1SYSCALL(2)                 Linux Programmer's Manual                SYSCALL(2)
2
3
4

NAME

6       syscall - indirect system call
7

SYNOPSIS

9       #include <unistd.h>
10       #include <sys/syscall.h>   /* For SYS_xxx definitions */
11
12       long syscall(long number, ...);
13
14   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
15       syscall():
16           Since glibc 2.19:
17               _DEFAULT_SOURCE
18           Before glibc 2.19:
19               _BSD_SOURCE || _SVID_SOURCE
20

DESCRIPTION

22       syscall()  is  a  small  library  function that invokes the system call
23       whose assembly language interface has the  specified  number  with  the
24       specified  arguments.  Employing syscall() is useful, for example, when
25       invoking a system call that has no wrapper function in the C library.
26
27       syscall() saves CPU registers before making the system  call,  restores
28       the  registers  upon  return from the system call, and stores any error
29       returned by the system call in errno(3).
30
31       Symbolic constants for system call numbers can be found in  the  header
32       file <sys/syscall.h>.
33

RETURN VALUE

35       The  return value is defined by the system call being invoked.  In gen‐
36       eral, a 0 return value indicates success.  A -1 return value  indicates
37       an error, and an error number is stored in errno.
38

NOTES

40       syscall() first appeared in 4BSD.
41
42   Architecture-specific requirements
43       Each architecture ABI has its own requirements on how system call argu‐
44       ments are passed to the kernel.  For system calls  that  have  a  glibc
45       wrapper (e.g., most system calls), glibc handles the details of copying
46       arguments to the right registers in a manner suitable for the architec‐
47       ture.   However, when using syscall() to make a system call, the caller
48       might need to handle architecture-dependent details;  this  requirement
49       is most commonly encountered on certain 32-bit architectures.
50
51       For  example,  on  the  ARM  architecture Embedded ABI (EABI), a 64-bit
52       value (e.g., long long) must be  aligned  to  an  even  register  pair.
53       Thus,  using  syscall()  instead  of the wrapper provided by glibc, the
54       readahead(2) system call would be invoked as follows on the ARM  archi‐
55       tecture with the EABI in little endian mode:
56
57           syscall(SYS_readahead, fd, 0,
58                   (unsigned int) (offset & 0xFFFFFFFF),
59                   (unsigned int) (offset >> 32),
60                   count);
61
62       Since  the  offset  argument is 64 bits, and the first argument (fd) is
63       passed in r0, the caller must manually split and align the 64-bit value
64       so  that it is passed in the r2/r3 register pair.  That means inserting
65       a dummy value into r1 (the second argument of 0).  Care  also  must  be
66       taken  so that the split follows endian conventions (according to the C
67       ABI for the platform).
68
69       Similar issues can occur on MIPS with  the  O32  ABI,  on  PowerPC  and
70       parisc with the 32-bit ABI, and on Xtensa.
71
72       Note  that  while the parisc C ABI also uses aligned register pairs, it
73       uses a shim layer to hide the issue from user space.
74
75       The  affected  system  calls   are   fadvise64_64(2),   ftruncate64(2),
76       posix_fadvise(2),      pread64(2),      pwrite64(2),      readahead(2),
77       sync_file_range(2), and truncate64(2).
78
79       This does not affect syscalls that manually split and  assemble  64-bit
80       values  such  as  _llseek(2),  preadv(2),  preadv2(2),  pwritev(2), and
81       pwritev2(2).  Welcome to the wonderful world of historical baggage.
82
83   Architecture calling conventions
84       Every architecture has its own way of invoking and passing arguments to
85       the  kernel.   The  details for various architectures are listed in the
86       two tables below.
87
88       The first table lists the instruction used to transition to kernel mode
89       (which  might  not be the fastest or best way to transition to the ker‐
90       nel, so you might have to refer to vdso(7)), the register used to indi‐
91       cate  the system call number, the register(s) used to return the system
92       call result, and the register used to signal an error.
93
94       Arch/ABI    Instruction           System  Ret  Ret  Error    Notes
95                                         call #  val  val2
96       ───────────────────────────────────────────────────────────────────
97       alpha       callsys               v0      v0   a4   a3       1, 6
98       arc         trap0                 r8      r0   -    -
99       arm/OABI    swi NR                -       r0   -    -        2
100       arm/EABI    swi 0x0               r7      r0   r1   -
101       arm64       svc #0                w8      x0   x1   -
102       blackfin    excpt 0x0             P0      R0   -    -
103       i386        int $0x80             eax     eax  edx  -
104       ia64        break 0x100000        r15     r8   r9   r10      1, 6
105       m68k        trap #0               d0      d0   -    -
106       microblaze  brki r14,8            r12     r3   -    -
107       mips        syscall               v0      v0   v1   a3       1, 6
108       nios2       trap                  r2      r2   -    r7
109       parisc      ble 0x100(%sr2, %r0)  r20     r28  -    -
110       powerpc     sc                    r0      r3   -    r0       1
111       powerpc64   sc                    r0      r3   -    cr0.SO   1
112       riscv       ecall                 a7      a0   a1   -
113       s390        svc 0                 r1      r2   r3   -        3
114       s390x       svc 0                 r1      r2   r3   -        3
115       superh      trap #0x17            r3      r0   r1   -        4, 6
116       sparc/32    t 0x10                g1      o0   o1   psr/csr  1, 6
117       sparc/64    t 0x6d                g1      o0   o1   psr/csr  1, 6
118       tile        swint1                R10     R00  -    R01      1
119       x86-64      syscall               rax     rax  rdx  -        5
120       x32         syscall               rax     rax  rdx  -        5
121       xtensa      syscall               a2      a2   -    -
122
123       Notes:
124
125       [1] On a few architectures, a register is used as a boolean (0 indicat‐
126           ing no error, and -1 indicating an error) to signal that the system
127           call failed.  The actual error value is still contained in the  re‐
128           turn register.  On sparc, the carry bit (csr) in the processor sta‐
129           tus register (psr) is used instead of a  full  register.   On  pow‐
130           erpc64,  the  summary overflow bit (SO) in field 0 of the condition
131           register (cr0) is used.
132
133       [2] NR is the system call number.
134
135       [3] For s390 and s390x, NR (the system call number) may be  passed  di‐
136           rectly with svc NR if it is less than 256.
137
138       [4] On SuperH, the trap number controls the maximum number of arguments
139           passed.  A trap #0x10 can  be  used  with  only  0-argument  system
140           calls, a trap #0x11 can be used with 0- or 1-argument system calls,
141           and so on up to trap #0x17 for 7-argument system calls.
142
143       [5] The x32 ABI shares syscall table with x86-64  ABI,  but  there  are
144           some nuances:
145
146           •  In  order to indicate that a system call is called under the x32
147              ABI, an additional bit, __X32_SYSCALL_BIT, is bitwise-ORed  with
148              the  system call number.  The ABI used by a process affects some
149              process behaviors, including  signal  handling  or  system  call
150              restarting.
151
152           •  Since  x32  has different sizes for long and pointer types, lay‐
153              outs of some (but not all; struct timeval or struct  rlimit  are
154              64-bit, for example) structures are different.  In order to han‐
155              dle this, additional system calls are added to the  system  call
156              table, starting from number 512 (without the __X32_SYSCALL_BIT).
157              For example, __NR_readv is defined as 19 for the x86-64 ABI  and
158              as __X32_SYSCALL_BIT | 515 for the x32 ABI.  Most of these addi‐
159              tional system calls are actually identical to the  system  calls
160              used  for  providing i386 compat.  There are some notable excep‐
161              tions, however, such as preadv2(2), which uses struct iovec  en‐
162              tities  with 4-byte pointers and sizes ("compat_iovec" in kernel
163              terms), but passes an 8-byte pos argument in a  single  register
164              and not two, as is done in every other ABI.
165
166       [6] Some  architectures  (namely, Alpha, IA-64, MIPS, SuperH, sparc/32,
167           and sparc/64) use an additional register ("Retval2"  in  the  above
168           table)  to  pass back a second return value from the pipe(2) system
169           call; Alpha uses this technique in the architecture-specific  getx‐
170           pid(2), getxuid(2), and getxgid(2) system calls as well.  Other ar‐
171           chitectures do not use the second return value register in the sys‐
172           tem call interface, even if it is defined in the System V ABI.
173
174       The second table shows the registers used to pass the system call argu‐
175       ments.
176
177       Arch/ABI      arg1  arg2  arg3  arg4  arg5  arg6  arg7  Notes
178       ──────────────────────────────────────────────────────────────
179       alpha         a0    a1    a2    a3    a4    a5    -
180       arc           r0    r1    r2    r3    r4    r5    -
181       arm/OABI      r0    r1    r2    r3    r4    r5    r6
182       arm/EABI      r0    r1    r2    r3    r4    r5    r6
183       arm64         x0    x1    x2    x3    x4    x5    -
184       blackfin      R0    R1    R2    R3    R4    R5    -
185       i386          ebx   ecx   edx   esi   edi   ebp   -
186       ia64          out0  out1  out2  out3  out4  out5  -
187       m68k          d1    d2    d3    d4    d5    a0    -
188       microblaze    r5    r6    r7    r8    r9    r10   -
189       mips/o32      a0    a1    a2    a3    -     -     -     1
190       mips/n32,64   a0    a1    a2    a3    a4    a5    -
191       nios2         r4    r5    r6    r7    r8    r9    -
192       parisc        r26   r25   r24   r23   r22   r21   -
193       powerpc       r3    r4    r5    r6    r7    r8    r9
194       powerpc64     r3    r4    r5    r6    r7    r8    -
195       riscv         a0    a1    a2    a3    a4    a5    -
196       s390          r2    r3    r4    r5    r6    r7    -
197       s390x         r2    r3    r4    r5    r6    r7    -
198
199       superh        r4    r5    r6    r7    r0    r1    r2
200       sparc/32      o0    o1    o2    o3    o4    o5    -
201       sparc/64      o0    o1    o2    o3    o4    o5    -
202       tile          R00   R01   R02   R03   R04   R05   -
203       x86-64        rdi   rsi   rdx   r10   r8    r9    -
204       x32           rdi   rsi   rdx   r10   r8    r9    -
205       xtensa        a6    a3    a4    a5    a8    a9    -
206
207       Notes:
208
209       [1] The mips/o32 system call convention passes arguments 5 through 8 on
210           the user stack.
211
212       Note  that  these tables don't cover the entire calling convention—some
213       architectures may indiscriminately clobber other registers  not  listed
214       here.
215

EXAMPLES

217       #define _GNU_SOURCE
218       #include <unistd.h>
219       #include <sys/syscall.h>
220       #include <sys/types.h>
221       #include <signal.h>
222
223       int
224       main(int argc, char *argv[])
225       {
226           pid_t tid;
227
228           tid = syscall(SYS_gettid);
229           syscall(SYS_tgkill, getpid(), tid, SIGHUP);
230       }
231

SEE ALSO

233       _syscall(2), intro(2), syscalls(2), errno(3), vdso(7)
234

COLOPHON

236       This  page  is  part of release 5.10 of the Linux man-pages project.  A
237       description of the project, information about reporting bugs,  and  the
238       latest     version     of     this    page,    can    be    found    at
239       https://www.kernel.org/doc/man-pages/.
240
241
242
243Linux                             2020-06-09                        SYSCALL(2)
Impressum