mirror of https://github.com/mkerrisk/man-pages
getgid.2, getpid.2, getuid.2, pipe.2, syscall.2: Describe 2nd return value peculiarity
Some architectures (ab)use second return value register for additional return value in some system calls. Let's describe this. Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Eugene Syromiatnikov
Michael Kerrisk
parent
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70ea1968cc
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@ -59,6 +59,25 @@ The glibc
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and
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.BR getegid ()
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wrapper functions transparently deal with the variations across kernel versions.
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.PP
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On Alpha, instead of pair of
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.BR getgid "() and " getegid ()
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system calls, single
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.BR getxgid ()
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system call is provided, which returns a pair of real and effective GIDs.
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The glibc
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.BR getgid ()
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and
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.BR getegid ()
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wrapper functions transparently deal with this.
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See
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.BR syscall (2)
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for details regarding register mapping.
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Other examples of such peculiarity are Alpha-specific
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.BR getxpid "(2) and " getxuid (2),
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along with
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.BR pipe (2)
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on several architectures.
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.SH SEE ALSO
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.BR getresgid (2),
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.BR setgid (2),
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@ -136,6 +136,25 @@ calls to
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always invoke the actual system call, rather than returning a cached value.
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.\" FIXME .
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.\" Review progress of https://bugzilla.redhat.com/show_bug.cgi?id=1469757
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.PP
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On Alpha, instead of pair of
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.BR getpid "() and " getppid ()
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system calls, single
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.BR getxpid ()
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system call is provided, which returns a pair of PID and parent PID.
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The glibc
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.BR getpid ()
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and
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.BR getppid ()
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wrapper functions transparently deal with this.
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See
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.BR syscall (2)
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for details regarding register mapping.
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Other examples of such peculiarity are Alpha-specific
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.BR getxuid "(2) and " getxgid (2),
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along with
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.BR pipe (2)
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on several architectures.
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.SH SEE ALSO
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.BR clone (2),
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.BR fork (2),
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@ -70,6 +70,25 @@ The glibc
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and
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.BR geteuid ()
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wrapper functions transparently deal with the variations across kernel versions.
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.PP
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On Alpha, instead of pair of
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.BR getuid "() and " geteuid ()
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system calls, single
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.BR getxuid ()
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system call is provided, which returns a pair of real and effective UIDs.
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The glibc
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.BR getuid ()
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and
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.BR geteuid ()
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wrapper functions transparently deal with this.
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See
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.BR syscall (2)
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for details regarding register mapping.
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Other examples of such peculiarity are Alpha-specific
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.BR getxpid "(2) and " getxgid (2),
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along with
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.BR pipe (2)
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on several architectures.
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.SH SEE ALSO
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.BR getresuid (2),
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.BR setreuid (2),
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26
man2/pipe.2
26
man2/pipe.2
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@ -40,6 +40,13 @@ pipe, pipe2 \- create pipe
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.nf
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.B #include <unistd.h>
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.PP
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/* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64; see NOTES */
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.B struct fd_pair {
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.B " long fd[2];"
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.B "};"
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.B struct fd_pair pipe();
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.PP
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/* On all other architectures */
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.BI "int pipe(int " pipefd "[2]);"
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.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
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@ -184,6 +191,25 @@ has been reached and the caller is not privileged; see
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was added to Linux in version 2.6.27;
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glibc support is available starting with
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version 2.9.
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.SH NOTES
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.\" See http://math-atlas.sourceforge.net/devel/assembly/64.psabi.1.33.ps.Z
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.\" for example, section 3.2.1 "Registers and the Stack Frame".
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SystemV ABI on some architectures allows using more than one register
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for returning multiple values; several architectures
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(namely, Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64)
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(ab)use this feature in order to implement
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.BR pipe ()
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system call in a functional manner: it doesn't take any arguments and returns
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a pair of file descriptors as a return value on success.
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The glibc
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.BR pipe ()
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wrapper function transparently deal with this.
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See
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.BR syscall (2)
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for information regarding registers used for storing second file descriptor.
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Other examples of such peculiarity are
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.BR getxpid (2)/ getxuid (2)/ getxgid (2)
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on Alpha.
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.SH CONFORMING TO
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.BR pipe ():
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POSIX.1-2001, POSIX.1-2008.
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@ -172,39 +172,39 @@ The first table lists the instruction used to transition to kernel mode
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so you might have to refer to
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.BR vdso (7)),
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the register used to indicate the system call number,
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the register used to return the system call result,
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the register(s) used to return the system call result,
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and the register used to signal an error.
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.if t \{\
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.ft CW
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\}
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.TS
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l2 l2 l2 l2 l2 l.
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arch/ABI instruction syscall # retval error Notes
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l2 l2 l2 l2 l2 l2 l.
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arch/ABI instruction syscall # retval retval2 error Notes
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_
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alpha callsys v0 v0 a3 [1]
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arc trap0 r8 r0 -
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arm/OABI swi NR - a1 - [2]
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arm/EABI swi 0x0 r7 r0 -
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arm64 svc #0 x8 x0 -
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blackfin excpt 0x0 P0 R0 -
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i386 int $0x80 eax eax -
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ia64 break 0x100000 r15 r8 r10 [1]
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m68k trap #0 d0 d0 -
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microblaze brki r14,8 r12 r3 -
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mips syscall v0 v0 a3 [1]
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nios2 trap r2 r2 r7
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parisc ble 0x100(%sr2, %r0) r20 r28 -
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powerpc sc r0 r3 r0 [1]
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riscv scall a7 a0 -
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s390 svc 0 r1 r2 - [3]
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s390x svc 0 r1 r2 - [3]
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superh trap #0x17 r3 r0 - [4]
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sparc/32 t 0x10 g1 o0 psr/csr [1]
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sparc/64 t 0x6d g1 o0 psr/csr [1]
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tile swint1 R10 R00 R01 [1]
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x86-64 syscall rax rax - [5]
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x32 syscall rax rax - [5]
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xtensa syscall a2 a2 -
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alpha callsys v0 v0 a4 a3 [1], [6]
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arc trap0 r8 r0 - -
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arm/OABI swi NR - a1 - - [2]
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arm/EABI swi 0x0 r7 r0 r1 -
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arm64 svc #0 x8 x0 x1 -
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blackfin excpt 0x0 P0 R0 - -
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i386 int $0x80 eax eax edx -
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ia64 break 0x100000 r15 r8 r9 r10 [1], [6]
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m68k trap #0 d0 d0 - -
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microblaze brki r14,8 r12 r3 - -
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mips syscall v0 v0 v1 a3 [1], [6]
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nios2 trap r2 r2 - r7
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parisc ble 0x100(%sr2, %r0) r20 r28 - -
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powerpc sc r0 r3 - r0 [1]
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riscv scall a7 a0 a1 -
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s390 svc 0 r1 r2 r3 - [3]
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s390x svc 0 r1 r2 r3 - [3]
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superh trap #0x17 r3 r0 r1 - [4], [6]
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sparc/32 t 0x10 g1 o0 o1 psr/csr [1], [6]
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sparc/64 t 0x6d g1 o0 o1 psr/csr [1], [6]
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tile swint1 R10 R00 - R01 [1]
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x86-64 syscall rax rax rdx - [5]
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x32 syscall rax rax rdx - [5]
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xtensa syscall a2 a2 - -
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.TE
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.PP
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Notes:
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@ -279,6 +279,17 @@ but passes an 8-byte
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.I pos
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argument in a single register and not two, as is done in every other ABI.
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.RE
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.IP [6]
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Some architectures
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(namely, Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64)
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use second return value register in order to pass second return value in
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.BR pipe (2)
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system call;
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Alpha uses it in the architecture-specific
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.BR getxpid "(2), " getxuid "(2), and " getxgid (2)
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system calls as well.
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Other architectures do not use the second return value register
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in the system call interface, even if it is defined in the SystemV ABI.
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.RE
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.if t \{\
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.in
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