sys_err_init.c

#include <stdlib.h>
#include <errno.h>

/* Reporting integer errno values is not nice, even in init, but linking
   sterror() from libc brings in the whole syserrlist. This is while init
   itself can only produce a handful of different errno values.

   The idea is to choose the middle ground, providing readable messages
   for expected errors and printing anything unexpected as a integer.
   Getting something unexpected in init is already a sign of a big trouble,
   so making the user go on and use strerror(3)/perror(1)/etc to get
   the message should not make things much worse.

   Finally, I don't like verbose messages like "No such file or directory",
   especially when they aren't accompanied by respective error code.
   Most man pages only list symbolic constants, ENOENT in this case, with
   a syscall-specific description, so searching for "No such file ..."
   is pointless. Showing symbolic constants, on the other hand, makes it
   easy to read both errno(3) and relevant section 2 pages
   (when it's clear from the message which syscall failed, that is) */

/* The whole symbol-or-int thing only works via sys_printf %m.
   dietlibc printf can't handle NULL return properly, and neither glibc
   nor musl printfs use strerror internally. */

/* For a long time this has been a switch(err), but it turns out gcc generates
   relatively large jump table here. The same does not seem to happen with
   telinit and run errlists, strange. */

struct errlist {
	unsigned char err;
	char name[11];	/* EADDRINUSE is 10+1 bytes long */
} errlist[] = {
#define r(e) { e, #e }
	r(ENOENT),	/* open */
	r(ELOOP),	/* open, unlikely */
	r(EADDRINUSE),	/* bind */
	r(ENFILE),	/* open, socket, accept */
	r(ENOSPC),	/* open(..., O_CREAT) for process logging */
	r(EPIPE),	/* sendmsg */
	r(ENOMEM),	/* mmap and a buch of other calls */
	r(EINTR),	/* poll, unlikely for other syscalls */
	r(EINVAL),	/* generic, unlikely */
	r(EAGAIN),	/* fork */
	r(EACCES),	/* execve */
	r(ENOEXEC),	/* execve */
	r(ENOTDIR),	/* execve */
	r(E2BIG),	/* execve, very large service file */
	r(EIO),		/* unlikely, bad news */
	r(EPERM)	/* reboot and some other calls */
};

#define endof(a) (a + sizeof(a)/sizeof(*a))

char* strerror(int err)
{
	struct errlist* p;

	for(p = errlist; p < endof(errlist); p++)
		if((int)p->err == err)
			return p->name;

	return NULL;
}