memcmp
- Compare two blocks of memory.
- Compares the first num bytes of the block of memory pointed by ptr1 to the first num bytes pointed by ptr2, returning zero if they all match or a value different from zero representing which is greater if they do not.
Notice that, unlike strcmp, the function does not stop comparing after finding a null character.
- 把儲存區 lhs 和儲存區 的前 count 個位元組進行比較。
- 比較 lhs 和 rhs 所指向物件的首 count 個位元組。比較按字典序進行。
- 結果的符號是在被比較物件中相異的首對位元組的值（都轉譯成 unsigned char ）的差。
- 若在 lhs 和 rhs 所指向的任一物件結尾後出現訪問，則行為未定義。若 lhs 或 rhs 為空指標則行為未定義。
int memcmp( const void* lhs, const void* rhs, size_t count );Parameters
lhs
- Pointer to block of memory.
- 指向記憶體塊的指標。
rhs
- Pointer to block of memory.
- 指向記憶體塊的指標。
count
- Number of bytes to compare.
- 要被比較的位元組數。
Return Value
-
Returns an integral value indicating the relationship between the content of the memory blocks:
- < 0 the first byte that does not match in both memory blocks has a lower value in lhs than in rhs (if evaluated as unsigned char values)
- 如果返回值 < 0，則表示 lhs 小於 rhs
- 0 the contents of both memory blocks are equal
- 如果返回值 > 0，則表示 lhs 小於 rhs
- > 0 the first byte that does not match in both memory blocks has a greater value in lhs than in rhs (if evaluated as unsigned char values)
- 如果返回值 = 0，則表示 lhs 等於 rhs
Example
//
// Created by zhangrongxiang on 2018/2/8 16:57
// File memcmp
//
#include <stdio.h>
#include <string.h>
//C 庫函式 int memcmp(const void *str1, const void *str2, size_t n)) 把儲存區 str1 和儲存區 str2 的前 n 個位元組進行比較。
//比較按字典序進行。
typedef struct {
char *name;
} Stu;
int main() {
char *str = "abc";
char *str2 = "abC";
int cmp = memcmp(str, str2, strlen(str) > strlen(str2) ? strlen(str) : strlen(str2));
printf("%d
", cmp); // windows mingw -> 1 linux gcc 32
int i = 0;
int i2 = 10;
cmp = memcmp(&i, &i2, sizeof(int));
printf("%d
", cmp);//windows mingw -> -1 linux gcc -10
printf("%d
", (int) sizeof(short));//2
Stu stu = {.name = "zing"};
Stu stu2 = {.name = "zing"};
Stu stu3 = {.name = "Zing"};
Stu stu4 = {.name = "aing"};
/***********************************************/
printf("%d
", (int) sizeof(Stu)); //8
printf("%d
", (int) sizeof(stu.name)); //8
printf("%d
", (int) strlen(stu.name)); //4
printf("%d
", (int) sizeof(char *)); //8
printf("%d
", (int) sizeof(int *)); //8
printf("%d
", (int) sizeof(long *)); //8
/***********************************************/
cmp = memcmp(&stu, &stu2, sizeof(Stu));
printf("struct %d
", cmp);//0
cmp = memcmp(stu.name, stu3.name, sizeof(Stu));
printf("struct %d
", cmp); // windows mingw -> -1 linux gcc 32
cmp = memcmp(stu.name, stu4.name, sizeof(Stu));
printf("struct %d
", cmp); // windows mingw -> -1 linux gcc 25
cmp = memcmp(&"abC", &"abc", sizeof(char *));
printf("%d
", cmp);//-32
printf("a -> %d;A -> %d
", `a`, `A`);//a -> 97;A -> 65
printf("z -> %d;a -> %d
", `z`, `a`);//a -> 97;A -> 65
return 0;
}