How is it possible to compile code from code - function

Question

I want to experiment with the programs that write programs in C code, and i want to use construction like following:
int main() {
char* srcCode="int f(int x) { return x+42; }";
int (*compiledFun)(int) = compile(srcCode);
printf("result=%d", (*compiledFun)(123));
return 0;
}
Desired output should be printed "result=165".
My question is about compile() function. I may try to put srcCode in a file, then invoke external compiler, like gcc, then try to read produced binary, probably fix some addresses, and so to fill the compiledFun memory. But I feel like that would be a very inefficient stub. Is there any way to compile a program from within a program, directly from memory to memory? Maybe some library or a subset that can be ripped off gcc sources, responsible for producting binary code from source text?
That may be important addition, all source code that should be compiled is a function that takes arguments and returns. It will not call any external libraries and function like printf, but only do some calculations and return.

Answer 1

Use libtcc an in-memory C compiler from TinyC.
A complete example from here https://github.com/TinyCC/tinycc/blob/mob/tests/libtcc_test.c
/*
* Simple Test program for libtcc
*
* libtcc can be useful to use tcc as a "backend" for a code generator.
*/
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "libtcc.h"
/* this function is called by the generated code */
int add(int a, int b)
{
return a + b;
}
/* this strinc is referenced by the generated code */
const char hello[] = "Hello World!";
char my_program[] =
"#include <tcclib.h>\n" /* include the "Simple libc header for TCC" */
"extern int add(int a, int b);\n"
"#ifdef _WIN32\n" /* dynamically linked data needs 'dllimport' */
" __attribute__((dllimport))\n"
"#endif\n"
"extern const char hello[];\n"
"int fib(int n)\n"
"{\n"
" if (n <= 2)\n"
" return 1;\n"
" else\n"
" return fib(n-1) + fib(n-2);\n"
"}\n"
"\n"
"int foo(int n)\n"
"{\n"
" printf(\"%s\\n\", hello);\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 0;\n"
"}\n";
int main(int argc, char **argv)
{
TCCState *s;
int i;
int (*func)(int);
s = tcc_new();
if (!s) {
fprintf(stderr, "Could not create tcc state\n");
exit(1);
}
/* if tcclib.h and libtcc1.a are not installed, where can we find them */
for (i = 1; i < argc; ++i) {
char *a = argv[i];
if (a[0] == '-') {
if (a[1] == 'B')
tcc_set_lib_path(s, a+2);
else if (a[1] == 'I')
tcc_add_include_path(s, a+2);
else if (a[1] == 'L')
tcc_add_library_path(s, a+2);
}
}
/* MUST BE CALLED before any compilation */
tcc_set_output_type(s, TCC_OUTPUT_MEMORY);
if (tcc_compile_string(s, my_program) == -1)
return 1;
/* as a test, we add symbols that the compiled program can use.
You may also open a dll with tcc_add_dll() and use symbols from that */
tcc_add_symbol(s, "add", add);
tcc_add_symbol(s, "hello", hello);
/* relocate the code */
if (tcc_relocate(s, TCC_RELOCATE_AUTO) < 0)
return 1;
/* get entry symbol */
func = tcc_get_symbol(s, "foo");
if (!func)
return 1;
/* run the code */
func(32);
/* delete the state */
tcc_delete(s);
return 0;
}