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 int syscall(int 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:
50 syscall(SYS_readahead, fd, 0,
52 (unsigned int) (offset >> 32),
53 (unsigned int) (offset & 0xFFFFFFFF),
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).
60 Similar issues can occur on MIPS with the O32 ABI, on PowerPC with the
62 32-bit ABI, and on Xtensa.
63 The affected system calls are fadvise64_64(2), ftruncate64(2),
65 posix_fadvise(2), pread64(2), pwrite64(2), readahead(2),
66 sync_file_range(2), and truncate64(2).
67 Architecture calling conventions
69 Every architecture has its own way of invoking and passing arguments to
70 the kernel. The details for various architectures are listed in the
71 two tables below.
72 The first table lists the instruction used to transition to kernel
74 mode, (which might not be the fastest or best way to transition to the
75 kernel, so you might have to refer to the VDSO), the register used to
76 indicate the system call number, and the register used to return the
77 system call result.
78 arch/ABI instruction syscall # retval Notes
80 ───────────────────────────────────────────────────────────────────────────────────
81 arm/OABI swi NR - a1 NR is syscall #
82 arm/EABI swi 0x0 r7 r1
83 blackfin excpt 0x0 P0 R0
84 i386 int $0x80 eax eax
85 ia64 break 0x100000 r15 r10/r8
86 parisc ble 0x100(%sr2, %r0) r20 r28
87 s390 svc 0 r1 r2 NR may be passed directly with
88 s390x svc 0 r1 r2 "svc NR" if NR is less than 256
89 sparc/32 t 0x10 g1 o0
90 sparc/64 t 0x6d g1 o0
91 x86_64 syscall rax rax
92 The second table shows the registers used to pass the system call argu‐
94 ments.
95 arch/ABI arg1 arg2 arg3 arg4 arg5 arg6 arg7
97 ──────────────────────────────────────────────────────────
98 arm/OABI a1 a2 a3 a4 v1 v2 v3
99 arm/EABI r1 r2 r3 r4 r5 r6 r7
100 blackfin R0 R1 R2 R3 R4 R5 -
101 i386 ebx ecx edx esi edi ebp -
102 ia64 r11 r9 r10 r14 r15 r13 -
103 parisc r26 r25 r24 r23 r22 r21 -
104 s390 r2 r3 r4 r5 r6 r7 -
105 s390x r2 r3 r4 r5 r6 r7 -
106 sparc/32 o0 o1 o2 o3 o4 o5 -
107 sparc/64 o0 o1 o2 o3 o4 o5 -
108 x86_64 rdi rsi rdx r10 r8 r9 -
109 Note that these tables don't cover the entire calling convention—some
111 architectures may indiscriminately clobber other registers not listed
112 here.
113
115 #define _GNU_SOURCE
116 #include <unistd.h>
117 #include <sys/syscall.h>
118 #include <sys/types.h>
119 int
121 main(int argc, char *argv[])
122 {
123 pid_t tid;
124 tid = syscall(SYS_gettid);
126 tid = syscall(SYS_tgkill, getpid(), tid);
127 }
128
130 _syscall(2), intro(2), syscalls(2)
131
133 This page is part of release 3.53 of the Linux man-pages project. A
134 description of the project, and information about reporting bugs, can
135 be found at http://www.kernel.org/doc/man-pages/.
136Linux 2013-06-21 SYSCALL(2)