List of articles

Depth analysis ：strcat & strncat
Depth analysis ：strchr & strstr
Depth analysis ：strcmp & strncmp
Depth analysis ：strcpy & strncpy
Depth analysis ：strlen & strtok
Depth analysis ：memcpy & memmove
Depth analysis ：memset & memcmp

Preface

Library function string It is also very important code that we often use , I will deeply analyze the usage and implementation principle of commonly used functions

One 、memcpy

1. Call structure

void *memcpy(void *dest, const void *src, size_t n);

2. The use of,

memcpy： from src Copy count Byte to dest

The principle, :

1. Memory copies one byte by one
2. Copy the value of each byte stored in memory
3. It is no longer limited to the copy of strings

3. Source analysis

void *Memcpy(void *dest, const void *src, size_t count) {
    
// Memory copy : from  src  Copy  count  Byte to  dest
    assert(dest && src);
    char *start = dest;                    // Record the address of the starting position 
    while (count--) {
                          // Number of bytes copied 
        *(char *) dest = *(char *) src;    // Copy byte by byte 
        dest = (char *) dest + 1;
        src = (char *) src + 1;
    }
    return start;                          // Return the address of the starting position 
}

Two 、memmove

1. Call structure

void *memmove (void *destination, const void *source, size_t num);

2. The use of,

memmove： from src Copy count Byte to dest

The principle, :

1. And memcpy The difference is that memmove The source and target memory blocks processed by the function can overlap
2. If the source space and the target space overlap , You have to use memmove Function processing

3. Source analysis

void *Memmove(void *dest, void *src, int count) {
    
// Memory copy : from  src  Copy  count  Byte to  dest
    assert(dest && src);                     // Prevent passing null pointers 
    char *start = dest;
    if (dest < src) {
                            // front -> after 
        while (count--) {
    
            *(char *) dest = *(char *) src;
            dest = (char *) dest + 1;
            src = (char *) src + 1;
        }
    } else {
                                     // after -> front 
        while (count--) {
    
            *((char *) dest + count) = *((char *) src + count);
        }
    }
    return start;
}