C Useful

String

String To Upper / Lower / Capitalize
Trim A String (And Return Trim String Length)
Check If A String Is A Positive (Or 0) Integer
Check If A String Is A Valid IP Address (IPv4/IPv6)
Check If A String Is A Network Port
Check If A String Is A Hexadecimal String
Convert String To Hexadecimal String (And Inverse)
Get The 'Real' Length Of UTF-8 String
Convert A String UTF-8 To A String ISO_8859-1
An Example Of A Function With Variable Arguments
An Example To Print All String Combinations (From A Charset And Length Limits)

String To Upper / Lower / Capitalize

(Warning For UTF-8 Multi-Bytes Character)
( man strlen )

Trim A String (And Return Trim String Length)

( man strlen | man strcpy | man isspace )

Check If A String Is A Positive (Or 0) Integer

( man strlen | man atoi )

Check If A String Is A Valid IP Address (IPv4/IPv6)

( man strlen | man inet_pton )

Check If A String Is A Network Port

( man strlen | man atoi )

Check If A String Is A Hexadecimal String

(Accept Upper And Lower Hexadecimal Characters)
( man strlen )

Convert String To Hexadecimal String (And Inverse)

(Be Careful Of Strings Memory Allocations, Because Not Checked)
( man sprintf | man strtol )

Get The 'Real' Length Of UTF-8 String

(Example If A String Have Accents)
( man strlen )

Convert A String UTF-8 To A String ISO_8859-1

( man 3 iconv )

An Example Of A Function With Variable Arguments

(Here To Print A Formatted String To 'stdout')
( man va_start | man vfprintf )

An Example To Print All String Combinations (From A Charset And Length Limits)

[Useful For BruteForce ;)]