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mini_printf - minimal, self-contained and tested printf()

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This is a minimal printf() implementation that doesn't depend on any other libraries. It's meant to be used for early bootstrapping of DSLs and embeddable programs, as well as bare-metal software. It supports %x and %d format string specifiers only. One can easily extend mini_printf.c for additional format strings.

How to use it

The mini_printf comes in 1 C file called mini_printf.c. It has a function called pf(const char *fmt, ...) which works exactly how printf does. It's named pf() to prevent from eventual collisions with other printf-alike API you may have in your programs.

To use it, include mini_printf.c in your program. It only wants one thing from you: you must provide it a io_putc(int c) function, which is used internally to do an actual I/O. You should be able to later call:


and get yours io_putc routine called for each character.

How it was tested

The biggest effort went into verification. Testing of printf is very important, since failure of printf can lead to many hours of frustration and debugging, so I tried to make it less likely.

The testprog.c has a randomized stress test suite proving my pf() is fairly robust. Stress test basically generates random format strings in a smart way and plays them against system printf() and mini_printf and then tries to compare the results. Mismatch means a failure of the test.

To start stress-testing, type:

make stresstest

It'll try to generate 1,000,000 random settings and play them against mini_printf.

Several other useful targets are:

  • make test.vv
    • very verbose and slow test. Mostly for development.
  • make test.v
    • less verbose test. Slightly slower due to not all test data printed out.
  • make test
    • long-running test for very stressful testing. Can make your MacBook Air boil
  • make unittest
    • unit-testing, for the cases when bugs are found

Underlying program for all tests is testprog. Its usage:

Flag Description
d turn debugging on. Can be specified more than once
l how many loops to run. 1 loop = 1 chunk of format string. Default: 6.
m print mask: frequency of printouts after each test execution
t start only unit test.
w wait nanoseconds after each test. For debugging only.
p print generated format strings
s stop after tests.
v verbose mode.


You've looked at the source and want to run only unit test number 11:

./testprog -d -t 11

You want to run all unit tests:

./testprog -d

You want to figure out what's going on: random format strings need to be short (-l 1), output must be slow (-w 500000) and verbose (-d -v -p), and every generated test must be printed out (-m 0):

./testprog -w 500000 -l 1 -d -v -m 0 -p



mini_printf - minimal, self-contained and verified printf()





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