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 syscall() first appeared in 4BSD.
45 Architecture-specific requirements
47 Each architecture ABI has its own requirements on how system call argu‐
48 ments are passed to the kernel. For system calls that have a glibc
49 wrapper (e.g., most system calls), glibc handles the details of copying
50 arguments to the right registers in a manner suitable for the architec‐
51 ture. However, when using syscall() to make a system call, the caller
52 might need to handle architecture-dependent details; this requirement
53 is most commonly encountered on certain 32-bit architectures.
54 For example, on the ARM architecture Embedded ABI (EABI), a 64-bit
56 value (e.g., long long) must be aligned to an even register pair.
57 Thus, using syscall() instead of the wrapper provided by glibc, the
58 readahead(2) system call would be invoked as follows on the ARM archi‐
59 tecture with the EABI in little endian mode:
60 syscall(SYS_readahead, fd, 0,
62 (unsigned int) (offset & 0xFFFFFFFF),
63 (unsigned int) (offset >> 32),
64 count);
65 Since the offset argument is 64 bits, and the first argument (fd) is
67 passed in r0, the caller must manually split and align the 64-bit value
68 so that it is passed in the r2/r3 register pair. That means inserting
69 a dummy value into r1 (the second argument of 0). Care also must be
70 taken so that the split follows endian conventions (according to the C
71 ABI for the platform).
72 Similar issues can occur on MIPS with the O32 ABI, on PowerPC and
74 parisc with the 32-bit ABI, and on Xtensa.
75 Note that while the parisc C ABI also uses aligned register pairs, it
77 uses a shim layer to hide the issue from user space.
78 The affected system calls are fadvise64_64(2), ftruncate64(2),
80 posix_fadvise(2), pread64(2), pwrite64(2), readahead(2),
81 sync_file_range(2), and truncate64(2).
82 This does not affect syscalls that manually split and assemble 64-bit
84 values such as _llseek(2), preadv(2), preadv2(2), pwritev(2), and
85 pwritev2(2). Welcome to the wonderful world of historical baggage.
86 Architecture calling conventions
88 Every architecture has its own way of invoking and passing arguments to
89 the kernel. The details for various architectures are listed in the
90 two tables below.
91 The first table lists the instruction used to transition to kernel mode
93 (which might not be the fastest or best way to transition to the ker‐
94 nel, so you might have to refer to vdso(7)), the register used to indi‐
95 cate the system call number, the register(s) used to return the system
96 call result, and the register used to signal an error.
97 Arch/ABI Instruction System Ret Ret Error Notes
99 call # val val2
100 ───────────────────────────────────────────────────────────────────
101 alpha callsys v0 v0 a4 a3 1, 6
102 arc trap0 r8 r0 - -
103 arm/OABI swi NR - r0 - - 2
104 arm/EABI swi 0x0 r7 r0 r1 -
105 arm64 svc #0 w8 x0 x1 -
106 blackfin excpt 0x0 P0 R0 - -
107 i386 int $0x80 eax eax edx -
108 ia64 break 0x100000 r15 r8 r9 r10 1, 6
109 loongarch syscall 0 a7 a0 - -
110 m68k trap #0 d0 d0 - -
111 microblaze brki r14,8 r12 r3 - -
112 mips syscall v0 v0 v1 a3 1, 6
113 nios2 trap r2 r2 - r7
114 parisc ble 0x100(%sr2, %r0) r20 r28 - -
115 powerpc sc r0 r3 - r0 1
116 powerpc64 sc r0 r3 - cr0.SO 1
117 riscv ecall a7 a0 a1 -
118 s390 svc 0 r1 r2 r3 - 3
119 s390x svc 0 r1 r2 r3 - 3
120 superh trapa #31 r3 r0 r1 - 4, 6
121 sparc/32 t 0x10 g1 o0 o1 psr/csr 1, 6
122 sparc/64 t 0x6d g1 o0 o1 psr/csr 1, 6
123 tile swint1 R10 R00 - R01 1
124 x86-64 syscall rax rax rdx - 5
125 x32 syscall rax rax rdx - 5
126 xtensa syscall a2 a2 - -
127 Notes:
129 • On a few architectures, a register is used as a boolean (0 indicat‐
131 ing no error, and -1 indicating an error) to signal that the system
132 call failed. The actual error value is still contained in the re‐
133 turn register. On sparc, the carry bit (csr) in the processor sta‐
134 tus register (psr) is used instead of a full register. On pow‐
135 erpc64, the summary overflow bit (SO) in field 0 of the condition
136 register (cr0) is used.
137 • NR is the system call number.
139 • For s390 and s390x, NR (the system call number) may be passed di‐
141 rectly with svc NR if it is less than 256.
142 • On SuperH additional trap numbers are supported for historic rea‐
144 sons, but trapa#31 is the recommended "unified" ABI.
145 • The x32 ABI shares syscall table with x86-64 ABI, but there are some
147 nuances:
148 • In order to indicate that a system call is called under the x32
150 ABI, an additional bit, __X32_SYSCALL_BIT, is bitwise-ORed with
151 the system call number. The ABI used by a process affects some
152 process behaviors, including signal handling or system call
153 restarting.
154 • Since x32 has different sizes for long and pointer types, layouts
156 of some (but not all; struct timeval or struct rlimit are 64-bit,
157 for example) structures are different. In order to handle this,
158 additional system calls are added to the system call table,
159 starting from number 512 (without the __X32_SYSCALL_BIT). For
160 example, __NR_readv is defined as 19 for the x86-64 ABI and as
161 __X32_SYSCALL_BIT | 515 for the x32 ABI. Most of these addi‐
162 tional system calls are actually identical to the system calls
163 used for providing i386 compat. There are some notable excep‐
164 tions, however, such as preadv2(2), which uses struct iovec enti‐
165 ties with 4-byte pointers and sizes ("compat_iovec" in kernel
166 terms), but passes an 8-byte pos argument in a single register
167 and not two, as is done in every other ABI.
168 • Some architectures (namely, Alpha, IA-64, MIPS, SuperH, sparc/32,
170 and sparc/64) use an additional register ("Retval2" in the above ta‐
171 ble) to pass back a second return value from the pipe(2) system
172 call; Alpha uses this technique in the architecture-specific getx‐
173 pid(2), getxuid(2), and getxgid(2) system calls as well. Other ar‐
174 chitectures do not use the second return value register in the sys‐
175 tem call interface, even if it is defined in the System V ABI.
176 The second table shows the registers used to pass the system call argu‐
178 ments.
179 Arch/ABI arg1 arg2 arg3 arg4 arg5 arg6 arg7 Notes
181 ──────────────────────────────────────────────────────────────
182 alpha a0 a1 a2 a3 a4 a5 -
183 arc r0 r1 r2 r3 r4 r5 -
184 arm/OABI r0 r1 r2 r3 r4 r5 r6
185 arm/EABI r0 r1 r2 r3 r4 r5 r6
186 arm64 x0 x1 x2 x3 x4 x5 -
187 blackfin R0 R1 R2 R3 R4 R5 -
188 i386 ebx ecx edx esi edi ebp -
189 ia64 out0 out1 out2 out3 out4 out5 -
190 loongarch a0 a1 a2 a3 a4 a5 a6
191 m68k d1 d2 d3 d4 d5 a0 -
192 microblaze r5 r6 r7 r8 r9 r10 -
193 mips/o32 a0 a1 a2 a3 - - - 1
194 mips/n32,64 a0 a1 a2 a3 a4 a5 -
195 nios2 r4 r5 r6 r7 r8 r9 -
196 parisc r26 r25 r24 r23 r22 r21 -
197 powerpc r3 r4 r5 r6 r7 r8 r9
198 powerpc64 r3 r4 r5 r6 r7 r8 -
200 riscv a0 a1 a2 a3 a4 a5 -
201 s390 r2 r3 r4 r5 r6 r7 -
202 s390x r2 r3 r4 r5 r6 r7 -
203 superh r4 r5 r6 r7 r0 r1 r2
204 sparc/32 o0 o1 o2 o3 o4 o5 -
205 sparc/64 o0 o1 o2 o3 o4 o5 -
206 tile R00 R01 R02 R03 R04 R05 -
207 x86-64 rdi rsi rdx r10 r8 r9 -
208 x32 rdi rsi rdx r10 r8 r9 -
209 xtensa a6 a3 a4 a5 a8 a9 -
210 Notes:
212 • The mips/o32 system call convention passes arguments 5 through 8 on
214 the user stack.
215 Note that these tables don't cover the entire calling convention—some
217 architectures may indiscriminately clobber other registers not listed
218 here.
219
221 #define _GNU_SOURCE
222 #include <signal.h>
223 #include <sys/syscall.h>
224 #include <unistd.h>
225 int
227 main(void)
228 {
229 pid_t tid;
230 tid = syscall(SYS_gettid);
232 syscall(SYS_tgkill, getpid(), tid, SIGHUP);
233 }
234
236 _syscall(2), intro(2), syscalls(2), errno(3), vdso(7)
237Linux man-pages 6.04 2023-02-05 syscall(2)