discovering tcc

categories: blog

Today I just discovered another great piece of software by Fabrice Bellard: tcc.

It is in orders of magnitude faster than gcc - lots of pieces of code of mine (albeit very small) were compiling 10-20 times faster.

Despite it being ANSI C compliant but not fully ISOC99 compliant all code of mine that I was trying it on compiled happily. Checking out the documentation, the missing ISOC99 parts turned out to be those I wouldnt use anyways (complex and imaginary numbers and variable length arrays).

Apart from being small and fast there are two killer features: C scripting support and dynamic code generation through libtcc.

By using the shebang line

#!/usr/local/bin/tcc -run

and setting the executable bit one can now "run" C source files just as scripts. tcc will compile and execute the code on the fly without even creating any temporary files but leaving the code in memory.

Since tcc is also extremely fast there is only little disadvantage over shell code. A simple helloworld.c "script" was "executed" in just 0.08 seconds where the same shell script did that in 0.03 seconds. The example "script" from the manpage reads:

#!/usr/bin/tcc -run
#include <stdio.h>

int main()
printf("Hello World\n");
return 0;

Another feature that is just the logical consequence of the above is tcc's ability to read C source from standard input and compile and run it on the fly:

echo 'main(){puts("hello");}' | tcc -run -

Now to the second amazing thing: dynamic code generation on the fly. The above is achieved using libtcc with which one can dynamically generate and compile C code through library calls to libtcc and execute the code right away from memory.

The following example program shows how to achieve this (inspired by libtcc_test.c from the tcc source):

#include <stdlib.h>
#include <stdio.h>
#include "libtcc.h"

int add(int a, int b) { return a + b; }

char my_program[] =
"int fib(int n) {\n"
" if (n <= 2) return 1;\n"
" else return fib(n-1) + fib(n-2);\n"
"int foobar(int n) {\n"
" printf(\"fib(%d) = %d\\n\", n, fib(n));\n"
" printf(\"add(%d, %d) = %d\\n\", n, 2 * n, add(n, 2 * n));\n"
" return 1337;\n"

int main(int argc, char **argv)
TCCState *s;
int (*foobar_func)(int);
void *mem;

s = tcc_new();
tcc_set_output_type(s, TCC_OUTPUT_MEMORY);
tcc_compile_string(s, my_program);
tcc_add_symbol(s, "add", add);

mem = malloc(tcc_relocate(s, NULL));
tcc_relocate(s, mem);

foobar_func = tcc_get_symbol(s, "foobar");


printf("foobar returned: %d\n", foobar_func(32));

return 0;

Two usecases of tcc already come to my mind. Firstly, there is an amazing movement going on that creates music from C oneliners. erlehman started a project on github where he is gathering a number of such oneliners. The workflow is to first dynamically generate the C source code by plugging the single line of algorithm into a simple for-loop wrapper, compiling this source with gcc and then piping the output of the generated executable into aplay or sox. Using tcc one could now just pipe the generated code into tcc which in turn would compile AND execute the code in one step. It would be faster than with gcc and would require no intermediary source files or executables but would just be one line that does everything.

Secondly there is a project I'm working on at Jacobs called Flowy where I struggle with optimizing performance of a parser of a processing language for network flow records. Performance is already quite good and to increase it, dynamic code generation has always been an option but would have been quite messy if done with gcc. With libtcc I would be able to dynamically construct the rules and execute them with just a number of library calls and without the complexity of gcc and the fact that I would have to call it as an executable.

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