1.strlen函数与sizeof关键字
strlen功能：计算给定字符串的长度，不包括‘\0’
strlen是一个函数，参数是char *
sizeof是关键字，取字节运算符
sizeof功能：返回一个对象或者类型所占的内存字节数
str.length()和str.size() 是求string类型对象的成员函数
#include <cstring>
#include <iostream>
#include <stdio.h>
using namespace std;
#define test "test string";
int main() {
char *s = "test string"; //warning: deprecated conversion from string constant to ‘char*’
cout << "strlen(s)=" << strlen(s) << endl; //strlen(s)=11
cout << "sizeof(s)=" << sizeof(s) << endl; //sizeof(s)=8
cout << endl;
char mystr[100] = "test string";
cout << "strlen(mystr)=" << strlen(mystr) << endl; //strlen(mystr)=11
cout << "sizeof(mystr)=" << sizeof(mystr) << endl; //sizeof(mystr)=100
cout << endl;
char ss[] = "test string";
cout << "strlen(ss)=" << strlen(ss) << endl; //strlen(ss)=11
cout << "sizeof(ss)=" << sizeof(ss) << endl; //sizeof(ss)=12
cout << endl;
char *p = mystr;
cout << "strlen(p)=" << strlen(p) << endl; //strlen(p)=11
cout << "sizeof(p)=" << sizeof(p) << endl; //sizeof(p)=8
cout << endl;
string cur = "test string";
cout << "strlen(cur)=" << strlen(cur.c_str()) << endl; //strlen(cur)=11
cout << "cur.size()=" << cur.size() << endl; //cur.size()=11
cout << "cur.length()=" << cur.length() << endl; //cur.length()=11
cout << "sizeof(cur)=" << sizeof(cur) << endl; //sizeof(cur)=8
cout << endl;
string now = test;
cout << "strlen(now)=" << strlen(now.c_str()) << endl;//strlen(now)=11
cout << "now.size()=" << now.size() << endl;//now.size()=11
cout << "now.length()=" << now.length() << endl;//now.length()=11
cout << "sizeof(now)=" << sizeof(now) << endl;//sizeof(now)=8
cout << endl;
return 0;
}
运行结果：
-
strlen函数求得的字符串长度是从字符串第一个元素到第一个’\0’之间元素的个数(如果字符串中间有’\0’,则结果不是整个字符串的长度)，同时不包括该’\0’
-
sizeof求得的结果是存储该字符串的变量占用的空间大小，因而一定会包括’\0’.若’\0’后还有空余的空间，也会包含到结果里面
2.strcpy,strncpy, strncpy_s（字符串复制）
2.1 strcpy
char * strcpy ( char * destination, const char * source );
strcpy 是依据 “\0” 作为结束判断的，如果 destination 的空间不够，则会引起 buffer overflow。
char str1[100];
char *s = "test string";
cout << "strlen(s)=" << strlen(s) << endl; //strlen(s)=11
cout << "sizeof(s)=" << sizeof(s) << endl; //sizeof(s)=8
strcpy(str1, s);
cout << "strcpy str1:" << str1 << endl; //strcpy str1:test string
cout << "strlen(str1)=" << strlen(str1) << endl; //strlen(str1)=11
cout << "sizeof(str1)=" << sizeof(str1) << endl; //sizeof(str1)=100
cout << endl;
2.2 strncpy
char * strncpy ( char * destination, const char * source, size_t num );
strncpy要安全一些。
拷贝你指定的个数或者碰到’\0’，不验证源缓冲区长度，可能造成越界。
如果遇到空字符（’\0’），则空字符后面为原来的字符。
char str2[100];
char str3[10];
char mystr[100] = "test string";
cout << "strlen(mystr)=" << strlen(mystr) << endl;//strlen(mystr)=11
cout << "sizeof(mystr)=" << sizeof(mystr) << endl;//sizeof(mystr)=100
strncpy(str2, mystr, sizeof(str2));
cout << "strncpy str2:" << str2 << endl;//strncpy str2:test string
cout << "strlen(str2)=" << strlen(str2) << endl; //strlen(str2)=11
cout << "sizeof(str2)=" << sizeof(str2) << endl; //sizeof(str2)=100
strncpy(str3, mystr, 4);
str3[4] = '\0';
cout << "strncpy str3:" << str3 << endl; //strncpy str3:test
cout << "strlen(str3)=" << strlen(str3) << endl;//strlen(str3)=4
cout << "sizeof(str3)=" << sizeof(str3) << endl;//sizeof(str3)=10
cout << endl;
2.3 strncpy_s
编译选项
g++ main.cpp -std=c++11 /home/lzy/code/taurus-root/src/3rdparty/securec/lib/libsecurec.a
头文件：#include “securec.h”
strncpy_s 不以零填充目标数组。这是转换既存代码到边界检查版本的常见错误源。
| dest | - | 指向要复制到的字符数组的指针 |
|---|---|---|
| src | - | 指向复制来源的字符数组的指针 |
| count | - | 要复制的最大字符数 |
| destsz | - | 目标缓冲区的大小 |
errno_t strncpy_s(char *restrict dest, rsize_t destsz,
const char *restrict src, rsize_t count);
char str4[100];
char str5[10];
char ss[] = "test string";
cout << "strlen(ss)=" << strlen(ss) << endl; //strlen(ss)=11
cout << "sizeof(ss)=" << sizeof(ss) << endl; //sizeof(ss)=12
strncpy_s(str4, sizeof(str4), ss, sizeof(ss));
cout << "strncpy_s str4:" << str4 << endl; //strncpy_s str4:test string
cout << "strlen(str4)=" << strlen(str4) << endl; //strlen(str4)=11
cout << "sizeof(str4)=" << sizeof(str4) << endl; //sizeof(str4)=100
strncpy_s(str5, 10, ss, 4);
str5[4] = '\0';
cout << "strncpy_s str5:" << str5 << endl; //strncpy_s str5:test
cout << "strlen(str5)=" << strlen(str5) << endl; //strlen(str5)=4
cout << "sizeof(str5)=" << sizeof(str5) << endl; //sizeof(str5)=10
cout << endl;
3. memcpy与memcpy_s（缓冲区复制）
3.1 memcpy
memcpy:不理’\0’，只拷贝你指定的个数，故strcpy可以不指定字符串长度，实现整串copy，而memcpy必定要指定长度。
void * memcpy（void * dest，const void * src，size_t count）;
char str6[100];
char str7[4];
char *p = mystr;
cout << "strlen(p)=" << strlen(p) << endl; //strlen(p)=11
cout << "sizeof(p)=" << sizeof(p) << endl; //sizeof(p)=8
memcpy(str6, p, strlen(p) + 1);
cout << "memcpy str6:" << str6 << endl; //memcpy str6:test string
cout << "strlen(str6)=" << strlen(str6) << endl; //strlen(str6)=11
cout << "sizeof(str6)=" << sizeof(str6) << endl; //sizeof(str6)=100
memcpy(str7, p, 4);
cout << "memcpy str7:" << str7 << endl; //memcpy str7:test
cout << "strlen(str7)=" << strlen(str7) << endl; //strlen(str7)=4
cout << "sizeof(str7)=" << sizeof(str7) << endl; //sizeof(str7)=4
cout << endl;
3.2 memcpy_s
count 大于 destsz （会发生缓冲区溢出）
| dest | - | 指向要复制的对象的指针 |
|---|---|---|
| destsz | - | 目标中要修改的最大字节数（典型地为目标对象的大小） |
| src | - | 指向复制来源对象的指针 |
| count | - | 复制的字节数 |
errno_t memcpy_s( void *restrict dest, rsize_t destsz,
const void *restrict src, rsize_t count );
char str8[100];
char str9[4];
char str10[4];
string cur = "test string";
string cur2 = "ab";
cout << "strlen(cur)=" << strlen(cur.c_str()) << endl;//strlen(cur)=11
cout << "cur.size()=" << cur.size() << endl; //cur.size()=11
cout << "cur.length()=" << cur.length() << endl; //cur.length()=11
cout << "sizeof(cur)=" << sizeof(cur) << endl; //sizeof(cur)=8
memcpy_s(str8, sizeof(str8), cur.c_str(), strlen(cur.c_str()) + 1);
cout << "memcpy_s str8:" << str8 << endl; //memcpy_s str8:test string
cout << "strlen(str8)=" << strlen(str8) << endl; //strlen(str8)=11
cout << "sizeof(str8)=" << sizeof(str8) << endl; //sizeof(str8)=100
memcpy_s(str9, 4, cur.c_str(), 4);
cout << "memcpy_s str9:" << str9 << endl; //memcpy_s str9:test
cout << "strlen(str9)=" << strlen(str9) << endl; //strlen(str9)=4
cout << "sizeof(str9)=" << sizeof(str9) << endl; //sizeof(str9)=4
memcpy_s(str10, 4, cur.c_str(), 3);
cout << "memcpy_s str10:" << str10 << endl; //memcpy_s str10:tes
cout << "strlen(str10)=" << strlen(str10) << endl; //strlen(str10)=3
cout << "sizeof(str10)=" << sizeof(str10) << endl; //sizeof(str10)=4
cout << endl;