Identify memory functions memset, memcmp, memmove, and memcpy

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1, memset is an initialization function, the function is to put a certain block Memory is all set to the specified value.

char a[5];memset(a,'a',5);puts(a);

• a points to the block of memory to fill.
• 'a' is the value to be set.
• 5 is the number of characters to be set to this value.

Run result:

Notes:

The memset function initializes the memory block according to bytes, so it cannot be used to initialize the int array to a value other than 0 and -1 (unless the value is high byte and lowbytes are the same).
In fact, the actual range of c should be 0~255, because the memset function can only take the last eight bits of c for each byte of the input range.That is to say no matter how big c is, only the last eight binary digits are valid.

The memset function is very convenient for initialization processing, but it also has its limitations, such as pay attention to the initialization value, pay attention to the number of bytes and so on.Of course, it is also possible to directly choose to use for loop or while loop to initialize, but memset is faster.

2, memcmp compares the first count bytes of the memory areas buf1 and buf2. This function compares by bytes.

Basic prototype: int memcmp(const void *buf1, const void *buf2, unsigned int count);

Main function: Compare the first count bytes of memory areas buf1 and buf2.

Return value:

When buf1

When buf1=buf2, return value=0

When buf1>buf2, return value>0

//a[0] 10 00 00 00//a[1] 20 00 00 00int a[2]={10,20};int b[2]={10,11};//a[0] 10 00 00 00//a[1] 11 00 00 00int ret=-1;ret=memcmp(a,b,5);//Memory comparison function, the same is 0, the difference is 1 (byte!!!)printf("ret=%d",ret);

Running result: ret=1

Note: This function compares by bytes.

Example:

When s1,s2 are strings, memcmp(s1,s2,1) is to compare the ascII code value of the first byte of s1 and s2;

memcmp(s1,s2,n) is to compare the ascII code values ​​of the first n bytes of s1 and s2, such as:

char *s1="abc";

char *s2="acd";

int r=memcmp(s1,s2,3);

It is to compare the first 3 bytes of s1 and s2, the first byte is equal, the size of the second byte comparison has been determined, and there is no need to continue to compare the third byte, so r=-1.

3, memmove memory copy function

memmove memory copy function, the function is to copy n bytes to the target address, the target memory and the source address memory can overlap.

Another memory copy function, memcpy, cannot overlap memory.
memmove is not a safe function, so pay attention to the generation of out-of-bounds when copying.An array of length n, using memmove to move backward one bit, can copy at most n-2 bytes.

 int a[10]={11,12,13,14,15,16,17,18,19,20};memmove(a,a+5,12);puts(a);

Run result:

4, memcpy memory copy function

Principle:

memcpy function is a function in C language forMemory copy function, declared in string.h.Its prototype is:

void test_01(void)//Test string, but memcpy copy is still copied one character by one{char str01[]="hello,ni,hao";char str02[20];memcpy(str02,str01,12);print_c(str02,12);}

Run result:

p[0]=h p[1]=e p[2]=l p[3]=l p[4]=o p[5]=, p[6]=n p[7]=i p[8]=, p[9]=h p[10]=a p[11]=o

The function is: take the address pointed to by source as the starting point, and copy consecutive n bytes of data to the address pointed to by destin as the starting point Memory.
The function has three parameters, the first is the destination address, the second is the source address, and the third is the data length.

When using the memcpy function, you need to pay attention:

• The unit of data length (the third parameter) is bytes (1byte = 8bit).
• Note that this function has a return value of type void*, which is a pointer to destin.

The length of the data copied by the memcpy function

When using the memcpy function, pay special attention to the data length.If the copied data type is char, then the data length is equal to the number of elements.And if the data type is other (such as int, double, custom structure, etc.), pay special attention to the value of the data length.
A good habit is to use n * sizeof(type_name) no matter what type of data is copied.