Add notes that caution users when passing arguments to syscall.2. Signed-off-by: Changhee Han <ch0.han@xxxxxxx> --- Modified notes as you suggested and added some example which show how to handle 64bit argument man2/syscall.2 | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) diff --git a/man2/syscall.2 b/man2/syscall.2 index 0675943..180a0e4 100644 --- a/man2/syscall.2 +++ b/man2/syscall.2 @@ -79,6 +79,36 @@ and an error code is stored in .BR syscall () first appeared in 4BSD. + +Each architecture ABI has its own requirements on how system call arguments are passed to the kernel. +For system calls that have a glibc wrapper (i.g., most system calls) glibc handles the details of copy arguments to the right registers in a manner suitable for the architecture. +However, when using +.BR syscall () +to make a system call, +the caller may need to handle architecture-dependent details. +For example, on ARM architecture, a +.I "long long" +argument is considered to be 8-byte aligned and to be split into two 4-byte arguments. + +.BR readahead () +system call could be called like below in ARM architecture. + +syscall(__NR_readahead, fd, +.I 0 +, (unsigned int)( +.I offset +>> 32), (unsigned int)( +.I offset +& 0xFFFFFFFF), count) + +.I offset +is 64 bit and should be 8-byte aligned. +Thus, a padding is inserted before +.I offset +and +.I offset +is split into two 32 bit arguments. + .SH EXAMPLE .nf #define _GNU_SOURCE -- 1.7.9.5 -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
