在Linux程序中,经常会看到形如下面的结构体定义
struct xfrm_algo {
char alg_name[64];
unsigned int alg_key_len; /* in bits */
char alg_key[0];
};
这里,最奇怪的是结构体最后一个元素, 是一个零长度的字符数组
这里先解释一下结构体的作用。
xfrm_algo是一个定义密钥算法的结构体,alg_name存放算法名称,alg_key_len存放密钥长度(单位是bit),alg_key存放密钥. 因为同一个算法,有可能会使用不同长度的密钥。
如AES, 就有128位、192位和256位三种密钥。 所以,在定义这样一个密钥算法的结构体时,就要求不定长的结构体,而零长数组就可实现这一点。
当然,我们也可以使用指针来代替
struct xfrm_algo {
char alg_name[64];
unsigned int alg_key_len; /* in bits */
char * alg_key;
};
下面,分别用指针和零长数组实现不定长结构体。
方法1:定义一个xfrm_algo结构体变量,再为alg_key成员动态创建内存
这种情况下,实际的xfrm_algo结构体和密钥是分离的
- #include<stdio.h>
- #include<stdlib.h>
- #include<string.h>
- void print_hex( unsigned char *buf, int len);
- struct xfrm_algo
- {
- char alg_name[64];
- unsigned int alg_key_len;
- unsigned char * alg_key;
- };
- int main( void )
- {
- char alg[] = "AES";
- unsigned char key[] = { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, /
- 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f };
- struct xfrm_algo algo1;
- memcpy( algo1.alg_name, alg, sizeof(alg) );
- algo1.alg_key_len = sizeof(key) * 8;
- if( ( algo1.alg_key = (unsigned char *)malloc( sizeof(key) ) ) == NULL )
- {
- perror("malloc");
- return -1;
- }
- memcpy( algo1.alg_key, key, sizeof(key) );
- printf("sizeof(struct xfrm_algo) = %d/n", sizeof(struct xfrm_algo));
- printf("algo1: 0x%08x/n", &algo1);
- printf("/talg_name : 0x%08x(%s)/n", algo1.alg_name, algo1.alg_name);
- printf("/talg_key_len: 0x%08x(%d)/n", &algo1.alg_key_len, algo1.alg_key_len);
- printf("/talg_key : 0x%08x", algo1.alg_key);
- print_hex( algo1.alg_key, sizeof(key) );
- free(algo1.alg_key);
- return 0;
- }
- void print_hex( unsigned char *buf, int len)
- {
- int i = 0;
- printf("(");
- for( i = 0; i < len; i++ )
- {
- printf("0x%02x ", buf[i]);
- }
- printf(")/n");
- }
执行结果:
$ ./struct_pointer1
sizeof(struct xfrm_algo) = 72
algo1: 0xbff54108
alg_name : 0xbff54108(AES)
alg_key_len: 0xbff54148(128)
alg_key : 0x09b2f008(0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0x09 0x0a 0x0b 0x0c 0x0d 0x0e 0x0f )
从输出可观察到, alg_key_len与alg_key是分离的
方法2: 直接为xfrm_algo和已知的密钥动态创建内存
此时,xfrm_algo结构体和密钥是连续的。
- #include<stdio.h>
- #include<stdlib.h>
- #include<string.h>
- void print_hex( unsigned char *buf, int len);
- struct xfrm_algo
- {
- char alg_name[64];
- unsigned int alg_key_len;
- unsigned char * alg_key;
- };
- int main( void )
- {
- char alg[] = "AES";
- unsigned char key[] = { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, /
- 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f };
- struct xfrm_algo *palgo = NULL;
- if( ( palgo = (struct xfrm_algo * )malloc( sizeof(struct xfrm_algo) + sizeof(key) ) ) == NULL )
- {
- perror("malloc");
- return -1;
- }
- memcpy( palgo->alg_name, alg, sizeof(alg) );
- palgo->alg_key_len = sizeof(key) * 8;
- palgo->alg_key = (unsigned char *)( palgo + 1 );
- memcpy( palgo->alg_key, key, sizeof(key) );
- printf("sizeof(struct xfrm_algo) = %d/n", sizeof(struct xfrm_algo));
- printf("palgo: 0x%08x/n", palgo);
- printf("/talg_name : 0x%08x(%s)/n", palgo->alg_name, palgo->alg_name);
- printf("/talg_key_len: 0x%08x(%d)/n", &palgo->alg_key_len, palgo->alg_key_len);
- printf("/talg_key : 0x%08x", palgo->alg_key);
- print_hex( palgo->alg_key, sizeof(key) );
- free(palgo);
- return 0;
- }
- void print_hex( unsigned char *buf, int len)
- {
- int i = 0;
- printf("(");
- for( i = 0; i < len; i++ )
- {
- printf("0x%02x ", buf[i]);
- }
- printf(")/n");
- }
执行结果:
$ ./struct_pointer2
sizeof(struct xfrm_algo) = 72
palgo: 0x096bd008
alg_name : 0x096bd008(AES)
alg_key_len: 0x096bd048(128)
alg_key : 0x096bd050(0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0x09 0x0a 0x0b 0x0c 0x0d 0x0e 0x0f )
从输出可观察到, alg_key_len与alg_key是连续的。这里,alg_key似乎是多余的,因为我们总能使用(unsigned char *)( palgo + 1 )得到key的首地址。
方法3:零长度数组
在标准C语言中,是不允许零长度数组的。但 GNU C 允许。
- #include<stdio.h>
- #include<stdlib.h>
- #include<string.h>
- void print_hex( unsigned char *buf, int len);
- struct xfrm_algo
- {
- char alg_name[64];
- unsigned int alg_key_len;
- unsigned char alg_key[0];
- };
- int main( void )
- {
- char alg[] = "AES";
- unsigned char key[] = { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, /
- 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f };
- struct xfrm_algo *palgo = NULL;
- if( ( palgo = (struct xfrm_algo *)malloc( sizeof(struct xfrm_algo) + sizeof(key) ) ) == NULL )
- {
- perror("malloc");
- return -1;
- }
- memcpy( palgo->alg_name, alg, sizeof(alg) );
- palgo->alg_key_len = sizeof(key) * 8;
- memcpy( palgo->alg_key, key, sizeof(key) );
- printf("sizeof(struct xfrm_algo) = %d/n", sizeof(struct xfrm_algo));
- printf("palgo: 0x%08x/n", palgo);
- printf("/talg_name : 0x%08x(%s)/n", palgo->alg_name, palgo->alg_name);
- printf("/talg_key_len: 0x%08x(%d)/n", &(palgo->alg_key_len), palgo->alg_key_len);
- printf("/talg_key : 0x%08x", palgo->alg_key);
- print_hex( palgo->alg_key, palgo->alg_key_len / 8 );
- free(palgo);
- return 0;
- }
- void print_hex( unsigned char *buf, int len)
- {
- int i = 0;
- printf("(");
- for( i = 0; i < len; i++ )
- {
- printf("0x%02x ", buf[i]);
- }
- printf(")/n");
- }
执行结果:
$ ./struct_array
sizeof(struct xfrm_algo) = 68
palgo: 0x0980d008
alg_name : 0x0980d008(AES)
alg_key_len: 0x0980d048(128)
alg_key : 0x0980d04c(0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0x09 0x0a 0x0b 0x0c 0x0d 0x0e 0x0f )
xfrm_algo结构体大小为68个字节, alg_key[0]不占存储空间,和方法1、2相比,少了个unsigned char *指针,但可移植性不比前两种方法。
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