cppreference.com > Standard C String and Character atof Syntax:
      #include <stdlib.h>
      double atof( const char *str );
The function atof() converts str into a double, then returns that value. str must start with a valid number, but can be terminated with any non-numerical character, other than "E" or "e". For example,
       x = atof( "42.0is_the_answer" );
results in x being set to 42.0. Related topics: atoi atol (Standard C I/O) sprintf strtod

isxdigit
Syntax:

      #include <ctype.h>
      int isxdigit( int ch );

The function isxdigit() returns non-zero if its argument is a hexidecimal digit (i.e. A-F, a-f, or 0-9). Otherwise, zero is returned.

Related topics:
isalnum
isalpha
iscntrl
isdigit
isgraph
ispunct
isspace

atoi
Syntax:

      #include <stdlib.h>
      int atoi( const char *str );

The atoi() function converts str into an integer, and returns that integer. str should start with some sort of number, and atoi() will stop reading from str as soon as a non-numerical character has been read. For example,

       i = atoi( "512.035" );

would result in i being set to 512.

You can use (Standard C I/O) sprintf() to convert a number into a string.

Related topics:
atof
atol
(Standard C I/O) sprintf

atol
Syntax:

      #include <stdlib.h>
      long atol( const char *str );

The function atol() converts str into a long, then returns that value. atol() will read from str until it finds any character that should not be in a long. The resulting truncated value is then converted and returned. For example,

       x = atol( "1024.0001" );

results in x being set to 1024L.

Related topics:
atof
atoi
(Standard C I/O) sprintf
strtol

isalnum
Syntax:

      #include <ctype.h>
      int isalnum( int ch );

The function isalnum() returns non-zero if its argument is a numeric digit or a letter of the alphabet. Otherwise, zero is returned.

       char c;
       scanf( "%c", &c );
       if( isalnum(c) )
         printf( "You entered the alphanumeric character %c\n", c );

Related topics:
isalpha
iscntrl
isdigit
isgraph
isprint
ispunct
isspace
isxdigit

isalpha
Syntax:

      #include <ctype.h>
      int isalpha( int ch );

The function isalpha() returns non-zero if its argument is a letter of the alphabet. Otherwise, zero is returned.

       char c;
       scanf( "%c", &c );
       if( isalpha(c) )
         printf( "You entered a letter of the alphabet\n" );

Related topics:
isalnum
iscntrl
isdigit
isgraph
isprint
ispunct
isspace
isxdigit

iscntrl
Syntax:

      #include <ctype.h>
      int iscntrl( int ch );

The iscntrl() function returns non-zero if its argument is a control character (between 0 and 0x1F or equal to 0x7F). Otherwise, zero is returned.

Related topics:
isalnum
isalpha
isdigit
isgraph
isprint
ispunct
isspace
isxdigit

isdigit
Syntax:

      #include <ctype.h>
      int isdigit( int ch );

The function isdigit() returns non-zero if its argument is a digit between 0 and 9. Otherwise, zero is returned.

       char c;
       scanf( "%c", &c );
       if( isdigit(c) )
         printf( "You entered the digit %c\n", c );

Related topics:
isalnum
isalpha
iscntrl
isgraph
isprint
ispunct
isspace
isxdigit

isgraph
Syntax:

      #include <ctype.h>
      int isgraph( int ch );

The function isgraph() returns non-zero if its argument is any printable character other than a space (if you can see the character, then isgraph() will return a non-zero value). Otherwise, zero is returned.

Related topics:
isalnum
isalpha
iscntrl
isdigit
isprint
ispunct
isspace
isxdigit

islower
Syntax:

      #include <ctype.h>
      int islower( int ch );

The islower() function returns non-zero if its argument is a lowercase letter. Otherwise, zero is returned.

Related topics:
isupper

isprint
Syntax:

      #include <ctype.h>
      int isprint( int ch );

The function isprint() returns non-zero if its argument is a printable character (including a space). Otherwise, zero is returned.

Related topics:
isalnum
isalpha
iscntrl
isdigit
isgraph
ispunct
isspace

ispunct
Syntax:

      #include <ctype.h>
      int ispunct( int ch );

The ispunct() function returns non-zero if its argument is a printing character but neither alphanumeric nor a space. Otherwise, zero is returned.

Related topics:
isalnum
isalpha
iscntrl
isdigit
isgraph
isprint
isspace
isxdigit

isspace
Syntax:

      #include <ctype.h>
      int isspace( int ch );

The isspace() function returns non-zero if its argument is some sort of space (i.e. single space, tab, vertical tab, form feed, carriage return, or newline). Otherwise, zero is returned.

Related topics:
isalnum
isalpha
iscntrl
isdigit
isgraph
isprint
ispunct
isxdigit

isupper
Syntax:

      #include <ctype.h>
      int isupper( int ch );

The isupper() function returns non-zero if its argument is an uppercase letter. Otherwise, zero is returned.

Related topics:
islower
tolower

strerror
Syntax:

      #include <string.h>
      char *strerror( int num );

The function strerror() returns an implementation defined string corresponding to num.

memchr
Syntax:

      #include <string.h>
      void *memchr( const void *buffer, int ch, size_t count );

The memchr() function looks for the first occurrence of ch within count characters in the array pointed to by buffer. The return value points to the location of the first occurrence of ch, or NULL if ch isn't found. For example:

       char names[] = "Alan Bob Chris X Dave";
       if( memchr(names,'X',strlen(names)) == NULL )
         printf( "Didn't find an X\n" );
       else
         printf( "Found an X\n" );

Related topics:
memcmp
memcpy
strstr

memcmp
Syntax:

      #include <string.h>
      int memcmp( const void *buffer1, const void *buffer2, size_t count );

The function memcmp() compares the first count characters of buffer1 and buffer2. The return values are as follows:

Value | Explanation
--- | ---
less than 0 | buffer1 is less than buffer2
equal to 0 | buffer1 is equal to buffer2
greater than 0 | buffer1 is greater than buffer2

Related topics:
memchr
memcpy
memset
strcmp

memcpy
Syntax:

      #include <string.h>
      void *memcpy( void *to, const void *from, size_t count );

The function memcpy() copies count characters from the array from to the array to. The return value of memcpy() is to. The behavior of memcpy() is undefined if to and from overlap.

Related topics:
memchr
memcmp
memmove
memset
strcpy
strlen
strncpy

memmove
Syntax:

      #include <string.h>
      void *memmove( void *to, const void *from, size_t count );

The memmove() function is identical to memcpy(), except that it works even if to and from overlap.

Related topics:
memcpy
memset

memset
Syntax:

      #include <string.h>
      void* memset( void* buffer, int ch, size_t count );

The function memset() copies ch into the first count characters of buffer, and returns buffer. memset() is useful for intializing a section of memory to some value. For example, this command:

       memset( the_array, '\0', sizeof(the_array) );

...is a very efficient way to set all values of the_array to zero.

The table below compares two different methods for initializing an array of characters: a for-loop versus memset(). As the size of the data being initialized increases, memset() clearly gets the job done much more quickly:

Input size | Initialized with a for-loop | Initialized with memset()
--- | --- | ---
1000 | 0.016 | 0.017
10000 | 0.055 | 0.013
100000 | 0.443 | 0.029
1000000 | 4.337 | 0.291

Related topics:
memcmp
memcpy
memmove

strcat
Syntax:

      #include <string.h>
      char *strcat( char *str1, const char *str2 );

The strcat() function concatenates str2 onto the end of str1, and returns str1. For example:

       printf( "Enter your name: " );
       scanf( "%s", name );
       title = strcat( name, " the Great" );
       printf( "Hello, %s\n", title );

Related topics:
strchr
strcmp
strcpy
strncat

strchr
Syntax:

      #include <string.h>
      char *strchr( const char *str, int ch );

The function strchr() returns a pointer to the first occurence of ch in str, or NULL if ch is not found.

Related topics:
strcat
strcmp
strcpy
strlen
strncat
strncmp
strncpy
strpbrk
strspn
strstr
strtok

strcmp
Syntax:

      #include <string.h>
      int strcmp( const char *str1, const char *str2 );

The function strcmp() compares str1 and str2, then returns:

Return value | Explanation
--- | ---
less than 0 | ''str1'' is less than ''str2''
equal to 0 | ''str1'' is equal to ''str2''
greater than 0 | ''str1'' is greater than ''str2''

For example:

       printf( "Enter your name: " );
       scanf( "%s", name );
       if( strcmp( name, "Mary" ) == 0 )
         printf( "Hello, Dr. Mary!\n" );

Related topics:
memcmp
strcat
strchr
strcoll
strcpy
strlen
strncmp
strxfrm

strcoll
Syntax:

      #include <string.h>
      int strcoll( const char *str1, const char *str2 );

The strcoll() function compares str1 and str2, much like strcmp(). However, strcoll() performs the comparison using the locale specified by the (Standard C Date & Time) setlocale() function.

Related topics:
(Standard C Date & Time) setlocale
strcmp
strxfrm

strcpy
Syntax:

      #include <string.h>
      char *strcpy( char *to, const char *from );

The strcpy() function copies characters in the string from to the string to, including the null termination. The return value is to.

Related topics:
memcpy
strcat
strchr
strcmp
strncmp
strncpy

strcspn
Syntax:

      #include <string.h>
      size_t strcspn( const char *str1, const char *str2 );

The function strcspn() returns the index of the first character in str1 that matches any of the characters in str2.

Related topics:
strpbrk
strrchr
strstr
strtok

strlen
Syntax:

      #include <string.h>
      size_t strlen( char *str );

The strlen() function returns the length of str (determined by the number of characters before null termination).

Related topics:
memcpy
strchr
strcmp
strncmp

strncat
Syntax:

      #include <string.h>
      char *strncat( char *str1, const char *str2, size_t count );

The function strncat() concatenates at most count characters of str2 onto str1, adding a null termination. The resulting string is returned.

Related topics:
strcat
strchr
strncmp
strncpy

strncmp
Syntax:

      #include <string.h>
      int strncmp( const char *str1, const char *str2, size_t count );

The strncmp() function compares at most count characters of str1 and str2. The return value is as follows:

Return value | Explanation
--- | ---
less than 0 | ''str1'' is less than ''str2''
equal to 0 | ''str1'' is equal to ''str2''
greater than 0 | ''str1'' is greater than str2''

If there are less than count characters in either string, then the comparison will stop after the first null termination is encountered.

Related topics:
strchr
strcmp
strcpy
strlen
strncat
strncpy

strncpy
Syntax:

      #include <string.h>
      char *strncpy( char *to, const char *from, size_t count );

The strncpy() function copies at most count characters of from to the string to. If from has less than count characters, the remainder is padded with '\0' characters. The return value is the resulting string.

Related topics:
memcpy
strchr
strcpy
strncat
strncmp

strpbrk
Syntax:

      #include <string.h>
      char* strpbrk( const char* str1, const char* str2 );

The function strpbrk() returns a pointer to the first ocurrence in str1 of any character in str2, or NULL if no such characters are present.

Related topics:
(C++ Algorithms) find_first_of
strchr
strcspn
strrchr
strspn
strstr
strtok

strrchr
Syntax:

      #include <string.h>
      char *strrchr( const char *str, int ch );

The function strrchr() returns a pointer to the last occurrence of ch in str, or NULL if no match is found.

Related topics:
strcspn
strpbrk
strspn
strstr
strtok

strspn
Syntax:

      #include <string.h>
      size_t strspn( const char *str1, const char *str2 );

The strspn() function returns the index of the first character in str1 that doesn't match any character in str2.

Related topics:
strchr
strpbrk
strrchr
strstr
strtok

strstr
Syntax:

      #include <string.h>
      char *strstr( const char *str1, const char *str2 );

The function strstr() returns a pointer to the first occurrence of str2 in str1, or NULL if no match is found.

Related topics:
memchr
strchr
strcspn
strpbrk
strrchr
strspn
strtok

strtod
Syntax:

      #include <stdlib.h>
      double strtod( const char *start, char **end );

The function strtod() returns whatever it encounters first in start as a double. end is set to point at whatever is left in start after that double. If overflow occurs, strtod() returns either HUGE_VAL or -HUGE_VAL.

Related topics:
atof

strtok
Syntax:

      #include <string.h>
      char *strtok( char *str1, const char *str2 );

The strtok() function returns a pointer to the next "token" in str1, where str2 contains the delimiters that determine the token. strtok() returns NULL if no token is found. In order to convert a string to tokens, the first call to strtok() should have str1 point to the string to be tokenized. All calls after this should have str1 be NULL.

For example:

       char str[] = "now # is the time for all # good men to come to the # aid of their country";
       char delims[] = "#";
       char *result = NULL;
       result = strtok( str, delims );
       while( result != NULL ) {
           printf( "result is \"%s\"\n", result );
           result = strtok( NULL, delims );
       }

The above code will display the following output:

       result is "now "
       result is " is the time for all "
       result is " good men to come to the "
       result is " aid of their country"

Related topics:
strchr
strcspn
strpbrk
strrchr
strspn
strstr

strtol
Syntax:

      #include <stdlib.h>
      long strtol( const char *start, char **end, int base );

The strtol() function returns whatever it encounters first in start as a long, doing the conversion to base if necessary. end is set to point to whatever is left in start after the long. If the result can not be represented by a long, then strtol() returns either LONG_MAX or LONG_MIN. Zero is returned upon error.

Related topics:
atol
strtoul

strtoul
Syntax:

      #include <stdlib.h>
      unsigned long strtoul( const char *start, char **end, int base );

The function strtoul() behaves exactly like strtol(), except that it returns an unsigned long rather than a mere long.

Related topics:
strtol

strxfrm
Syntax:

      #include <string.h>
      size_t strxfrm( char *str1, const char *str2, size_t num );

The strxfrm() function manipulates the first num characters of str2 and stores them in str1. The result is such that if a strcoll() is performed on str1 and the old str2, you will get the same result as with a strcmp().

Related topics:
strcmp
strcoll

tolower
Syntax:

      #include <ctype.h>

The function tolower() returns the lowercase version of the character ch.

Related topics:
isupper
toupper

toupper
Syntax:

      #include <ctype.h>
      int toupper( int ch );

The toupper() function returns the uppercase version of the character ch.

Related topics:
tolower