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