compatible header functions

Hi all!
I am new to writing c programs in linux and I have trouble finding 
the following functions in the header files.
1)itoa: to convert an integer to a string.
2)ltoa: to convert long integer to a string.
3)strset: to set the whole string to a given character.
Please tell me if these are present in linux or if they have any 
equivalent functions.
Thanks in advance.
mishra.