Changing Entry Point of a C Program

By default compilers (gcc and llvm) looks for the function main for the entry point. To have a different function as an entry point,

• --redefine-sym flag of objcopy on the object file can be used
• -nostartfiles and -Wl,--entry=<function name> flags of gcc can be used.
• compiler directive can be used as -D<function name>=main in the gcc command can be used.

for example, if the c code looks like

// hello.c
#include <stdio.h>
int say_hello(int argc, char **argv){
    if (argc >= 2){
	    printf("Hello %s\n", argv[1]);
    }
    else{
	    printf("Hell oh world!\n");
    }
    return 0;
}

To compile and execute this code where say_hello is used as the entry point.

# create an object file
gcc -c hello.c -o hello.o
# redefine the symbol for main (entry function)
objcopy --redefine-sym say_hello=main hello.o
# create a binary
gcc hello.o -o hello

or the gcc can be used as follows

gcc -nostartfiles -Wl,--entry=say_hello -o hello hello.c

or

gcc -Dsay_hello=main -o hello hello.c