字符串函数是最问常用的库函数之一，本文整理了常用的字符串函数，其来源为互联网

函数名: stpcpy

功 能: 拷贝一个字符串到另一个
用 法: char *stpcpy(char *destin, char *source);
程序例:

#include <stdio.h>
#include <string.h>
int main(void) 
{
   char string[10];
   char *str1 = "abcdefghi";
   stpcpy(string, str1);
   printf("%sn", string);
   return 0;
} 

函数名: strcat

功 能: 字符串拼接函数
用 法: char *strcat(char *destin, char *source);
程序例:

#include <string.h>
#include <stdio.h>
int main(void)
{
   char destination[25];
   char *blank = " ", *c = "C++", *Borland = "Borland";
   strcpy(destination, Borland);
   strcat(destination, blank);
   strcat(destination, c);
   printf("%sn", destination);
   return 0;
}

函数名: strchr

功 能: 在一个串中查找给定字符的第一个匹配之处
用 法: char *strchr(char *str, char c);
程序例:

#include <string.h>
#include <stdio.h>
int main(void)
 {
    char string[15];
    char *ptr, c = 'r';
    strcpy(string, "This is a string");
    ptr = strchr(string, c);
    if (ptr)
       printf("The character %c is at position: %dn", c, ptr-string);
    else
       printf("The character was not foundn");
    return 0;
 }

函数名: strcmp

功 能: 串比较
用 法: int strcmp(char *str1, char *str2);
看Asic码，str1>str2，返回值 > 0；两串相等，返回0
程序例:

#include <string.h> 
#include <stdio.h> 
int main(void) 
 { 
    char *buf1 = "aaa", *buf2 = "bbb", *buf3 = "ccc"; 
    int ptr; 
    ptr = strcmp(buf2, buf1); 
    if (ptr > 0) 
       printf("buffer 2 is greater than buffer 1n"); 
    else 
       printf("buffer 2 is less than buffer 1n"); 
    ptr = strcmp(buf2, buf3); 
    if (ptr > 0) 
       printf("buffer 2 is greater than buffer 3n"); 
    else 
       printf("buffer 2 is less than buffer 3n"); 
    return 0; 
 } 

函数名: strncmpi

功 能: 将一个串中的一部分与另一个串比较, 不管大小写
用 法: int strncmpi(char *str1, char *str2, unsigned maxlen);
程序例:

#include <string.h> 
#include <stdio.h> 
int main(void) 
{ 
   char *buf1 = "BBB", *buf2 = "bbb"; 
   int ptr; 
   ptr = strcmpi(buf2, buf1); 
   if (ptr > 0) 
      printf("buffer 2 is greater than buffer 1n"); 
   if (ptr < 0) 
      printf("buffer 2 is less than buffer 1n"); 
   if (ptr == 0) 
      printf("buffer 2 equals buffer 1n"); 
   return 0; 
} 

函数名: strcpy

功 能: 串拷贝
用 法: char *strcpy(char *str1, char *str2);
程序例:

#include <stdio.h> 
#include <string.h> 
int main(void) 
{ 
    char string[10]; 
    char *str1 = "abcdefghi"; 
    strcpy(string, str1); 
    printf("%sn", string); 
    return 0; 
} 

函数名: strcspn

功 能: 在串中查找第一个给定字符集内容的段
用 法: int strcspn(char *str1, char *str2);
程序例:

#include <stdio.h> 
#include <string.h> 
#include <alloc.h> 
int main(void) 
{ 
	char *string1 = "1234567890"; 
	char *string2 = "747DC8"; 
	int length; 
	length = strcspn(string1, string2); 
	printf("Character where strings intersect is at position %dn", length); 
	return 0; 
} 

函数名: strdup

功 能: 将串拷贝到新建的位置处
用 法: char *strdup(char *str);
程序例:

#include <stdio.h> 
#include <string.h> 
#include <alloc.h> 
int main(void) 
{ 
	char *dup_str, *string = "abcde"; 
	dup_str = strdup(string); 
	printf("%sn", dup_str); 
	free(dup_str); 
	return 0; 
} 

函数名: stricmp

功 能: 以大小写不敏感方式比较两个串
用 法: int stricmp(char *str1, char *str2);
程序例:

#include <string.h> 
#include <stdio.h> 
int main(void) 
{ 
	char *buf1 = "BBB", *buf2 = "bbb"; 
	int ptr; 
	ptr = stricmp(buf2, buf1); 
	if (ptr > 0) 
	  printf("buffer 2 is greater than buffer 1n"); 
	if (ptr < 0) 
	  printf("buffer 2 is less than buffer 1n"); 
	if (ptr == 0) 
	  printf("buffer 2 equals buffer 1n"); 
	return 0; 
} 

函数名: strerror

功 能: 返回指向错误信息字符串的指针
用 法: char *strerror(int errnum);
程序例:

#include <stdio.h> 
#include <errno.h> 
int main(void) 
{ 
	char *buffer; 
	buffer = strerror(errno); 
	printf("Error: %sn", buffer); 
	return 0; 
} 

函数名: strcmpi

功 能: 将一个串与另一个比较, 不管大小写
用 法: int strcmpi(char *str1, char *str2);
程序例:

#include <string.h> 
#include <stdio.h> 
int main(void) 
{ 
	char *buf1 = "BBB", *buf2 = "bbb"; 
	int ptr; 
	ptr = strcmpi(buf2, buf1); 
	if (ptr > 0) 
	  printf("buffer 2 is greater than buffer 1n"); 
	if (ptr < 0) 
	  printf("buffer 2 is less than buffer 1n"); 
	if (ptr == 0) 
	  printf("buffer 2 equals buffer 1n"); 
	return 0; 
} 

函数名: strncmp

功 能: 串比较
用 法: int strncmp(char *str1, char *str2, int maxlen);
程序例:

#include <string.h> 
#include <stdio.h> 
int  main(void) 
{ 
	char *buf1 = "aaabbb", *buf2 = "bbbccc", *buf3 = "ccc"; 
	int ptr; 
	ptr = strncmp(buf2,buf1,3); 
	if (ptr > 0) 
	  printf("buffer 2 is greater than buffer 1n"); 
	else 
	  printf("buffer 2 is less than buffer 1n"); 
	ptr = strncmp(buf2,buf3,3); 
	if (ptr > 0) 
	  printf("buffer 2 is greater than buffer 3n"); 
	else 
	  printf("buffer 2 is less than buffer 3n"); 
	return(0); 
} 

函数名: strncmpi

功 能: 把串中的一部分与另一串中的一部分比较, 不管大小写
用 法: int strncmpi(char *str1, char *str2);
程序例:

#include <string.h> 
#include <stdio.h> 
int main(void) 
{ 
	char *buf1 = "BBBccc", *buf2 = "bbbccc"; 
	int ptr; 
	ptr = strncmpi(buf2,buf1,3); 
	if (ptr > 0) 
	  printf("buffer 2 is greater than buffer 1n"); 
	if (ptr < 0) 
	  printf("buffer 2 is less than buffer 1n"); 
	if (ptr == 0) 
	  printf("buffer 2 equals buffer 1n"); 
	return 0; 
} 

函数名: strncpy

功 能: 串拷贝
用 法: char *strncpy(char *destin, char *source, int maxlen);
程序例:

#include <stdio.h> 
#include <string.h> 
int main(void) 
{ 
	char string[10]; 
	char *str1 = "abcdefghi"; 
	strncpy(string, str1, 3); 
	string[3] = ''; 
	printf("%sn", string); 
	return 0; 
} 

函数名: strnicmp

功 能: 不注重大小写地比较两个串
用 法: int strnicmp(char *str1, char *str2, unsigned maxlen);
程序例:

#include <string.h> 
#include <stdio.h> 
int main(void) 
{ 
	char *buf1 = "BBBccc", *buf2 = "bbbccc"; 
	int ptr; 
	ptr = strnicmp(buf2, buf1, 3); 
	if (ptr > 0) 
	  printf("buffer 2 is greater than buffer 1n"); 
	if (ptr < 0) 
	  printf("buffer 2 is less than buffer 1n"); 
	if (ptr == 0) 
	  printf("buffer 2 equals buffer 1n"); 
	return 0; 
} 

函数名: strnset

功 能: 将一个串中的所有字符都设为指定字符
用 法: char *strnset(char *str, char ch, unsigned n);
程序例:

#include <stdio.h> 
#include <string.h> 
int main(void) 
{ 
	char *string = "abcdefghijklmnopqrstuvwxyz"; 
	char letter = 'x'; 
	printf("string before strnset: %sn", string); 
	strnset(string, letter, 13); 
	printf("string after  strnset: %sn", string); 
	return 0; 
} 

函数名: strpbrk

功 能: 在串中查找给定字符集中的字符
用 法: char *strpbrk(char *str1, char *str2);
程序例:

#include <stdio.h> 
#include <string.h> 
int main(void) 
{ 
	char *string1 = "abcdefghijklmnopqrstuvwxyz"; 
	char *string2 = "onm"; 
	char *ptr; 
	ptr = strpbrk(string1, string2); 
	if (ptr) 
	  printf("strpbrk found first character: %cn", *ptr); 
	else 
	  printf("strpbrk didn't find character in setn"); 
	return 0; 
} 

函数名: strrchr

功 能: 在串中查找指定字符的最后一个出现
用 法: char *strrchr(char *str, char c);
程序例:

#include <string.h> 
#include <stdio.h> 
int main(void) 
{ 
	char string[15]; 
	char *ptr, c = 'r'; 
	strcpy(string, "This is a string"); 
	ptr = strrchr(string, c); 
	if (ptr) 
	  printf("The character %c is at position: %dn", c, ptr-string); 
	else 
	  printf("The character was not foundn"); 
	return 0; 
} 

函数名: strrev

功 能: 串倒转
用 法: char *strrev(char *str);
程序例:

#include <string.h> 
#include <stdio.h> 
int main(void) 
{ 
	char *forward = "string"; 
	printf("Before strrev(): %sn", forward); 
	strrev(forward); 
	printf("After strrev():  %sn", forward); 
	return 0; 
} 

函数名: strset

功 能: 将一个串中的所有字符都设为指定字符
用 法: char *strset(char *str, char c);
程序例:

#include <stdio.h> 
#include <string.h> 
int main(void) 
{ 
	char string[10] = "123456789"; 
	char symbol = 'c'; 
	printf("Before strset(): %sn", string); 
	strset(string, symbol); 
	printf("After strset():  %sn", string); 
	return 0; 
} 

函数名: strspn

功 能: 在串中查找指定字符集的子集的第一次出现
用 法: int strspn(char *str1, char *str2);
程序例:

#include <stdio.h> 
#include <string.h> 
#include <alloc.h> 
int main(void) 
{ 
	char *string1 = "1234567890"; 
	char *string2 = "123DC8"; 
	int length; 
	length = strspn(string1, string2); 
	printf("Character where strings differ is at position %dn", length); 
	return 0; 
} 

函数名: strstr

功 能: 在串中查找指定字符串的第一次出现
用 法: char *strstr(char *str1, char *str2);
程序例:

#include <stdio.h> 
#include <string.h> 
int main(void) 
{ 
	char *str1 = "Borland International", *str2 = "nation", *ptr; 
	ptr = strstr(str1, str2); 
	printf("The substring is: %sn", ptr); 
	return 0; 
} 

函数名: strtod

功 能: 将字符串转换为double型值
用 法: double strtod(char *str, char **endptr);
程序例:

#include <stdio.h> 
#include <stdlib.h> 
int main(void) 
{ 
	char input[80], *endptr; 
	double value; 
	printf("Enter a floating point number:"); 
	gets(input); 
	value = strtod(input, &endptr); 
	printf("The string is %s the number is %lfn", input, value); 
	return 0; 
} 

函数名: strtok

功 能: 查找由在第二个串中指定的分界符分隔开的单词
用 法: char *strtok(char *str1, char *str2);
程序例:

#include <string.h> 
#include <stdio.h> 
int main(void) 
{ 
	char input[16] = "abc,d"; 
	char *p; 
	/* strtok places a NULL terminator 
	in front of the token, if found */ 
	p = strtok(input, ","); 
	if (p)   printf("%sn", p); 
	/* A second call to strtok using a NULL 
	as the first parameter returns a pointer 
	to the character following the token  */ 
	p = strtok(NULL, ","); 
	if (p)   printf("%sn", p); 
	return 0; 
} 

函数名: strtol

功 能: 将串转换为长整数
用 法: long strtol(char *str, char **endptr, int base);
程序例:

#include <stdlib.h> 
#include <stdio.h> 
int main(void) 
{ 
	char *string = "87654321", *endptr; 
	long lnumber; 
	/* strtol converts string to long integer  */ 
	lnumber = strtol(string, &endptr, 10); 
	printf("string = %s  long = %ldn", string, lnumber); 
	return 0; 
} 

函数名: strupr

功 能: 将串中的小写字母转换为大写字母
用 法: char *strupr(char *str);
程序例:

#include <stdio.h> 
#include <string.h> 
int main(void) 
{ 
	char *string = "abcdefghijklmnopqrstuvwxyz", *ptr; 
	/* converts string to upper case characters */ 
	ptr = strupr(string); 
	printf("%sn", ptr); 
	return 0; 
} 

函数名: swab

功 能: 交换字节
用 法: void swab (char *from, char *to, int nbytes);
程序例:

#include <stdlib.h> 
#include <stdio.h> 
#include <string.h> 
char source[15] = "rFna koBlrna d"; 
char target[15]; 
int main(void) 
{ 
	swab(source, target, strlen(source)); 
	printf("This is target: %sn", target); 
	return 0; 
}

isalpha()

PS:isalpha()是字符函数，不是字符串函数
原型：extern int isalpha(int c);
用法：#include <ctype.h>
功能：判断字符c是否为英文字母
说明：当c为英文字母a-z或A-Z时，返回非零值，否则返回零。
举例：

#include <syslib.h>
#include <ctype.h>
#include <stdio.h>

main()
{
	int c;
	clrscr();        // clear screen
	printf("Press a key");
	for(;;)
	{
	    c=getchar();
	    clrscr();
	    printf("%c: %s letter",c,isalpha(c)?"is":"not");
	}
	return 0; // just to avoid warnings by compiler
}