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/ c-putc-vs-fputc.rst
Last active
March 2, 2023 07:21
The difference between ``putc`` and ``fputc`` in C: notes from a conversation with Shevek
From the putc man page, "putc() is equivalent to fputc() except that it may be implemented as a macro which evaluates stream more than once."
I wasn't sure what this meant so I asked Shevek.
He launched into a lambda calculus explanation that eventually boiled down to: putc may evaluate it's arguments more than once, but fputc evaluates them exactly once.
This is because putc is implemented as a macro and the C preprocessor just does string substitution, leading to examples like the ones below.
Ignoring what putc and fputc actually do, let's look at a simplified version where they both simply do one operation twice and another operation once: