内存对齐和char d[0], char *d_数组d[0]-优快云博客

本文链接：https://blog.youkuaiyun.com/hwizhao/article/details/5608472

本文通过四个不同结构体的实例，详细探讨了在C语言中结构体成员的内存对齐方式及其对结构体大小的影响。展示了如何使用 offsetof 宏来获取成员偏移，并通过实际运行结果对比了默认对齐与指定对齐参数（-fpack-struct=2）的情况。

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#include <stdio.h> #include <stdlib.h> int main() { struct aa{ int a; int b; char c; char d[0]; }; #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) // #define offsetof(TYPE,MEMBER) __compiler_offsetof(TYPE,MEMBER) struct bb{ int a; int b; char c; }; struct cc{ int a; int b; char c; char d[21]; }; struct dd{ int a; int b; char c; char *d; }; int i; int len; struct aa *pA; struct cc objC; struct dd *pD; printf("-------------struct aa------------------/n"); pA = (struct aa*)malloc(sizeof(struct aa) + 20); pA->c = 'c'; len = sizeof(struct aa) + 20 - offsetof(struct aa, d); for(i=0; i< len; i++) pA->d[i] = 'd' + i; printf("size of aa is %d, offset of d=%d, offst of c=%d, c=%c/n", sizeof(struct aa), offsetof(struct aa, d), offsetof(struct aa, c), *((char*)pA + offsetof(struct aa, c))); printf("pA->d[].len===%d/n/t", len); for(i=0; i<len ; i++){ printf("d[%2d]=%c, ", i, *((char*)pA + offsetof(struct aa, d) + i)); if((i+1)%4 == 0) printf("/n/t"); } printf("/n"); printf("-------------struct bb------------------/n"); printf("size of bb is %d/n", sizeof(struct bb)); printf("-------------struct cc------------------/n"); objC.c = 'c'; len = sizeof(struct cc) - offsetof(struct cc, d); for(i=0; i< len; i++) objC.d[i] = 'd' + i; printf("size of cc is %d, offset of d=%d, offst of c=%d, c=%c/n", sizeof(struct cc), offsetof(struct cc, d), offsetof(struct cc, c), *((char*)&objC + offsetof(struct cc, c))); printf("objC.d[]===%d/n/t", len); for(i=0; i<len ; i++){ printf("d[%2d]=%c, ", i, *((char*)&objC + offsetof(struct cc, d) + i)); if((i+1)%4 == 0) printf("/n/t"); } printf("/n"); printf("-------------struct dd------------------/n"); pD = (struct dd*)malloc(sizeof(struct dd) + 20); pD->c = 'c'; printf("size of dd is %d, offset of d=%d, offst of c=%d, c=%c/n", sizeof(struct dd), offsetof(struct dd, d), offsetof(struct dd, c), *((char*)pD + offsetof(struct dd, c))); pD->d = (char*)pD + offsetof(struct dd, d) + sizeof(char*); len = sizeof(struct dd) + 20 - offsetof(struct dd, d) - sizeof(char *); for(i=0; i< len; i++) pD->d[i] = 'd' + i; printf("pD->d[].len===%d/n/t", len); for(i=0; i<len ; i++){ printf("d[%2d]=%c, ", i, *((char*)pD + offsetof(struct dd, d) + sizeof(char*) + i)); if((i+1)%4 == 0) printf("/n/t"); } printf("/n"); };

[root@joy-desktop align-test]#gcc -o align_test align_test.c
[root@joy-desktop align-test]#./align_test
-------------struct aa------------------
size of aa is 12, offset of d=9, offst of c=8, c=c
pA->d[].len===23
d[ 0]=d, d[ 1]=e, d[ 2]=f, d[ 3]=g,
d[ 4]=h, d[ 5]=i, d[ 6]=j, d[ 7]=k,
d[ 8]=l, d[ 9]=m, d[10]=n, d[11]=o,
d[12]=p, d[13]=q, d[14]=r, d[15]=s,
d[16]=t, d[17]=u, d[18]=v, d[19]=w,
d[20]=x, d[21]=y, d[22]=z,
-------------struct bb------------------
size of bb is 12
-------------struct cc------------------
size of cc is 32, offset of d=9, offst of c=8, c=c
objC.d[]===23
d[ 0]=d, d[ 1]=e, d[ 2]=f, d[ 3]=g,
d[ 4]=h, d[ 5]=i, d[ 6]=j, d[ 7]=k,
d[ 8]=l, d[ 9]=m, d[10]=n, d[11]=o,
d[12]=p, d[13]=q, d[14]=r, d[15]=s,
d[16]=t, d[17]=u, d[18]=v, d[19]=w,
d[20]=x, d[21]=y, d[22]=z,
-------------struct dd------------------
size of dd is 16, offset of d=12, offst of c=8, c=c
pD->d[].len===20
d[ 0]=d, d[ 1]=e, d[ 2]=f, d[ 3]=g,
d[ 4]=h, d[ 5]=i, d[ 6]=j, d[ 7]=k,
d[ 8]=l, d[ 9]=m, d[10]=n, d[11]=o,
d[12]=p, d[13]=q, d[14]=r, d[15]=s,
d[16]=t, d[17]=u, d[18]=v, d[19]=w,

[root@joy-desktop align-test]#gcc -o align_test align_test.c -fpack-struct=2
[root@joy-desktop align-test]#./align_test
-------------struct aa------------------
size of aa is 10, offset of d=9, offst of c=8, c=c
pA->d[].len===21
d[ 0]=d, d[ 1]=e, d[ 2]=f, d[ 3]=g,
d[ 4]=h, d[ 5]=i, d[ 6]=j, d[ 7]=k,
d[ 8]=l, d[ 9]=m, d[10]=n, d[11]=o,
d[12]=p, d[13]=q, d[14]=r, d[15]=s,
d[16]=t, d[17]=u, d[18]=v, d[19]=w,
d[20]=x,
-------------struct bb------------------
size of bb is 10
-------------struct cc------------------
size of cc is 30, offset of d=9, offst of c=8, c=c
objC.d[]===21
d[ 0]=d, d[ 1]=e, d[ 2]=f, d[ 3]=g,
d[ 4]=h, d[ 5]=i, d[ 6]=j, d[ 7]=k,
d[ 8]=l, d[ 9]=m, d[10]=n, d[11]=o,
d[12]=p, d[13]=q, d[14]=r, d[15]=s,
d[16]=t, d[17]=u, d[18]=v, d[19]=w,
d[20]=x,
-------------struct dd------------------
size of dd is 14, offset of d=10, offst of c=8, c=c
pD->d[].len===20
d[ 0]=d, d[ 1]=e, d[ 2]=f, d[ 3]=g,
d[ 4]=h, d[ 5]=i, d[ 6]=j, d[ 7]=k,
d[ 8]=l, d[ 9]=m, d[10]=n, d[11]=o,
d[12]=p, d[13]=q, d[14]=r, d[15]=s,
d[16]=t, d[17]=u, d[18]=v, d[19]=w,