使用file命令查看动态链接库，得到如下结果：
#/usr/libx32$ file ../lib32/libgomp.so.1.0.0
../lib32/libgomp.so.1.0.0: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, BuildID[sha1]=57ec5c404cd4f781ac341332437a6960784e4581, stripped
#:/usr/libx32$ file ../libx32/libgcc_s.so.1
../libx32/libgcc_s.so.1: ELF 32-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=4713f15844e2b3e13f741bd5acef9aaab24cff03, stripped
其中比较让我困惑的红字部分。
ELF 32-bit表示这个elf文件是elf32的，ELF64-bit表示elf文件是elf64的。
typedef struct elf32_hdr {
unsigned char e_ident[EI_NIDENT];
Elf32_Half e_type;
Elf32_Half e_machine;
Elf32_Word e_version;
Elf32_Addr e_entry; /* Entry point */
Elf32_Off e_phoff;
Elf32_Off e_shoff;
Elf32_Word e_flags;
Elf32_Half e_ehsize;
Elf32_Half e_phentsize;
Elf32_Half e_phnum;
Elf32_Half e_shentsize;
Elf32_Half e_shnum;
Elf32_Half e_shstrndx;
} Elf32_Ehdr;
typedef struct elf64_hdr {
unsigned char e_ident[16]; /* ELF "magic number" */
Elf64_Half e_type;
Elf64_Half e_machine;
Elf64_Word e_version;
Elf64_Addr e_entry; /* Entry point virtual address */
Elf64_Off e_phoff; /* Program header table file offset */
Elf64_Off e_shoff; /* Section header table file offset */
Elf64_Word e_flags;
Elf64_Half e_ehsize;
Elf64_Half e_phentsize;
Elf64_Half e_phnum;
Elf64_Half e_shentsize;
Elf64_Half e_shnum;
Elf64_Half e_shstrndx;
} Elf64_Ehdr;
对于elf32和elf64头，二者的前16字节是Magic, Magic第五个字节定义了elf文件的格式.
Intel 80386和x86-64则定义了目标机器的类型,也就是目标机器的指令集.