c memcpy 同内存

The memcpy() function in C/C++ is used to copy data from one memory location to another. This is a common way of copying data using pointers.

C / C ++中的memcpy （）函数用于将数据从一个存储位置复制到另一个存储位置。 这是使用指针复制数据的常用方法。

Let’s understand how we can use this function in this article.

让我们了解如何在本文中使用此功能。

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句法 (Syntax)

The memcpy() function takes in two memory address locations (src and dst) as arguments, along with the number of bytes (n) to be copied.

memcpy （）函数接受两个内存地址位置（ src和dst ）作为参数，以及要复制的字节数（ n ）。

Since C/C++ typically uses a void* pointer to denote memory location addresses, the invocation for the source and destination addresses are void* src and void* dst.

由于C / C ++通常使用void *指针表示内存位置地址，因此对源地址和目标地址的调用为void* src和void* dst 。

The function returns a pointer to the destination memory address dst, but in practice, we do not usually capture the return value. (Since we already have the pointer with us)

该函数返回一个指向目标内存地址dst的指针，但实际上，我们通常不捕获返回值。 （因为我们已经有了指针）

Format:

格式 ：

#include 
    
     void* memcpy(void* dst, const void* src, size_t n)
    

The header file <string.h> is necessary to load this library function.

头文件<string.h>是加载此库函数所必需的。

Arguments:

参数 ：

• dst -> Pointer to the destination address
  dst >指向目标地址的指针
• src -> Pointer to the source address
  src >指向源地址的指针
• n -> Number of bytes to be copied
  n >要复制的字节数

Return Value

返回值

This returns the pointer to the destination location (dst).

这会将指针返回到目标位置（ dst ）。

NOTE: To avoid any kind of overflow, the size of the arrays pointed to by both the destination and source arguments must be at least n bytes and cannot overlap.

注意 ：为避免任何类型的溢出， 目标和源参数所指向的数组大小必须至少为n个字节，并且不能重叠。

Let’s understand the function using some examples.

让我们使用一些示例来了解该函数。

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一些例子 (Some Examples)

Here is an example which copies a string from one location to another using memcpy().

这是一个使用memcpy()将字符串从一个位置复制到另一个位置的示例。

#include 
    
     #include 
     
      int main () {    // This is constant, since memcpy() will not modify this   const char src[50] = "JournalDev";   char dest[50];   strcpy(dest, "Sample");   printf("Before memcpy(), dest: %s\n", dest);   // It is strlen(src) + 1 since we also copy the null terminator '\0'   memcpy(dest, src, strlen(src)+1);   printf("After memcpy(), dest: %s\n", dest);      return(0);}
     
    

Output

输出量

Before memcpy(), dest: SampleAfter memcpy(), dest: JournalDev

Here is another example which appends a string using memcpy() using appropriate offsets.

这是另一个示例，该示例使用memcpy()使用适当的偏移量附加字符串。

#include 
    
     #include 
     
      int main() {    // This is constant, since memcpy() will not modify this    const char src[100] = "This is a JournalDev article.";    char dst[100] = "memcpy";        printf("Before memcpy(), dst is: %s\n", dst);        // Set offsets for both src and dst    size_t offset_src = 0, offset_dst = 0;    offset_src += 4;    offset_dst += strlen(dst);    memcpy(dst + offset_dst, src + offset_src, strlen(src));    printf("After memcpy(), dst is: %s\n", dst);    return 0;}
     
    

The output will be the destination string appended with ” is a JournalDev article“.

输出将是目标字符串，后面附加“” 是JournalDev文章 ”。

Output

输出量

Before memcpy(), dst is: memcpyAfter memcpy(), dst is: memcpy is a JournalDev article.

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一个简单的memcpy（）实现 (A Simple memcpy() Implementation)

Here is a simple implementation of memcpy() in C/C++ which tries to replicate some of the mechanisms of the function.

这是C / C ++中memcpy()一个简单实现，它试图复制该函数的某些机制。

We first typecast src and dst to char* pointers, since we cannot de-reference a void* pointer. void* pointers are only used to transfer data across functions, threads, but not access them.

我们首先将src和dst类型转换为char*指针，因为我们无法取消引用void*指针。 void*指针仅用于在函数，线程之间传输数据，而不能访问它们。

Now we can directly copy the data byte by byte and return the void* destination address.

现在，我们可以逐字节直接复制数据并返回void*目标地址。

void* memcpy_usr(void* dst, const void* src, size_t n) {    // Copies n bytes from src to dst        // Since we cannot dereference a void* ptr,    // we first typecast it to a char* ptr    // and then do the copying byte by byte,    // since a char* ptr references a single byte    char* char_dst = (char*) dst;    char* char_src = (char*) src;    for (int i=0; i

You can verify that by replacing memcpy() with our new function, we will get the same output.

您可以通过用新函数替换memcpy()来验证是否将获得相同的输出。

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结论 (Conclusion)

In this article, we learned about the memcpy() library function in C/C++, which is useful to copy data from a source address to a destination address.

在本文中，我们了解了C / C ++中的memcpy()库函数，该函数对于将数据从源地址复制到目标地址很有用。

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参考资料 (References)

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c memcpy 同内存