size_t strlen ( const char * str );
'\0'
作为结束标志,strlen函数返回的是在字符串中 ‘\0’ 前面出现的字符个数(不包 含’\0’` )'\0'
结束。size_t
,是无符号的( 易错 )注:
#include <stdio.h> int main() { const char* str1 = "abcdef"; const char* str2 = "bbb"; if (strlen(str2) - strlen(str1) > 0) { printf("str2>str1\n"); } else { printf("srt1>str2\n"); } return 0; }
输出结果:
char* strcpy(char * destination, const char* source);
'\0'
结束'\0'
拷贝到目标空间char * strcat ( char * destination, const char * source );
int strcmp ( const char * str1, const char * str2 );
0
的数字0
0
的数字ASCII
码值char * strncpy ( char * destination, const char * source, size_t num );
num
个字符从源字符串到目标空间。num
,则拷贝完源字符串之后,在目标的后边追加0
,直到num
个char * strncat ( char * destination, const char * source, size_t num );
strncpy
对strcpy
的改进一样,strncat
也是对strcat
的改进,而且可以自我追加strncpy
无异#include <stdio.h> #include <string.h> int main() { char str1[20]; char str2[20]; strcpy(str1, "To be "); strcpy(str2, "or not to be"); strncat(str1, str2, 6); puts(str1); return 0; }
输出结果:
int strncmp ( const char * str1, const char * str2, size_t num );
num
个字符全部比较完strcmp
的改进#include <stdio.h> #include <string.h> int main() { char str[][5] = { "R2D2" , "C3PO" , "R2A6" }; int n; puts("Looking for R2 astromech droids..."); for (n = 0; n < 3; n++) if (strncmp(str[n], "R2xx", 2) == 0) { printf("found %s\n", str[n]); } return 0; }
输出结果:
char * strstr ( const char *str1, const char * str2);
str1
寻找与str2
相等的子串,然后把该子串的起始位置返回#include <stdio.h> #include <string.h> int main() { char str[] = "This is a simple string"; char* pch; pch = strstr(str, "simple"); puts(pch); return 0; }
char * strtok ( char * str, const char * sep );
#include <stdio.h> #include <string.h> int main() { char str[] = "- This, a sample string."; char* pch; printf("Splitting string \"%s\" into tokens:\n", str); pch = strtok(str, " ,.-"); while (pch != NULL) { printf("%s\n", pch); pch = strtok(NULL, " ,.-"); } return 0; }
输出结果:
#include <stdio.h> #include <string.h> #include <errno.h>//必须包含的头文件 int main() { FILE* pFile; pFile = fopen("unexist.ent", "r"); if (pFile == NULL) printf("Error opening file unexist.ent: %s\n", strerror(errno)); //errno: Last error number return 0; }
输出结果:
字符转换:
int tolower ( int c ); int toupper ( int c );
#include <stdio.h> #include <ctype.h> int main() { int i = 0; char str[] = "Test String.\n"; char c; while (str[i]) { c = str[i]; if (isupper(c)) c = tolower(c); putchar(c); i++; } return 0; }
输出结果:
void * memcpy ( void * destination, const void * source, size_t num );
memcpy
从source
的位置开始向后复制num
个字节的数据到destination
的内存位置。'\0'
的时候并不会停下来。source
和destination
有任何的重叠,复制的结果都是未定义的#include <stdio.h> #include <string.h> struct { char name[40]; int age; } person, person_copy; int main() { char myname[] = "Pierre de Fermat"; /* using memcpy to copy string: */ memcpy(person.name, myname, strlen(myname) + 1); person.age = 46; /* using memcpy to copy structure: */ memcpy(&person_copy, &person, sizeof(person)); printf("person_copy: %s, %d \n", person_copy.name, person_copy.age); return 0; }
输出结果:
void * memmove ( void * destination, const void * source, size_t num );
memcpy
的差别就是memmove
函数处理的源内存块和目标内存块是可以重叠的memmove
函数处理#include <stdio.h> #include <string.h> int main() { char str[] = "memmove can be very useful......"; memmove(str + 20, str + 15, 11); puts(str); return 0; }
输出结果:
int memcmp ( const void * ptr1, const void * ptr2, size_t num );
#include <stdio.h> #include <string.h> int main() { char buffer1[] = "DWgaOtP12df0"; char buffer2[] = "DWGAOTP12DF0"; int n; n = memcmp(buffer1, buffer2, sizeof(buffer1)); if (n > 0) printf("'%s' is greater than '%s'.\n", buffer1, buffer2); else if (n < 0) printf("'%s' is less than '%s'.\n", buffer1, buffer2); else printf("'%s' is the same as '%s'.\n", buffer1, buffer2); return 0; }
输出结果:
三种方式:
方式1:
//计数器方式 int my_strlen(const char* str) { int count = 0; while (*str) { count++; str++; } return count; }
方式2:
//不能创建临时变量计数器 int my_strlen(const char * str) { if(*str == '\0') return 0; else return 1+my_strlen(str+1); }
方式3:
//指针-指针的方式 int my_strlen(char *s) { char *p = s; while(*p != ‘\0' ) p++; return p-s; }
参考代码:
//1.参数顺序 //2.函数的功能,停止条件 //3.assert //4.const修饰指针 //5.函数返回值 //6.题目出自《高质量C/C++编程》书籍最后的试题部分 char* my_strcpy(char* dest, const char* src) { char* ret = dest; assert(dest != NULL); assert(src != NULL); while ((*dest++ = *src++)) { ; } return ret; }
参考代码:
char* my_strcat(char* dest, const char* src) { char* ret = dest; assert(dest != NULL); assert(src != NULL); while (*dest) { dest++; } while ((*dest++ = *src++)) { ; } return ret; }
参考代码:
char* strstr(const char* str1, const char* str2) { char* cp = (char*)str1; char* s1, * s2; if (!*str2) return((char*)str1); while (*cp) { s1 = cp; s2 = (char*)str2; while (*s1 && *s2 && !(*s1 - *s2)) s1++, s2++; if (!*s2) return(cp); cp++; } return(NULL); }
参考代码:
int my_strcmp(const char* src, const char* dst) { int ret = 0; assert(src != NULL); assert(dest != NULL); while (!(ret = *(unsigned char*)src - *(unsigned char*)dst) && *dst) ++src, ++dst; if (ret < 0) ret = -1; else if (ret > 0) ret = 1; return(ret); }
参考代码:
void* memcpy(void* dst, const void* src, size_t count) { void* ret = dst; assert(dst); assert(src); /* * copy from lower addresses to higher addresses */ while (count--) { *(char*)dst = *(char*)src; dst = (char*)dst + 1; src = (char*)src + 1; } return(ret); }
参考代码:
void* memmove(void* dst, const void* src, size_t count) { void* ret = dst; if (dst <= src || (char*)dst >= ((char*)src + count)) { /* * Non-Overlapping Buffers * copy from lower addresses to higher addresses */ while (count--) { *(char*)dst = *(char*)src; dst = (char*)dst + 1; src = (char*)src + 1; } } else { /* * Overlapping Buffers * copy from higher addresses to lower addresses */ dst = (char*)dst + count - 1; src = (char*)src + count - 1; while (count--) { *(char*)dst = *(char*)src; dst = (char*)dst - 1; src = (char*)src - 1; } } return(ret); }