Make sure you really know what you're asking. Integers are stored internally
in binary, although for most purposes it is not incorrect to think of them as being
in octal, decimal, or hexadecimal, whichever is convenient. The base in which a
number is expressed matters only when that number is read in from or written out
to the outside world, either in the form of a source code constant or in the form
of I/O performed by a program.
In source code, a non-decimal base is indicated by a leading 0 or 0x (for octal
or hexadecimal, respectively). During I/O, the base of a formatted number is controlled
in the printf and scanf family of functions by the choice of format specifier (%d,
%o, %x, etc.) and in the strtol and strtoul functions by the third argument. During
binary I/O, however, the base again becomes immaterial: if numbers are being read
or written as individual bytes (typically with getc or putc), or as multi-byte words
(typically with fread or fwrite), it is meaningless to ask what ``base''
they are in.
If what you need is formatted binary conversion, it's easy enough to do. Here
is a little function for formatting a number in a requested base: