working of syscall handling

Dan Nelson dnelson at
Wed Apr 8 07:55:19 PDT 2009

In the last episode (Apr 08), Mehul Chadha said:
> In the program given below the function readlink gets called up when
> printf is executed and the program ends without any output.
> readlink is a system call (syscall number = 58) which is being made by the
> printf function, but according to my understanding of system call, it is
> made by putting the handler number in eax register and then interrupting
> the processor, so that it can enter the kernel mode and execute the
> required function, but in this case(dont know why) my readlink function
> gets called up which should not have happened.

Readlink is not only a syscall, but a POSIX library function.  You are
overriding that, and FreeBSD's malloc function uses readlink to read the
/etc/malloc.conf settings file.  printf calls malloc, so that's why your
program exits.
> I will be very thankful if you can help me with it.
> #include<stdio.h>
> int readlink(void *a, void *b)
> {
>  exit(0);
> }
> int main(int argc, char **argv)
> {
>   printf("Hello World");
> }

	Dan Nelson
	dnelson at

More information about the freebsd-hackers mailing list