C (nonportable), cracks Pyautogui's answer

const char main[]="\x48\x8d\x35\xf9\xff\xff\xff\xbf\x01\x00\x00\x00\x89\xf8\x89\xfa\x0f\x05\xeb\xf8"; 

Very platform- and compiler-specific. This worked on old versions of tcc, but not on the current version. On my computer, it works on current gcc with the -zexecstack command-line option, but not without. On TIO, it doesn't work at all, not even with -zexecstack (thus no TIO link). On platforms with no memory protection (e.g. DOS), this should work on pretty much any compiler (although you would of course need to change the contents of the string literal to fit the platform you were running on).

The basic idea is to run arbitrary machine code by misinterpreting a string literal as a function (the "function" given above prints the letter H – its own first byte – infinitely many times, if run on a Linux x86-64 system). In C, type-checking is done only by the compiler, not the linker, so if you place a string literal in one translation unit (i.e. file) and a function call to it in a different translation unit, neither the compiler nor the linker will notice that one unit declared the symbol in question to be a function but the other declared it to be a character array. (Function pointers and character array pointers are the same width on most platforms, which is all the linker cares about, so it'll happily misinterpret one as the other.)

How to call a function without (? We can't write the function call, but luckily there's an implicit call to main at the start of the program, and as a bonus, the implicit call is in a different file (one provided by the C implementation) than our program is, so neither the compiler nor the linker will produce an error (although several compilers have a warning for this sort of thing nowadays). So as long as our string literal is used to initialise a variable named main, it'll get called automatically.

Further reading: @Dennis discussing how to various get C implementations to run a variable as though it were a function.

C (nonportable), cracks Pyautogui's answer

const char main[]="\x48\x8d\x35\xf9\xff\xff\xff\xbf\x01\x00\x00\x00\x89\xf8\x89\xfa\x0f\x05\xeb\xf8"; 

Try it online!

Very platform- and compiler-specific. This worked on old versions of tcc, but not on the current version. On my computer and on TIO, it works on current gcc with the -zexecstack command-line option, but not without. On platforms with no memory protection (e.g. DOS), this should work on pretty much any compiler (although you would of course need to change the contents of the string literal to fit the platform you were running on).

The basic idea is to run arbitrary machine code by misinterpreting a string literal as a function (the "function" given above prints the letter H – its own first byte – infinitely many times, if run on a Linux x86-64 system). In C, type-checking is done only by the compiler, not the linker, so if you place a string literal in one translation unit (i.e. file) and a function call to it in a different translation unit, neither the compiler nor the linker will notice that one unit declared the symbol in question to be a function but the other declared it to be a character array. (Function pointers and character array pointers are the same width on most platforms, which is all the linker cares about, so it'll happily misinterpret one as the other.)

How to call a function without (? We can't write the function call, but luckily there's an implicit call to main at the start of the program, and as a bonus, the implicit call is in a different file (one provided by the C implementation) than our program is, so neither the compiler nor the linker will produce an error (although several compilers have a warning for this sort of thing nowadays). So as long as our string literal is used to initialise a variable named main, it'll get called automatically.

Further reading: @Dennis discussing how to various get C implementations to run a variable as though it were a function.