padding bytes count toward offsets

4. Output Offset Table

Use hexadecimal representation for offsets, format:

| Offset(0x) | Member | Type | Size |
|------------|--------|------|------|
| 0x00 | field1 | int | 4 |
| 0x04 | *(padding)* | - | 4 |
| 0x08 | field2 | void * | 8 |

Common Search Patterns

# Find struct member definition
grep -n "struct xxx_st" **/*.h

# Find type definition
grep -n "typedef.*XXX" **/*.h

# Find constant definition
grep -n "#define.*SIZE" **/*.h

# Find enum definition
grep -n "typedef enum" **/*.h

Example: OpenSSL ssl_st Analysis

Analyzing client_app_traffic_secret member offset:

1. Locate struct: ssl/ssl_local.h:1068
2. Find constant: EVP_MAX_MD_SIZE = 64 (include/openssl/evp.h:19)
3. Calculate layout, note that unsigned char arrays need no padding
4. Result: offset 0x33c (828 bytes)

Notes

• Confirm target platform (32-bit vs 64-bit)
• Note that conditional compilation (#ifdef) may affect struct layout
• Check for #pragma pack directives that may change alignment
• Union members share the same offset