Efficient 2 pass using heap memory

Question

I have an algorithm that requires making two passes a file's data. The file may be stdin or stream (like a |) since this is a command line tool, which makes me unfortunately (to the best of my knowledge) rule out mmap.

I require the information from the 1st pass in order to perform a write operation on the 2nd pass. This is because I need a sum of all the bytes on the first pass for a specific cipher on the second pass.

One way I have thought of to do this is to use the heap as a single contiguous region of memory, and to allocate additional size once the end has been reached with sbrk (similar to what I believe the first implementation of bash) did. Is there a simple way to do this?

Specifically, how can I avoid stdlib's set-up of the heap and do so myself?

• Jabberwocky: You probably either need to read the whole input into malloced memory, or you need copy it into a temporary file. OTOH, once you have copied the input into a file, you can use mmap on that file.
• Some programmer dude: Read into a buffer, doing all your first-pass processing while doing that. Then use the buffer for your second pass. If you read from stdin (no matter if it's from a terminal or a pipe) that's the only way to do multiple passes over the input, because once the data is read you can't seek back. If the buffer is in memory or as a temporary file doesn't matter, but you need some kind of buffer to store the input.
• user207421: Two-pass problems are rare. What's yours? Entire compilers can be written in one pass.
• chux: @user129393192, "Do I simply have to allocate memory for the entire length of the data as I read, or is there an alternative?" --> Yes. Post details of your task to find useful alternatives.
• Some programmer dude: Also please note that whenever "efficiency" is mentioned, its more often than not a red herring. Gather requirements, do analysis of them, and do a good design from the analysis. Then concentrate to write a good, simple, maintainable program from the design. If there are "efficiency" requirements, they should already be in the analysis and the design, and the code should reflect that.