Messages in this thread | | | Date | Thu, 8 Jun 2023 18:06:16 +0200 | From | Thomas Weißschuh <> | Subject | Re: [PATCH v2 1/4] tools/nolibc: sys.h: add __syscall() and __sysret() helpers |
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Hi David,
On 2023-06-08 14:35:49+0000, David Laight wrote: > From: Zhangjin Wu > > Sent: 06 June 2023 09:10 > > > > most of the library routines share the same code model, let's add two > > helpers to simplify the coding and shrink the code lines too. > > > ... > > +/* Syscall return helper, set errno as -ret when ret < 0 */ > > +static inline __attribute__((always_inline)) long __sysret(long ret) > > +{ > > + if (ret < 0) { > > + SET_ERRNO(-ret); > > + ret = -1; > > + } > > + return ret; > > +} > > If that right? > I thought that that only the first few (1024?) negative values > got used as errno values. > > Do all Linux architectures even use negatives for error? > I thought at least some used the carry flag. > (It is the historic method of indicating a system call failure.)
I guess you are thinking about the architectures native systemcall ABI.
In nolibc these are abstracted away in the architecture-specific assembly wrappers: my_syscall0 to my_syscall6. (A good example would be arch-mips.h)
These normalize the architecture systemcall ABI to negative errornumbers which then are returned from the sys_* wrapper functions.
The sys_* wrapper functions in turn are used by the libc function which translate the negative error number to the libc-style "return -1 and set errno" mechanism. At this point the new __sysret function is used.
Returning negative error numbers in between has the advantage that it can be used without having to set up a global/threadlocal errno variable.
In hope this helped, Thomas
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