所以,在BRPocock的帮助下,我会在这里汇报我的发现。
Linux内核会为沿线的所有的系统调用一个错误检查(从unistd.h):
#define __syscall_return(type, res) \
do { \
if ((unsigned long)(res) >= (unsigned long)(-125)) { \
errno = -(res); \
res = -1; \
} \
return (type) (res); \
} while (0)
libc库也将做沿(从syscall.S)的行所有的系统调用错误检查:
.text
ENTRY (syscall)
PUSHARGS_6 /* Save register contents. */
_DOARGS_6(44) /* Load arguments. */
movl 20(%esp), %eax /* Load syscall number into %eax. */
ENTER_KERNEL /* Do the system call. */
POPARGS_6 /* Restore register contents. */
cmpl $-4095, %eax /* Check %eax for error. */
jae SYSCALL_ERROR_LABEL /* Jump to error handler if error. */
ret /* Return to caller. */
PSEUDO_END (syscall)
的Glibc给出了4096值的原因(来自sysdep.h):
/* Linux uses a negative return value to indicate syscall errors,
unlike most Unices, which use the condition codes' carry flag.
Since version 2.1 the return value of a system call might be
negative even if the call succeeded. E.g., the `lseek' system call
might return a large offset. Therefore we must not anymore test
for < 0, but test for a real error by making sure the value in %eax
is a real error number. Linus said he will make sure the no syscall
returns a value in -1 .. -4095 as a valid result so we can savely
test with -4095. */
__syscall_return似乎从新内核中丢失了,我还没有研究过。
有人可以将此问题迁移到SO吗? –