1SYSCALL(2) Linux Programmer's Manual SYSCALL(2)
2
6 syscall - indirect system call
7
9 #define _GNU_SOURCE /* See feature_test_macros(7) */
10 #include <unistd.h>
11 #include <sys/syscall.h> /* For SYS_xxx definitions */
12 long syscall(long number, ...);
14
16 syscall() is a small library function that invokes the system call
17 whose assembly language interface has the specified number with the
18 specified arguments. Employing syscall() is useful, for example, when
19 invoking a system call that has no wrapper function in the C library.
20 syscall() saves CPU registers before making the system call, restores
22 the registers upon return from the system call, and stores any error
23 code returned by the system call in errno(3) if an error occurs.
24 Symbolic constants for system call numbers can be found in the header
26 file <sys/syscall.h>.
27
29 The return value is defined by the system call being invoked. In gen‐
30 eral, a 0 return value indicates success. A -1 return value indicates
31 an error, and an error code is stored in errno.
32
34 syscall() first appeared in 4BSD.
35 Architecture-specific requirements
37 Each architecture ABI has its own requirements on how system call argu‐
38 ments are passed to the kernel. For system calls that have a glibc
39 wrapper (e.g., most system calls), glibc handles the details of copying
40 arguments to the right registers in a manner suitable for the architec‐
41 ture. However, when using syscall() to make a system call, the caller
42 might need to handle architecture-dependent details; this requirement
43 is most commonly encountered on certain 32-bit architectures.
44 For example, on the ARM architecture Embedded ABI (EABI), a 64-bit
46 value (e.g., long long) must be aligned to an even register pair.
47 Thus, using syscall() instead of the wrapper provided by glibc, the
48 readahead() system call would be invoked as follows on the ARM archi‐
49 tecture with the EABI in little endian mode:
50 syscall(SYS_readahead, fd, 0,
52 (unsigned int) (offset & 0xFFFFFFFF),
53 (unsigned int) (offset >> 32),
54 count);
55 Since the offset argument is 64 bits, and the first argument (fd) is
57 passed in r0, the caller must manually split and align the 64-bit value
58 so that it is passed in the r2/r3 register pair. That means inserting
59 a dummy value into r1 (the second argument of 0). Care also must be
60 taken so that the split follows endian conventions (according to the C
61 ABI for the platform).
62 Similar issues can occur on MIPS with the O32 ABI, on PowerPC with the
64 32-bit ABI, and on Xtensa.
65 Note that while the parisc C ABI also uses aligned register pairs, it
67 uses a shim layer to hide the issue from userspace.
68 The affected system calls are fadvise64_64(2), ftruncate64(2),
70 posix_fadvise(2), pread64(2), pwrite64(2), readahead(2),
71 sync_file_range(2), and truncate64(2).
72 This does not affect syscalls that manually split and assemble 64-bit
74 values such as _llseek(2), preadv(2), preadv2(2), pwritev(2). and
75 pwritev2(2). Welcome to the wonderful world of historical baggage.
76 Architecture calling conventions
78 Every architecture has its own way of invoking and passing arguments to
79 the kernel. The details for various architectures are listed in the
80 two tables below.
81 The first table lists the instruction used to transition to kernel mode
83 (which might not be the fastest or best way to transition to the ker‐
84 nel, so you might have to refer to vdso(7)), the register used to indi‐
85 cate the system call number, the register used to return the system
86 call result, and the register used to signal an error.
87 arch/ABI instruction syscall # retval error Notes
89 ────────────────────────────────────────────────────────────────────
90 alpha callsys v0 a0 a3 [1]
91 arc trap0 r8 r0 -
92 arm/OABI swi NR - a1 - [2]
93 arm/EABI swi 0x0 r7 r0 -
94 arm64 svc #0 x8 x0 -
95 blackfin excpt 0x0 P0 R0 -
96 i386 int $0x80 eax eax -
97 ia64 break 0x100000 r15 r8 r10 [1]
98 m68k trap #0 d0 d0 -
99 microblaze brki r14,8 r12 r3 -
100 mips syscall v0 v0 a3 [1]
101 nios2 trap r2 r2 r7
102 parisc ble 0x100(%sr2, %r0) r20 r28 -
103 powerpc sc r0 r3 r0 [1]
104 s390 svc 0 r1 r2 - [3]
105 s390x svc 0 r1 r2 - [3]
106 superh trap #0x17 r3 r0 - [4]
107 sparc/32 t 0x10 g1 o0 psr/csr [1]
108 sparc/64 t 0x6d g1 o0 psr/csr [1]
109 tile swint1 R10 R00 R01 [1]
110 x86-64 syscall rax rax - [5]
111 x32 syscall rax rax - [5]
112 xtensa syscall a2 a2 -
113 Notes:
115 [1] On a few architectures, a register is used as a boolean (0
117 indicating no error, and -1 indicating an error) to signal that
118 the system call failed. The actual error value is still con‐
119 tained in the return register. On sparc, the carry bit (csr)
120 in the processor status register (psr) is used instead of a
121 full register.
122 [2] NR is the system call number.
124 [3] For s390 and s390x, NR (the system call number) may be passed
126 directly with svc NR if it is less than 256.
127 [4] On SuperH, the trap number controls the maximum number of argu‐
129 ments passed. A trap #0x10 can be used with only 0-argument
130 system calls, a trap #0x11 can be used with 0- or 1-argument
131 system calls, and so on up to trap #0x17 for 7-argument system
132 calls.
133 [5] The x32 ABI uses the same instruction as the x86-64 ABI and is
135 used on the same processors. To differentiate between them,
136 the bit mask __X32_SYSCALL_BIT is bitwise-ORed into the system
137 call number for system calls under the x32 ABI. Both system
138 call tables are available though, so setting the bit is not a
139 hard requirement.
140 The second table shows the registers used to pass the system call argu‐
142 ments.
143 arch/ABI arg1 arg2 arg3 arg4 arg5 arg6 arg7 Notes
145 ──────────────────────────────────────────────────────────────
146 alpha a0 a1 a2 a3 a4 a5 -
147 arc r0 r1 r2 r3 r4 r5 -
148 arm/OABI a1 a2 a3 a4 v1 v2 v3
149 arm/EABI r0 r1 r2 r3 r4 r5 r6
150 arm64 x0 x1 x2 x3 x4 x5 -
151 blackfin R0 R1 R2 R3 R4 R5 -
152 i386 ebx ecx edx esi edi ebp -
153 ia64 out0 out1 out2 out3 out4 out5 -
154 m68k d1 d2 d3 d4 d5 a0 -
155 microblaze r5 r6 r7 r8 r9 r10 -
156 mips/o32 a0 a1 a2 a3 - - - [1]
157 mips/n32,64 a0 a1 a2 a3 a4 a5 -
158 nios2 r4 r5 r6 r7 r8 r9 -
159 parisc r26 r25 r24 r23 r22 r21 -
160 powerpc r3 r4 r5 r6 r7 r8 r9
161 s390 r2 r3 r4 r5 r6 r7 -
162 s390x r2 r3 r4 r5 r6 r7 -
163 superh r4 r5 r6 r7 r0 r1 r2
164 sparc/32 o0 o1 o2 o3 o4 o5 -
165 sparc/64 o0 o1 o2 o3 o4 o5 -
166 tile R00 R01 R02 R03 R04 R05 -
167 x86-64 rdi rsi rdx r10 r8 r9 -
168 x32 rdi rsi rdx r10 r8 r9 -
169 xtensa a6 a3 a4 a5 a8 a9 -
170 Notes:
172 [1] The mips/o32 system call convention passes arguments 5 through
174 8 on the user stack.
175 Note that these tables don't cover the entire calling convention—some
177 architectures may indiscriminately clobber other registers not listed
178 here.
179
181 #define _GNU_SOURCE
182 #include <unistd.h>
183 #include <sys/syscall.h>
184 #include <sys/types.h>
185 #include <signal.h>
186 int
188 main(int argc, char *argv[])
189 {
190 pid_t tid;
191 tid = syscall(SYS_gettid);
193 syscall(SYS_tgkill, getpid(), tid, SIGHUP);
194 }
195
197 _syscall(2), intro(2), syscalls(2), errno(3), vdso(7)
198
200 This page is part of release 4.15 of the Linux man-pages project. A
201 description of the project, information about reporting bugs, and the
202 latest version of this page, can be found at
203 https://www.kernel.org/doc/man-pages/.
204Linux 2017-09-15 SYSCALL(2)