C语言中对字符串进行操作的标准库函数有哪些
C\u8bed\u8a00\u4e2d\u5bf9\u5b57\u7b26\u4e32\u8fdb\u884c\u64cd\u4f5c\u7684\u6807\u51c6\u5e93\u51fd\u6570\u6709\u54ea\u4e9b1\uff09\u5b57\u7b26\u4e32\u64cd\u4f5c
strcpy(p, p1) \u590d\u5236\u5b57\u7b26\u4e32
strncpy(p, p1, n) \u590d\u5236\u6307\u5b9a\u957f\u5ea6\u5b57\u7b26\u4e32
strcat(p, p1) \u9644\u52a0\u5b57\u7b26\u4e32
strncat(p, p1, n) \u9644\u52a0\u6307\u5b9a\u957f\u5ea6\u5b57\u7b26\u4e32
strlen(p) \u53d6\u5b57\u7b26\u4e32\u957f\u5ea6
strcmp(p, p1) \u6bd4\u8f83\u5b57\u7b26\u4e32
strcasecmp\u5ffd\u7565\u5927\u5c0f\u5199\u6bd4\u8f83\u5b57\u7b26\u4e32
strncmp(p, p1, n) \u6bd4\u8f83\u6307\u5b9a\u957f\u5ea6\u5b57\u7b26\u4e32
strchr(p, c) \u5728\u5b57\u7b26\u4e32\u4e2d\u67e5\u627e\u6307\u5b9a\u5b57\u7b26
strrchr(p, c) \u5728\u5b57\u7b26\u4e32\u4e2d\u53cd\u5411\u67e5\u627e
strstr(p, p1) \u67e5\u627e\u5b57\u7b26\u4e32
strpbrk(p, p1) \u4ee5\u76ee\u6807\u5b57\u7b26\u4e32\u7684\u6240\u6709\u5b57\u7b26\u4f5c\u4e3a\u96c6\u5408\uff0c\u5728\u5f53\u524d\u5b57\u7b26\u4e32\u67e5\u627e\u8be5\u96c6\u5408\u7684\u4efb\u4e00\u5143\u7d20
strspn(p, p1) \u4ee5\u76ee\u6807\u5b57\u7b26\u4e32\u7684\u6240\u6709\u5b57\u7b26\u4f5c\u4e3a\u96c6\u5408\uff0c\u5728\u5f53\u524d\u5b57\u7b26\u4e32\u67e5\u627e\u4e0d\u5c5e\u4e8e\u8be5\u96c6\u5408\u7684\u4efb\u4e00\u5143\u7d20\u7684\u504f\u79fb
strcspn(p, p1) \u4ee5\u76ee\u6807\u5b57\u7b26\u4e32\u7684\u6240\u6709\u5b57\u7b26\u4f5c\u4e3a\u96c6\u5408\uff0c\u5728\u5f53\u524d\u5b57\u7b26\u4e32\u67e5\u627e\u5c5e\u4e8e\u8be5\u96c6\u5408\u7684\u4efb\u4e00\u5143\u7d20\u7684\u504f\u79fb
* \u5177\u6709\u6307\u5b9a\u957f\u5ea6\u7684\u5b57\u7b26\u4e32\u5904\u7406\u51fd\u6570\u5728\u5df2\u5904\u7406\u7684\u5b57\u7b26\u4e32\u4e4b\u540e\u586b\u8865\u96f6\u7ed3\u5c3e\u7b26
2\uff09\u5b57\u7b26\u4e32\u5230\u6570\u503c\u7c7b\u578b\u7684\u8f6c\u6362
strtod(p, ppend) \u4ece\u5b57\u7b26\u4e32 p \u4e2d\u8f6c\u6362 double \u7c7b\u578b\u6570\u503c\uff0c\u5e76\u5c06\u540e\u7eed\u7684\u5b57\u7b26\u4e32\u6307\u9488\u5b58\u50a8\u5230 ppend \u6307\u5411\u7684 char* \u7c7b\u578b\u5b58\u50a8\u3002
strtol(p, ppend, base) \u4ece\u5b57\u7b26\u4e32 p \u4e2d\u8f6c\u6362 long \u7c7b\u578b\u6574\u578b\u6570\u503c\uff0cbase \u663e\u5f0f\u8bbe\u7f6e\u8f6c\u6362\u7684\u6574\u578b\u8fdb\u5236\uff0c\u8bbe\u7f6e\u4e3a 0 \u4ee5\u6839\u636e\u7279\u5b9a\u683c\u5f0f\u5224\u65ad\u6240\u7528\u8fdb\u5236\uff0c0x, 0X \u524d\u7f00\u4ee5\u89e3\u91ca\u4e3a\u5341\u516d\u8fdb\u5236\u683c\u5f0f\u6574\u578b\uff0c0 \u524d\u7f00\u4ee5\u89e3\u91ca\u4e3a\u516b\u8fdb\u5236\u683c\u5f0f\u6574\u578b
atoi(p) \u5b57\u7b26\u4e32\u8f6c\u6362\u5230 int \u6574\u578b
atof(p) \u5b57\u7b26\u4e32\u8f6c\u6362\u5230 double \u7b26\u70b9\u6570
atol(p) \u5b57\u7b26\u4e32\u8f6c\u6362\u5230 long \u6574\u578b
3\uff09\u5b57\u7b26\u68c0\u67e5
isalpha() \u68c0\u67e5\u662f\u5426\u4e3a\u5b57\u6bcd\u5b57\u7b26
isupper() \u68c0\u67e5\u662f\u5426\u4e3a\u5927\u5199\u5b57\u6bcd\u5b57\u7b26
islower() \u68c0\u67e5\u662f\u5426\u4e3a\u5c0f\u5199\u5b57\u6bcd\u5b57\u7b26
isdigit() \u68c0\u67e5\u662f\u5426\u4e3a\u6570\u5b57
isxdigit() \u68c0\u67e5\u662f\u5426\u4e3a\u5341\u516d\u8fdb\u5236\u6570\u5b57\u8868\u793a\u7684\u6709\u6548\u5b57\u7b26
isspace() \u68c0\u67e5\u662f\u5426\u4e3a\u7a7a\u683c\u7c7b\u578b\u5b57\u7b26
iscntrl() \u68c0\u67e5\u662f\u5426\u4e3a\u63a7\u5236\u5b57\u7b26
ispunct() \u68c0\u67e5\u662f\u5426\u4e3a\u6807\u70b9\u7b26\u53f7
isalnum() \u68c0\u67e5\u662f\u5426\u4e3a\u5b57\u6bcd\u548c\u6570\u5b57
isprint() \u68c0\u67e5\u662f\u5426\u662f\u53ef\u6253\u5370\u5b57\u7b26
isgraph() \u68c0\u67e5\u662f\u5426\u662f\u56fe\u5f62\u5b57\u7b26\uff0c\u7b49\u6548\u4e8e isalnum() | ispunct()
\u51fd\u6570\u540d: strrchr
\u529f \u80fd: \u5728\u4e32\u4e2d\u67e5\u627e\u6307\u5b9a\u5b57\u7b26\u7684\u6700\u540e\u4e00\u4e2a\u51fa\u73b0
\u7528 \u6cd5: char *strrchr(char *str, char c);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
char fullname="./lib/lib1.so";
char *ptr;
ptr = strrchr(fullname,'/');
printf("filename is %s",++ptr);
//\u8fd0\u884c\u7ed3\u679c\uff1afilename is lib1.so
\u51fd\u6570\u540d: strchr
\u529f \u80fd: \u5728\u4e32\u4e2d\u67e5\u627e\u6307\u5b9a\u5b57\u7b26\u7684\u7b2c\u4e00\u4e2a\u51fa\u73b0
\u7528 \u6cd5: char *strchr(char *str, char c);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
char fullname="./lib/lib1.so";
char *ptr;
ptr = strrchr(fullname,'.');
printf("after strchr() is %s",++ptr);
//\u8fd0\u884c\u7ed3\u679c\uff1aafter strchr() is /lib/lib1.so
\u51fd\u6570\u540d: strtok
\u529f \u80fd: \u5728\u4e32\u4e2d\u67e5\u627e\u6307\u5b9a\u5b57\u7b26\u7684\u7b2c\u4e00\u4e2a\u51fa\u73b0
\u7528 \u6cd5: char *strtok(char *s, char *delim);
\u8bf4\u660e\uff1a
1.strtok\u51fd\u6570\u7684\u5b9e\u8d28\u4e0a\u7684\u5904\u7406\u662f\uff0cstrtok\u5728s\u4e2d\u67e5\u627e\u5305\u542b\u5728delim\u4e2d\u7684\u5b57\u7b26\u5e76\u7528NULL(\u2019/0\u2032)\u6765\u66ff\u6362,\u76f4\u5230\u627e\u904d\u6574\u4e2a\u5b57\u7b26\u4e32\u3002\u8fd9\u53e5\u8bdd\u6709\u4e24\u5c42\u542b\u4e49\uff1a\uff081\uff09\u6bcf\u6b21\u8c03\u7528strtok\u51fd\u6570\u53ea\u80fd\u83b7\u5f97\u4e00\u4e2a\u5206\u5272\u5355\u4f4d\u3002\uff082\uff09\u8981\u83b7\u5f97\u6240\u6709\u7684\u5206\u5272\u5355\u5143\u5fc5\u987b\u53cd\u590d\u8c03\u7528strtok\u51fd\u6570\u3002
2.strtok\u51fd\u6570\u4ee5\u540e\u7684\u8c03\u7528\u65f6\u7684\u9700\u7528NULL\u6765\u66ff\u6362s.
3.\u5f62\u53c2s(\u8981\u5206\u5272\u7684\u5b57\u7b26\u4e32)\u5bf9\u5e94\u7684\u53d8\u91cf\u5e94\u7528char s[]=\u201d\u2026.\u201d\u5f62\u5f0f\uff0c\u800c\u4e0d\u80fd\u7528char *s=\u201d\u2026.\u201d\u5f62\u5f0f\u3002
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
void main()
{
char buf[]=\u201dGolden Global View\u201d;
char* token = strtok( buf, \u201d \u201c);
while( token != NULL )
{
printf( \u201d%s \u201c, token );
token = strtok( NULL, \u201d \u201c);
}
return 0;
}
/*\u5176\u7ed3\u679c\u4e3a\uff1a
Golden
Global
View
*/
\u51fd\u6570\u540d\uff1astrncpy
\u529f\u80fd\uff1a\u628asrc\u6240\u6307\u7531NULL\u7ed3\u675f\u7684\u5b57\u7b26\u4e32\u7684\u524dn\u4e2a\u5b57\u8282\u590d\u5236\u5230dest\u6240\u6307\u7684\u6570\u7ec4\u4e2d
\u7528\u6cd5\uff1achar *strncpy(char *dest, char *src, int n);
\u8bf4\u660e\uff1a
\u5982\u679csrc\u7684\u524dn\u4e2a\u5b57\u8282\u4e0d\u542bNULL\u5b57\u7b26\uff0c\u5219\u7ed3\u679c\u4e0d\u4f1a\u4ee5NULL\u5b57\u7b26\u7ed3\u675f\u3002
\u5982\u679csrc\u7684\u957f\u5ea6\u5c0f\u4e8en\u4e2a\u5b57\u8282\uff0c\u5219\u4ee5NULL\u586b\u5145dest\u76f4\u5230\u590d\u5236\u5b8cn\u4e2a\u5b57\u8282\u3002
src\u548cdest\u6240\u6307\u5185\u5b58\u533a\u57df\u4e0d\u53ef\u4ee5\u91cd\u53e0\u4e14dest\u5fc5\u987b\u6709\u8db3\u591f\u7684\u7a7a\u95f4\u6765\u5bb9\u7eb3src\u7684\u5b57\u7b26\u4e32\u3002
\u8fd4\u56de\u6307\u5411dest\u7684\u6307\u9488\u3002
\u4e3e\u4f8b\uff1a
[c-sharp] view plain copy
#include
#include
main()
{
char buf[4];
char *s="abcdefg";
strncpy(buf,s,4);
printf("%s/n",buf);
return 0;
}
/*\u8fd0\u884c\u7ed3\u679c\uff1a
abcd
*/
\u51fd\u6570\u540d: stpcpy
\u529f \u80fd: \u62f7\u8d1d\u4e00\u4e2a\u5b57\u7b26\u4e32\u5230\u53e6\u4e00\u4e2a
\u7528 \u6cd5: char *stpcpy(char *destin, char *source);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char string[10];
char *str1 = "abcdefghi";
stpcpy(string, str1);
printf("%s/n", string);
return 0;
}
/*\u8fd0\u884c\u7ed3\u679c
abcdefghi
*/
\u51fd\u6570\u540d: strcat
\u529f \u80fd: \u5b57\u7b26\u4e32\u62fc\u63a5\u51fd\u6570
\u7528 \u6cd5: char *strcat(char *destin, char *source);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char destination[25];
char *blank = " ", *c = "C++", *Borland = "Borland";
strcpy(destination, Borland);
strcat(destination, blank);
strcat(destination, c);
printf("%s/n", destination);
return 0;
}
/*\u8fd0\u884c\u7ed3\u679c\uff1a
Borland C++
*/
\u51fd\u6570\u540d: strcmp
\u529f \u80fd: \u4e32\u6bd4\u8f83
\u7528 \u6cd5: int strcmp(char *str1, char *str2);
\u770bAsic\u7801\uff0cstr1>str2\uff0c\u8fd4\u56de\u503c > 0\uff1b\u4e24\u4e32\u76f8\u7b49\uff0c\u8fd4\u56de0
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char *buf1 = "aaa", *buf2 = "bbb"\uff1b
int ptr;
ptr = strcmp(buf2, buf1);
if (ptr > 0)
printf("buffer 2 is greater than buffer 1/n");
else if(ptr < 0)
printf("buffer 2 is less than buffer 1/n");
else
printf("buffer 2 is equal with buffer 1/n");
return 0;
}
/*\u8fd0\u884c\u7ed3\u679c\uff1a
buffer 2 is greater than buffer 1
*/
\u51fd\u6570\u540d: strncmpi
\u529f \u80fd: \u5c06\u4e00\u4e2a\u4e32\u4e2d\u7684\u4e00\u90e8\u5206\u4e0e\u53e6\u4e00\u4e2a\u4e32\u6bd4\u8f83, \u4e0d\u7ba1\u5927\u5c0f\u5199
\u7528 \u6cd5: int strncmpi(char *str1, char *str2, unsigned maxlen);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char *buf1 = "BBB", *buf2 = "bbb";
int ptr;
ptr = strcmpi(buf2, buf1);
if (ptr > 0)
printf("buffer 2 is greater than buffer 1/n");
if (ptr < 0)
printf("buffer 2 is less than buffer 1/n");
if (ptr == 0)
printf("buffer 2 equals buffer 1/n");
return 0;
}
\u51fd\u6570\u540d: strcspn
\u529f \u80fd: \u5728\u4e32\u4e2d\u67e5\u627e\u7b2c\u4e00\u4e2a\u7ed9\u5b9a\u5b57\u7b26\u96c6\u5185\u5bb9\u7684\u6bb5
\u7528 \u6cd5: int strcspn(char *str1, char *str2);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
#include
int main(void)
{
char *string1 = "1234567890";
char *string2 = "747DC8";
int length;
length = strcspn(string1, string2);
printf("Character where strings intersect is at position %d/n", length);
return 0;
}
\u51fd\u6570\u540d: strdup
\u529f \u80fd: \u5c06\u4e32\u62f7\u8d1d\u5230\u65b0\u5efa\u7684\u4f4d\u7f6e\u5904
\u7528 \u6cd5: char *strdup(char *str);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
#include
int main(void)
{
char *dup_str, *string = "abcde";
dup_str = strdup(string);
printf("%s/n", dup_str);
free(dup_str);
return 0;
}
\u51fd\u6570\u540d: stricmp
\u529f \u80fd: \u4ee5\u5927\u5c0f\u5199\u4e0d\u654f\u611f\u65b9\u5f0f\u6bd4\u8f83\u4e24\u4e2a\u4e32
\u7528 \u6cd5: int stricmp(char *str1, char *str2);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char *buf1 = "BBB", *buf2 = "bbb";
int ptr;
ptr = stricmp(buf2, buf1);
if (ptr > 0)
printf("buffer 2 is greater than buffer 1/n");
if (ptr < 0)
printf("buffer 2 is less than buffer 1/n");
if (ptr == 0)
printf("buffer 2 equals buffer 1/n");
return 0;
}
\u51fd\u6570\u540d: strerror
\u529f \u80fd: \u8fd4\u56de\u6307\u5411\u9519\u8bef\u4fe1\u606f\u5b57\u7b26\u4e32\u7684\u6307\u9488
\u7528 \u6cd5: char *strerror(int errnum);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char *buffer;
buffer = strerror(errno);
printf("Error: %s/n", buffer);
return 0;
}
\u51fd\u6570\u540d: strncmp
\u529f \u80fd: \u4e32\u6bd4\u8f83
\u7528 \u6cd5: int strncmp(char *str1, char *str2, int maxlen);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
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 1/n");
else
printf("buffer 2 is less than buffer 1/n");
ptr = strncmp(buf2,buf3,3);
if (ptr > 0)
printf("buffer 2 is greater than buffer 3/n");
else
printf("buffer 2 is less than buffer 3/n");
return(0);
}
\u51fd\u6570\u540d: strncmpi
\u529f \u80fd: \u628a\u4e32\u4e2d\u7684\u4e00\u90e8\u5206\u4e0e\u53e6\u4e00\u4e32\u4e2d\u7684\u4e00\u90e8\u5206\u6bd4\u8f83, \u4e0d\u7ba1\u5927\u5c0f\u5199
\u7528 \u6cd5: int strncmpi(char *str1, char *str2, int len);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
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 1/n");
if (ptr < 0)
printf("buffer 2 is less than buffer 1/n");
if (ptr == 0)
printf("buffer 2 equals buffer 1/n");
return 0;
}
\u51fd\u6570\u540d: strnset
\u529f \u80fd: \u5c06\u4e00\u4e2a\u4e32\u4e2d\u7684\u6240\u6709\u5b57\u7b26\u90fd\u8bbe\u4e3a\u6307\u5b9a\u5b57\u7b26
\u7528 \u6cd5: char *strnset(char *str, char ch, unsigned n);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char *string = "abcdefghijklmnopqrstuvwxyz";
char letter = 'x';
printf("string before strnset: %s/n", string);
strnset(string, letter, 13);
printf("string after strnset: %s/n", string);
return 0;
}
\u51fd\u6570\u540d: strpbrk
\u529f \u80fd: \u5728\u4e32\u4e2d\u67e5\u627e\u7ed9\u5b9a\u5b57\u7b26\u96c6\u4e2d\u7684\u5b57\u7b26
\u7528 \u6cd5: char *strpbrk(char *str1, char *str2);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char *string1 = "abcdefghijklmnopqrstuvwxyz";
char *string2 = "onm";
char *ptr;
ptr = strpbrk(string1, string2);
if (ptr)
printf("strpbrk found first character: %c/n", *ptr);
else
printf("strpbrk didn't find character in set/n");
return 0;
}
\u51fd\u6570\u540d: strrev
\u529f \u80fd: \u4e32\u5012\u8f6c
\u7528 \u6cd5: char *strrev(char *str);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char *forward = "string";
printf("Before strrev(): %s/n", forward);
strrev(forward);
printf("After strrev(): %s/n", forward);
return 0;
}
/*\u8fd0\u884c\u7ed3\u679c\uff1a
Before strrev(): string
After strrev(): gnirts
*/
\u51fd\u6570\u540d: strstr
\u529f \u80fd: \u5728\u4e32\u4e2d\u67e5\u627e\u6307\u5b9a\u5b57\u7b26\u4e32\u7684\u7b2c\u4e00\u6b21\u51fa\u73b0
\u7528 \u6cd5: char *strstr(char *str1, char *str2);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char *str1 = "Borland International", *str2 = "nation", *ptr;
ptr = strstr(str1, str2);
printf("The substring is: %s/n", ptr);
return 0;
}
\u51fd\u6570\u540d: strtod
\u529f \u80fd: \u5c06\u5b57\u7b26\u4e32\u8f6c\u6362\u4e3adouble\u578b\u503c
\u7528 \u6cd5: double strtod(char *str, char **endptr);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
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 %lf/n", input, value);
return 0;
}
\u51fd\u6570\u540d: strtol
\u529f \u80fd: \u5c06\u4e32\u8f6c\u6362\u4e3a\u957f\u6574\u6570
\u7528 \u6cd5: long strtol(char *str, char **endptr, int base);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char *string = "87654321", *endptr;
long lnumber;
/* strtol converts string to long integer */
lnumber = strtol(string, &endptr, 10);
printf("string = %s long = %ld/n", string, lnumber);
return 0;
}
\u51fd\u6570\u540d: strupr
\u529f \u80fd: \u5c06\u4e32\u4e2d\u7684\u5c0f\u5199\u5b57\u6bcd\u8f6c\u6362\u4e3a\u5927\u5199\u5b57\u6bcd
\u7528 \u6cd5: char *strupr(char *str);
\u4e3e\u4f8b\uff1a
[cpp] view plain copy
#include
#include
int main(void)
{
char *string = "abcdefghijklmnopqrstuvwxyz", *ptr;
/* converts string to upper case characters */
ptr = strupr(string);
printf("%s/n", ptr);
return 0;
}
strcpy(p, p1) 复制字符串
strncpy(p, p1, n) 复制指定长度字符串
strcat(p, p1) 附加字符串
strncat(p, p1, n) 附加指定长度字符串
strlen(p) 取字符串长度
strcmp(p, p1) 比较字符串
strcasecmp忽略大小写比较字符串
strncmp(p, p1, n) 比较指定长度字符串
strchr(p, c) 在字符串中查找指定字符
strrchr(p, c) 在字符串中反向查找
strstr(p, p1) 查找字符串
strpbrk(p, p1) 以目标字符串的所有字符作为集合，在当前字符串查找该集合的任一元素
strspn(p, p1) 以目标字符串的所有字符作为集合，在当前字符串查找不属于该集合的任一元素的偏移
strcspn(p, p1) 以目标字符串的所有字符作为集合，在当前字符串查找属于该集合的任一元素的偏移
* 具有指定长度的字符串处理函数在已处理的字符串之后填补零结尾符
2）字符串到数值类型的转换
strtod(p, ppend) 从字符串 p 中转换 double 类型数值，并将后续的字符串指针存储到 ppend 指向的 char* 类型存储。
strtol(p, ppend, base) 从字符串 p 中转换 long 类型整型数值，base 显式设置转换的整型进制，设置为 0 以根据特定格式判断所用进制，0x, 0X 前缀以解释为十六进制格式整型，0 前缀以解释为八进制格式整型
atoi(p) 字符串转换到 int 整型
atof(p) 字符串转换到 double 符点数
atol(p) 字符串转换到 long 整型
3）字符检查
isalpha() 检查是否为字母字符
isupper() 检查是否为大写字母字符
islower() 检查是否为小写字母字符
isdigit() 检查是否为数字
isxdigit() 检查是否为十六进制数字表示的有效字符
isspace() 检查是否为空格类型字符
iscntrl() 检查是否为控制字符
ispunct() 检查是否为标点符号
isalnum() 检查是否为字母和数字
isprint() 检查是否是可打印字符
isgraph() 检查是否是图形字符，等效于 isalnum() | ispunct()
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