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__syscall(2) [netbsd man page]

SYSCALL(2)						      BSD System Calls Manual							SYSCALL(2)

syscall, __syscall -- indirect system call LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/syscall.h> #include <unistd.h> int syscall(int number, ...); quad_t __syscall(quad_t number, ...); DESCRIPTION
syscall() performs the system call whose assembly language interface has the specified number with the specified arguments. Symbolic con- stants for system calls can be found in the header file <sys/syscall.h>. The __syscall form should be used when one or more of the parame- ters is a 64-bit argument to ensure that argument alignment is correct. This system call is useful for testing new system calls that do not have entries in the C library. It should not be used in normal applica- tions. RETURN VALUES
The return values are defined by the system call being invoked. In general, a 0 return value indicates success. A -1 return value indicates an error, and an error code is stored in errno. HISTORY
The syscall() function call appeared in 4.0BSD. BUGS
There is no way to simulate system calls that have multiple return values such as pipe(2). Since architectures return 32 bit and 64 bit results in different registers, it may be impossible to portably convert the result of __syscall() to a 32bit value. For instance sparc returns 32 bit values in %o0 and 64 bit values in %o0:%o1 (with %o0 containing the most significant part) so a 32 bit right shift of the result is needed to get a correct 32 bit result. Many architectures mask off the unwanted high bits of the syscall number, rather than returning an error. Due to ABI implementation differences in passing struct or union type arguments to system calls between different processors, all system calls pass instead pointers to such structs or unions, even when the documentation of the system call mentions otherwise. The conversion between passing structs and unions is handled normally via userland stubs. The correct arguments for the kernel entry points for each system call can be found in the header file <sys/syscallargs.h> BSD
August 7, 2009 BSD

Check Out this Related Man Page

INTRO(2)						     Linux Programmer's Manual							  INTRO(2)

intro - Introduction to system calls DESCRIPTION
Section 2 of the manual describes the Linux system calls. A system call is an entry point into the Linux kernel. Usually, system calls are not invoked directly: instead, most system calls have corresponding C library wrapper functions which perform the steps required (e.g., trapping to kernel mode) in order to invoke the system call. Thus, making a system call looks the same as invoking a normal library func- tion. For a list of the Linux system calls, see syscalls(2). RETURN VALUE
On error, most system calls return a negative error number (i.e., the negated value of one of the constants described in errno(3)). The C library wrapper hides this detail from the caller: when a system call returns a negative value, the wrapper copies the absolute value into the errno variable, and returns -1 as the return value of the wrapper. The value returned by a successful system call depends on the call. Many system calls return 0 on success, but some can return nonzero values from a successful call. The details are described in the individual manual pages. In some cases, the programmer must define a feature test macro in order to obtain the declaration of a system call from the header file specified in the man page SYNOPSIS section. (Where required, these feature test macros must be defined before including any header files.) In such cases, the required macro is described in the man page. For further information on feature test macros, see fea- ture_test_macros(7). CONFORMING TO
Certain terms and abbreviations are used to indicate Unix variants and standards to which calls in this section conform. See standards(7). NOTES
Calling Directly In most cases, it is unnecessary to invoke a system call directly, but there are times when the Standard C library does not implement a nice wrapper function for you. In this case, the programmer must manually invoke the system call using syscall(2). Historically, this was also possible using one of the _syscall macros described in _syscall(2). Authors and Copyright Conditions Look at the header of the manual page source for the author(s) and copyright conditions. Note that these can be different from page to page! SEE ALSO
_syscall(2), syscall(2), errno(3), feature_test_macros(7), standards(7) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at Linux 2010-09-10 INTRO(2)
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