数据宽度和存储容量的单位数据在内存中的存放顺序字节顺序大端小端对齐网络传输一般采用大端序 BigEndian ByteOrder

    char x2;
    int x1;
    short x3;
    long long x4;
1,4,2,8

struct test
{
    char x2;
    int x1;
    short x3;
    long long x4;
} __attribute__((packed));
struct test1
{
    char x2;
    int x1;
    short x3;
    long long x4;
};
struct test2
{
    char x2;
    int x1;
    short x3;
    long long x4;
} __attribute__((aligned(4)));
struct test3
{
    char x2;
    int x1;
    short x3;
    long long x4;
} __attribute__((aligned(8)));
struct test4
{
    char x2;
    int x1;
    short x3;
    long long x4;
} __attribute__((aligned(16)));

Linux  short char 2字节对齐，其他4字节对齐 

当cpu字长64位=8byte

1,4,2,8

#pragma pack()
1,4,2,8=15
1,4,2,8=0-0,4-7,8-9,16-23=24
1,4,2,8=0-0,4-7,8-9,12-19=20->24
1,4,2,8=0-0,4-7,8-9,12-19=20->24
1,4,2,8=0-0,4-7,8-9,12-19=20->32

# pragma pack(1)
1,4,2,8=15
1,4,2,8=15
1,4,2,8=15->16
1,4,2,8=15->16
1,4,2,8=15->16

# pragma pack(2)
1,4,2,8=15
1,4,2,8=0-0,2-5,6-7,8-15=16
1,4,2,8=0-0,2-5,6-7,8-15=16
1,4,2,8=0-0,2-5,6-7,8-15=16
1,4,2,8=0-0,2-5,6-7,8-15=16

# pragma pack(4)
1,4,2,8=15
1,4,2,8=0-0,4-7,8-9,12-19=20
1,4,2,8=0-0,4-7,8-9,12-19=20
1,4,2,8=0-0,4-7,8-9,12-19=20->24
1,4,2,8=0-0,4-7,8-9,12-19=20->32

#include <stdio.h>
#pragma pack()
struct test
{
    char x2;
    int x1;
    short x3;
    long long x4;
} __attribute__((packed));
struct test1
{
    char x2;
    int x1;
    short x3;
    long long x4;
};
struct test2
{
    char x2;
    int x1;
    short x3;
    long long x4;
} __attribute__((aligned(4)));
struct test3
{
    char x2;
    int x1;
    short x3;
    long long x4;
} __attribute__((aligned(8)));
struct test4
{
    char x2;
    int x1;
    short x3;
    long long x4;
} __attribute__((aligned(16)));

void main()
{
    printf("%lu \n", sizeof(struct test));
    printf("%lu \n", sizeof(struct test1));
    printf("%lu \n", sizeof(struct test2));
    printf("%lu \n", sizeof(struct test3));
    printf("%lu \n", sizeof(struct test4));
}
     1          .file   "alignment_pragma_pack.c"
     2          .text
     3          .section        .rodata
     4  .LC0:
     5          .string "%lu \n"
     6          .text
     7          .globl  main
     8          .type   main, @function
     9  main:
    10  .LFB0:
    11          .cfi_startproc
    12          endbr64
    13          pushq   %rbp
    14          .cfi_def_cfa_offset 16
    15          .cfi_offset 6, -16
    16          movq    %rsp, %rbp
    17          .cfi_def_cfa_register 6
    18          movl    $15, %esi
    19          leaq    .LC0(%rip), %rax
    20          movq    %rax, %rdi
    21          movl    $0, %eax
    22          call    printf@PLT
    23          movl    $24, %esi
    24          leaq    .LC0(%rip), %rax
    25          movq    %rax, %rdi
    26          movl    $0, %eax
    27          call    printf@PLT
    28          movl    $24, %esi
    29          leaq    .LC0(%rip), %rax
    30          movq    %rax, %rdi
    31          movl    $0, %eax
    32          call    printf@PLT
    33          movl    $24, %esi
    34          leaq    .LC0(%rip), %rax
    35          movq    %rax, %rdi
    36          movl    $0, %eax
    37          call    printf@PLT
    38          movl    $32, %esi
    39          leaq    .LC0(%rip), %rax
    40          movq    %rax, %rdi
    41          movl    $0, %eax
    42          call    printf@PLT
    43          nop
    44          popq    %rbp
    45          .cfi_def_cfa 7, 8
    46          ret
    47          .cfi_endproc
    48  .LFE0:
    49          .size   main, .-main
    50          .ident  "GCC: (Ubuntu 11.2.0-19ubuntu1) 11.2.0"
    51          .section        .note.GNU-stack,"",@progbits
    52          .section        .note.gnu.property,"a"
    53          .align 8
    54          .long   1f - 0f
    55          .long   4f - 1f
    56          .long   5
    57  0:
    58          .string "GNU"
    59  1:
    60          .align 8
    61          .long   0xc0000002
    62          .long   3f - 2f
    63  2:
    64          .long   0x3
    65  3:
    66          .align 8
    67  4:
15
24
24
24
32
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 142
model name      : Intel(R) Core(TM) i3-10110U CPU @ 2.10GHz
stepping        : 12
cpu MHz         : 2592.000
cache size      : 4096 KB
physical id     : 0
siblings        : 1
core id         : 0
cpu cores       : 1
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 22
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx rdtscp lm constant_tsc rep_good nopl xtopology nonstop_tsc cpuid tsc_known_freq pni pclmulqdq monitor ssse3 cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx rdrand hypervisor lahf_lm abm 3dnowprefetch invpcid_single fsgsbase avx2 invpcid rdseed clflushopt md_clear flush_l1d arch_capabilities
bugs            : spectre_v1 spectre_v2 spec_store_bypass swapgs itlb_multihit srbds mmio_stale_data retbleed
bogomips        : 5184.00
clflush size    : 64
cache_alignment : 64
address sizes   : 39 bits physical, 48 bits virtual
power management: