When you create an elf file with another program and this file uses standard C functions like printf(), tcc does not recognize them as functions until the function flag is set for the symbol.

A more detailed description of the problem can be found in this stackoverflow answer:

"It appears that TCC requires specific type information on functions that are external linkage like printf. By default NASM creates references to symbols with a NOTYPE attribute in the ELF objects. This seems to confuse TCC as it seems to expect external function symbols to be marked with a FUNCTION type."
https://stackoverflow.com/a/45924381/3657155

The SO question is specific for nasm, but the problem should appear with other programs too.