1syscall(2) System Calls Manual syscall(2)
2
6 syscall - indirect system call
7
9 Standard C library (libc, -lc)
10
12 #include <sys/syscall.h> /* Definition of SYS_* constants */
13 #include <unistd.h>
14 long syscall(long number, ...);
16 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18 syscall():
20 Since glibc 2.19:
21 _DEFAULT_SOURCE
22 Before glibc 2.19:
23 _BSD_SOURCE || _SVID_SOURCE
24
26 syscall() is a small library function that invokes the system call
27 whose assembly language interface has the specified number with the
28 specified arguments. Employing syscall() is useful, for example, when
29 invoking a system call that has no wrapper function in the C library.
30 syscall() saves CPU registers before making the system call, restores
32 the registers upon return from the system call, and stores any error
33 returned by the system call in errno(3).
34 Symbolic constants for system call numbers can be found in the header
36 file <sys/syscall.h>.
37
39 The return value is defined by the system call being invoked. In gen‐
40 eral, a 0 return value indicates success. A -1 return value indicates
41 an error, and an error number is stored in errno.
42
44 ENOSYS The requested system call number is not implemented.
45 Other errors are specific to the invoked system call.
47
49 syscall() first appeared in 4BSD.
50 Architecture-specific requirements
52 Each architecture ABI has its own requirements on how system call argu‐
53 ments are passed to the kernel. For system calls that have a glibc
54 wrapper (e.g., most system calls), glibc handles the details of copying
55 arguments to the right registers in a manner suitable for the architec‐
56 ture. However, when using syscall() to make a system call, the caller
57 might need to handle architecture-dependent details; this requirement
58 is most commonly encountered on certain 32-bit architectures.
59 For example, on the ARM architecture Embedded ABI (EABI), a 64-bit
61 value (e.g., long long) must be aligned to an even register pair.
62 Thus, using syscall() instead of the wrapper provided by glibc, the
63 readahead(2) system call would be invoked as follows on the ARM archi‐
64 tecture with the EABI in little endian mode:
65 syscall(SYS_readahead, fd, 0,
67 (unsigned int) (offset & 0xFFFFFFFF),
68 (unsigned int) (offset >> 32),
69 count);
70 Since the offset argument is 64 bits, and the first argument (fd) is
72 passed in r0, the caller must manually split and align the 64-bit value
73 so that it is passed in the r2/r3 register pair. That means inserting
74 a dummy value into r1 (the second argument of 0). Care also must be
75 taken so that the split follows endian conventions (according to the C
76 ABI for the platform).
77 Similar issues can occur on MIPS with the O32 ABI, on PowerPC and
79 parisc with the 32-bit ABI, and on Xtensa.
80 Note that while the parisc C ABI also uses aligned register pairs, it
82 uses a shim layer to hide the issue from user space.
83 The affected system calls are fadvise64_64(2), ftruncate64(2),
85 posix_fadvise(2), pread64(2), pwrite64(2), readahead(2),
86 sync_file_range(2), and truncate64(2).
87 This does not affect syscalls that manually split and assemble 64-bit
89 values such as _llseek(2), preadv(2), preadv2(2), pwritev(2), and
90 pwritev2(2). Welcome to the wonderful world of historical baggage.
91 Architecture calling conventions
93 Every architecture has its own way of invoking and passing arguments to
94 the kernel. The details for various architectures are listed in the
95 two tables below.
96 The first table lists the instruction used to transition to kernel mode
98 (which might not be the fastest or best way to transition to the ker‐
99 nel, so you might have to refer to vdso(7)), the register used to indi‐
100 cate the system call number, the register(s) used to return the system
101 call result, and the register used to signal an error.
102 Arch/ABI Instruction System Ret Ret Error Notes
104 call # val val2
105 ───────────────────────────────────────────────────────────────────
106 alpha callsys v0 v0 a4 a3 1, 6
107 arc trap0 r8 r0 - -
108 arm/OABI swi NR - r0 - - 2
109 arm/EABI swi 0x0 r7 r0 r1 -
110 arm64 svc #0 w8 x0 x1 -
111 blackfin excpt 0x0 P0 R0 - -
112 i386 int $0x80 eax eax edx -
113 ia64 break 0x100000 r15 r8 r9 r10 1, 6
114 loongarch syscall 0 a7 a0 - -
115 m68k trap #0 d0 d0 - -
116 microblaze brki r14,8 r12 r3 - -
117 mips syscall v0 v0 v1 a3 1, 6
118 nios2 trap r2 r2 - r7
119 parisc ble 0x100(%sr2, %r0) r20 r28 - -
120 powerpc sc r0 r3 - r0 1
121 powerpc64 sc r0 r3 - cr0.SO 1
122 riscv ecall a7 a0 a1 -
123 s390 svc 0 r1 r2 r3 - 3
124 s390x svc 0 r1 r2 r3 - 3
125 superh trapa #31 r3 r0 r1 - 4, 6
126 sparc/32 t 0x10 g1 o0 o1 psr/csr 1, 6
127 sparc/64 t 0x6d g1 o0 o1 psr/csr 1, 6
128 tile swint1 R10 R00 - R01 1
129 x86-64 syscall rax rax rdx - 5
130 x32 syscall rax rax rdx - 5
131 xtensa syscall a2 a2 - -
132 Notes:
134 • On a few architectures, a register is used as a boolean (0 indicat‐
136 ing no error, and -1 indicating an error) to signal that the system
137 call failed. The actual error value is still contained in the re‐
138 turn register. On sparc, the carry bit (csr) in the processor sta‐
139 tus register (psr) is used instead of a full register. On pow‐
140 erpc64, the summary overflow bit (SO) in field 0 of the condition
141 register (cr0) is used.
142 • NR is the system call number.
144 • For s390 and s390x, NR (the system call number) may be passed di‐
146 rectly with svc NR if it is less than 256.
147 • On SuperH additional trap numbers are supported for historic rea‐
149 sons, but trapa#31 is the recommended "unified" ABI.
150 • The x32 ABI shares syscall table with x86-64 ABI, but there are some
152 nuances:
153 • In order to indicate that a system call is called under the x32
155 ABI, an additional bit, __X32_SYSCALL_BIT, is bitwise ORed with
156 the system call number. The ABI used by a process affects some
157 process behaviors, including signal handling or system call
158 restarting.
159 • Since x32 has different sizes for long and pointer types, layouts
161 of some (but not all; struct timeval or struct rlimit are 64-bit,
162 for example) structures are different. In order to handle this,
163 additional system calls are added to the system call table,
164 starting from number 512 (without the __X32_SYSCALL_BIT). For
165 example, __NR_readv is defined as 19 for the x86-64 ABI and as
166 __X32_SYSCALL_BIT | 515 for the x32 ABI. Most of these addi‐
167 tional system calls are actually identical to the system calls
168 used for providing i386 compat. There are some notable excep‐
169 tions, however, such as preadv2(2), which uses struct iovec enti‐
170 ties with 4-byte pointers and sizes ("compat_iovec" in kernel
171 terms), but passes an 8-byte pos argument in a single register
172 and not two, as is done in every other ABI.
173 • Some architectures (namely, Alpha, IA-64, MIPS, SuperH, sparc/32,
175 and sparc/64) use an additional register ("Retval2" in the above ta‐
176 ble) to pass back a second return value from the pipe(2) system
177 call; Alpha uses this technique in the architecture-specific getx‐
178 pid(2), getxuid(2), and getxgid(2) system calls as well. Other ar‐
179 chitectures do not use the second return value register in the sys‐
180 tem call interface, even if it is defined in the System V ABI.
181 The second table shows the registers used to pass the system call argu‐
183 ments.
184 Arch/ABI arg1 arg2 arg3 arg4 arg5 arg6 arg7 Notes
186 ──────────────────────────────────────────────────────────────
187 alpha a0 a1 a2 a3 a4 a5 -
188 arc r0 r1 r2 r3 r4 r5 -
189 arm/OABI r0 r1 r2 r3 r4 r5 r6
190 arm/EABI r0 r1 r2 r3 r4 r5 r6
191 arm64 x0 x1 x2 x3 x4 x5 -
192 blackfin R0 R1 R2 R3 R4 R5 -
193 i386 ebx ecx edx esi edi ebp -
194 ia64 out0 out1 out2 out3 out4 out5 -
195 loongarch a0 a1 a2 a3 a4 a5 a6
196 m68k d1 d2 d3 d4 d5 a0 -
197 microblaze r5 r6 r7 r8 r9 r10 -
198 mips/o32 a0 a1 a2 a3 - - - 1
200 mips/n32,64 a0 a1 a2 a3 a4 a5 -
201 nios2 r4 r5 r6 r7 r8 r9 -
202 parisc r26 r25 r24 r23 r22 r21 -
203 powerpc r3 r4 r5 r6 r7 r8 r9
204 powerpc64 r3 r4 r5 r6 r7 r8 -
205 riscv a0 a1 a2 a3 a4 a5 -
206 s390 r2 r3 r4 r5 r6 r7 -
207 s390x r2 r3 r4 r5 r6 r7 -
208 superh r4 r5 r6 r7 r0 r1 r2
209 sparc/32 o0 o1 o2 o3 o4 o5 -
210 sparc/64 o0 o1 o2 o3 o4 o5 -
211 tile R00 R01 R02 R03 R04 R05 -
212 x86-64 rdi rsi rdx r10 r8 r9 -
213 x32 rdi rsi rdx r10 r8 r9 -
214 xtensa a6 a3 a4 a5 a8 a9 -
215 Notes:
217 • The mips/o32 system call convention passes arguments 5 through 8 on
219 the user stack.
220 Note that these tables don't cover the entire calling convention—some
222 architectures may indiscriminately clobber other registers not listed
223 here.
224
226 #define _GNU_SOURCE
227 #include <signal.h>
228 #include <sys/syscall.h>
229 #include <unistd.h>
230 int
232 main(void)
233 {
234 pid_t tid;
235 tid = syscall(SYS_gettid);
237 syscall(SYS_tgkill, getpid(), tid, SIGHUP);
238 }
239
241 _syscall(2), intro(2), syscalls(2), errno(3), vdso(7)
242Linux man-pages 6.05 2023-05-03 syscall(2)