Search for a value in an XML file from a C program and then write to a new tag in another XML file

I am new to XML and have a very sketchy knowledge of C. I am writing a C program where I need to read strings (each string on a separate line) from a text file, match that string with a value (unique for each record) enclosed within a certain tag in an XML file in order to get the value of another tag in the same XML file, and then write that value to another XML file in a new tag.
I will really appreciate any help.
Thanks.