C语言 strncpy 和 memcpy 的区别？

Difference between strncpy and memcpy?

提问by user559208

How can i access s[7]in s?

我如何才能获得s[7]的s？

I didn't observe any difference between strncpyand memcpy. If I want to print the output s, along with s[7](like qwertyA), what are the changes I have to made in the following code:

我没有观察到strncpy和之间的任何区别memcpy。如果我想打印输出s以及s[7]（如qwertyA），我必须在以下代码中进行哪些更改：

#include <stdio.h>
#include <stdlib.h>
int main()
{
    char s[10] = "qwerty", str[10], str1[10];
    s[7] = 'A';
    printf("%s\n",s);
    strncpy(str,s,8);
    printf("%s\n",str);
    memcpy(str1,s,8);
    printf("%s\n",str1);
    return 0;
}
/*
O/P
qwerty
qwerty
qwerty
*/

回答by Philip Potter

Others have pointed out your null-termination problems. You need to understand null-termination before you understand the difference between memcpyand strncpy.

其他人已经指出了您的空终止问题。你需要了解空终止你明白之间的差别之前memcpy和strncpy。

The difference is that memcpywill copy all N characters you ask for, while strncpywill copy up to the first null terminator inclusive, or N characters, whichever is fewer. (If it copies less than N characters, it will pad the rest out with null characters.)

不同之处在于memcpy将复制您要求的所有 N 个字符，而strncpy将复制到第一个空终止符或 N 个字符，以较少者为准。（如果它复制的字符少于 N 个，它将用空字符填充其余的字符。）

回答by moinudin

You are getting the output quertybecause the index 7is incorrect (arrays are indexed from 0, not 1). There is a null-terminatorat index 6to signal the end of the string, and whatever comes after it will have no effect.

您之所以得到输出，querty是因为索引7不正确（数组从 0 开始索引，而不是从 1 开始）。index 处有一个空终止符6来表示字符串的结束，它后面的任何内容都无效。

Two things you need to fix:

你需要解决的两件事：

1. Change the 7in s[7]to 6
2. Add a null-terminator at s[7]

1. 更改7在s[7]以6
2. 在处添加一个空终止符 s[7]

The result will be:

结果将是：

char s[10] = "qwerty";
s[6] = 'A';
s[7] = 0;

Original not workingand fixed working.

原来不工作和固定工作。

As for the question of strncpyversus memcpy, the difference is that strncpyadds a null-terminator for you. BUT, only if the source string has one before n. So strncpyis what you want to use here, but be very careful of the big BUT.

至于strncpyvs的问题memcpy，区别在于strncpy为您添加了一个空终止符。但是，仅当源字符串在n. 这strncpy就是你想在这里使用的东西，但要非常小心大 BUT。

回答by GOBI

Strncpy will copy up to NULLeven you specified the number of bytes to copy , but memcpy will copy up to specified number of bytes .

NULL即使您指定要复制的字节数，Strncpy 也将复制，但 memcpy 将复制至指定的字节数。

printf statement will print up to NULL , so you will try to print a single charater , it will show ,

printf 语句最多打印 NULL ，因此您将尝试打印单个字符，它将显示，

printf("\t%c %c %c\t",s[7],str[7],str1[7]);

OUTPUT

输出

  7              7

回答by Matt Ellen

To make "qwertyA" you need to set s[6] = 'A'and s[7]='\0'

要制作“qwertyA”，您需要设置s[6] = 'A'和s[7]='\0'

Strings are indexed from 0, so s[0] == 'q', and they need to be null terminated.

字符串从 0 开始索引，所以s[0] == 'q'，它们需要以空字符结尾。

回答by nos

When you have:

当你有：

char s[10] = "qwerty";

this is what that array contains:

这是该数组包含的内容：

s[0]  'q'
s[1]  'w'
s[2]  'e'
s[3]  'r'
s[4]  't'
s[5]  'y'
s[6]   0
s[7]   0
s[8]   0
s[9]   0

If you want to add an 'A' to the end of your string, that's at index 6, since array indexes start at 0

如果你想在你的字符串末尾添加一个 'A'，那就是在索引 6 处，因为数组索引从 0 开始

 s[6] = 'A';

Note that when you initialize an array this way, the remaining space is set to 0 (a nul terminator), While not needed in this case, generally be aware that you need to make your strings nul terminated. e.g.

请注意，当您以这种方式初始化数组时，剩余空间将设置为 0（一个 nul 终止符），虽然在这种情况下不需要，但通常要注意您需要使您的字符串以 nul 终止。例如

char s[10];
strcpy(s,"qwerty");
s[6] = 'A';
s[7] = 0;

In the above example "qwerty" , including its nul terminator is copied to s. s[6] overwrites that nul terminator. Since the rest of sis not initialized we need to add a nul terminator ourselves with s[7] = 0;

在上面的例子 "qwerty" 中，包括它的 nul 终止符被复制到s. s[6] 覆盖了那个 nul 终止符。由于其余的s未初始化，我们需要自己添加一个 nul 终止符s[7] = 0;