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34 .\" @(#)syscall.2 8.1 (Berkeley) 6/16/93
37 .\" 2002-03-20 Christoph Hellwig <hch@infradead.org>
38 .\" - adopted for Linux
40 .TH SYSCALL 2 2013-04-07 "Linux" "Linux Programmer's Manual"
42 syscall \- indirect system call
45 .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
46 .B #include <unistd.h>
47 .BR "#include <sys/syscall.h> " "/* For SYS_xxx definitions */"
49 .BI "int syscall(int " number ", ...);"
53 is a small library function that invokes
54 the system call whose assembly language
55 interface has the specified
57 with the specified arguments.
60 is useful, for example,
61 when invoking a system call that has no wrapper function in the C library.
64 saves CPU registers before making the system call,
65 restores the registers upon return from the system call,
66 and stores any error code returned by the system call in
70 Symbolic constants for system call numbers can be found in the header file
73 The return value is defined by the system call being invoked.
74 In general, a 0 return value indicates success.
75 A \-1 return value indicates an error,
76 and an error code is stored in
82 .SS Architecture-specific requirements
83 Each architecture ABI has its own requirements on how
84 system call arguments are passed to the kernel.
85 For system calls that have a glibc wrapper (e.g., most system calls),
86 glibc handles the details of copying arguments to the right registers
87 in a manner suitable for the architecture.
90 to make a system call,
91 the caller might need to handle architecture-dependent details;
92 this requirement is most commonly encountered on certain 32-bit architectures.
94 For example, on the ARM architecture Embedded ABI (EABI), a
97 must be aligned to an even register pair.
100 instead of the wrapper provided by glibc,
103 system call would be invoked as follows on the ARM architecture with the EABI:
107 syscall(SYS_readahead, fd, 0,
108 (unsigned int) (offset >> 32),
109 (unsigned int) (offset & 0xFFFFFFFF),
114 Since the offset argument is 64 bits, and the first argument
118 the caller must manually split and align the 64-bit value
119 so that it is passed in the
122 That means inserting a dummy value into
124 (the second argument of 0).
126 Similar issues can occur on MIPS with the O32 ABI,
127 on PowerPC with the 32-bit ABI, and on Xtensa.
128 .\" Mike Frysinger: this issue ends up forcing MIPS
129 .\" O32 to take 7 arguments to syscall()
131 The affected system calls are
132 .BR fadvise64_64 (2),
134 .BR posix_fadvise (2),
138 .BR sync_file_range (2),
141 .SS Architecture calling conventions
142 Every architecture has its own way of invoking and passing arguments to the
144 The details for various architectures are listed in the two tables below.
146 The first table lists the instruction used to transition to kernel mode,
147 (which might not be the fastest or best way to transition to the kernel,
148 so you might have to refer to the VDSO),
149 the register used to indicate the system call number,
150 and the register used to return the system call result.
156 arch/ABI instruction syscall # retval Notes
158 arm/OABI swi NR - a1 NR is syscall #
159 arm/EABI swi 0x0 r7 r1
160 blackfin excpt 0x0 P0 R0
161 i386 int $0x80 eax eax
162 ia64 break 0x100000 r15 r10/r8
163 parisc ble 0x100(%sr2, %r0) r20 r28
164 sparc/32 t 0x10 g1 o0
165 sparc/64 t 0x6d g1 o0
166 x86_64 syscall rax rax
173 The second table shows the registers used to pass the system call arguments.
179 arch/ABI arg1 arg2 arg3 arg4 arg5 arg6 arg7
181 arm/OABI a1 a2 a3 a4 v1 v2 v3
182 arm/EABI r1 r2 r3 r4 r5 r6 r7
183 blackfin R0 R1 R2 R3 R4 R5 -
184 i386 ebx ecx edx esi edi ebp -
185 ia64 r11 r9 r10 r14 r15 r13 -
186 parisc r26 r25 r24 r23 r22 r21 -
187 sparc/32 o0 o1 o2 o3 o4 o5 -
188 sparc/64 o0 o1 o2 o3 o4 o5 -
189 x86_64 rdi rsi rdx r10 r8 r9 -
196 Note that these tables don't cover the entire calling convention\(emsome
197 architectures may indiscriminately clobber other registers not listed here.
202 #include <sys/syscall.h>
203 #include <sys/types.h>
206 main(int argc, char *argv[])
210 tid = syscall(SYS_gettid);
211 tid = syscall(SYS_tgkill, getpid(), tid);