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2.3 errno.h

2.2 ctype.h

The ctype header is used for testing and converting characters. A control character refers to a character that is not part of the normal printing set. In the ASCII character set, the control characters are the characters from 0 (NUL) through 0x1F (US), and the character 0x7F (DEL). Printable characters are those from 0x20 (space) to 0x7E (tilde).

Functions:

isalnum();
isalpha();
iscntrl();
isdigit();
isgraph();
islower();
isprint();
ispunct();
isspace();
isupper();
isxdigit();
tolower();
toupper();

2.2.1 is... Functions

Declarations:

int isalnum(int character);
int isalpha(int character);
int iscntrl(int character);
int isdigit(int character);
int isgraph(int character);
int islower(int character);
int isprint(int character);
int ispunct(int character);
int isspace(int character);
int isupper(int character);
int isxdigit(int character);
The is... functions test the given character and return a nonzero (true) result if it satisfies the following conditions. If not, then 0 (false) is returned.

Conditions:
isalnum a letter (A to Z or a to z) or a digit (0 to 9)
isalpha a letter (A to Z or a to z)
iscntrl any control character (0x00 to 0x1F or 0x7F)
isdigit a digit (0 to 9)
isgraph any printing character except for the space character (0x21 to 0x7E)
islower a lowercase letter (a to z)
isprint any printing character (0x20 to 0x7E)
ispunct any punctuation character (any printing character except for space character or isalnum)
isspace a whitespace character (space, tab, carriage return, new line, vertical tab, or formfeed)
isupper an uppercase letter (A to Z)
isxdigit a hexadecimal digit (0 to 9, A to F, or a to f)

2.2.2 to... Functions

Declarations:

int tolower(int character);
int toupper(int character);
The to... functions provide a means to convert a single character. If the character matches the appropriate condition, then it is converted. Otherwise the character is returned unchanged.

Conditions:
tolower If the character is an uppercase character (A to Z), then it is converted to lowercase (a to z)
toupper If the character is a lowercase character (a to z), then it is converted to uppercase (A to Z)
Example: 

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

int main(void)
{
  int loop;
  char string[]="THIS IS A TEST";

  for(loop=0;loop<strlen(string);loop++)
    string[loop]=tolower(string[loop]);

  printf("%s\n",string);
  return 0;
}
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2.3 errno.h