Write a function in C to merge text files

I have written a program in C that process lines from a text file. I would like to expand the program with a function that includes the text from referenced files and text from files referenced by the referenced files and writes the output to a new file. Where a file is referenced the calling line should be written to the output file and preceded with a %. I would like to write this but don’t have the time.

The function is to be written in C with emphasis on reliability, readability and future maintenance. The files can be large so speed is a consideration. Memory and disk space are not an issue but any memory used should be released before leaving the function

Checks are to be included to pick up any errors with the errors written to a text string that is passed back to the calling program. The referenced files are called with the command \vimport{path/file.txt}

My preference is to format the code GNU style with 3 character tab.

I use Linux so the path is formatted accordingly.

See the attached files for example input and output files. Due to the pph limits, three input files are combined into one file.

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