sprintf accelerator for GCC and Clang
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kazuho Merge pull request #25
Add support for %hh modifier
Latest commit 0eba385 Mar 13, 2017


qrintf - sprintf accelerator

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The sprintf(3) family is a great set of functions for stringifying various kinds of data. The drawback is that they are slow. In certain applications, more than 10% of CPU time is consumed by the functions. The reason why it is slow is because it parses the given format at run-time.

qrintf is a preprocessor (and a set of runtime functions) that precompiles invocations of sprintf (and snprintf) with constant format strings into specialized forms.

The benchmark below shows the power of qrintf; converting IPv4 address to string becomes more than 10x faster when the preprocessor is applied to the source code.

$ gcc -O2 examples/ipv4addr.c
$ time ./a.out 1234567890

real	0m2.514s
user	0m2.503s
sys	0m0.003s
$ qrintf gcc -O2 examples/ipv4addr.c
$ time ./a.out 1234567890

real	0m0.175s
user	0m0.170s
sys	0m0.002s


make install PREFIX=/usr/local



qrintf is the command that wraps the C compiler (GCC or clang).

It preprocesses the source files using the preprocessor (by calling the compiler with -E option), applies qrintf-pp, and then once again invokes the compiler to compile the processed file.


qrintf-pp is the filter program that rewrites invocations of sprintf to optimized forms.

The command reads a C source file from the standard input, applies the necessary transformations, and prints the result to standard output.


Q. License?

The software is provided under the MIT license.

Q. Why did you develop qrintf?

Because sprintf is the bottleneck in some of my applications. I plan to use it in H2O, an optimized HTTP server/library implementation with support for HTTP/1.x, HTTP/2, websocket.

Q. Which functions are optimized?

sprintf and snprintf.

Q. Is there a list of conversion specifiers that get optimized?

  • %c
  • %s
  • %d (modifiers: h, l, ll)
  • %u,%x,%X (modifiers: h, l, ll, z)

Field widths (including *) and 0 flag (for zero padding) are also recognized.

note: Invocations of sprintf using other conversion specifiers are left as is.

Q. How do I run the tests?

make test CC=gcc
make test CC=clang

Q. Shouldn't such feature be implemented in the compiler?


Q. Where can I find more information?

Please refer to the developer's weblog.