Is it reasonable to expect errno to be 0 when entering main?

[Leo Liang <ycliang@andestech.com>]

Hello guys,

According to the ANSI C standard "7.5 Errors <errno.h>" section 3[1], 
"The value of errno is zero at program startup,
but is never set to zero by any library function."

The "program startup" is also defined in ANSI C standard "5.1.2.2.1 Program startup" section 1[2],
"The function called at program startup is named main."

Therefore, it should be reasonable to expect errno be zero when main is executed.

However, we found that the following program would get non-zero errno under some circumstances.

```
/* errno.c */
#include <errno.h>
#include <stdio.h>

int main()
{
	printf("errno: %d\n", errno);
	return 0;
}


$ riscv64-linux-gcc -O0 -g -static -o errno_st errno.c

[   51.968765] random: fast init done
/mnt # ./errno
errno: 11

[   51.968765] random: fast init done
[  262.517056] crng init done
/mnt # ./errno
errno: 0
```

If the above program is statically-linked, and if the program is executed before "crng init done",
the errno will not be zero when entering main.

There seems to be syscall (__getrandom) before main is entered,
and if the crng is not initialized, the syscall will fail causing the errno to be set.
(_dl_get_origin -> __libc_malloc -> ptmalloc_init -> tcache_key_initialize -> __getrandom)

So we are wondering if we should set errno to zero before entering main.

Best regards,
Leo



The environment we are using is 
Linux Kernel 5.4
GLIBC 2.35
Binutil 2.38

Reference:
[1] ISO/IEC 9899:1999 "7.5 Errors <errno.h>": p.186:3 https://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf
[2] ISO/IEC 9899:1999 "5.1.2.2.1 Program startup": p.12:1 https://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf