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intro(3) [linux man page]

INTRO(3)						     Linux Programmer's Manual							  INTRO(3)

NAME
intro - Introduction to library functions DESCRIPTION
Section 3 of the manual describes all library functions excluding the library functions (system call wrappers) described in section 2, which implement system calls. Many of the functions described in the section are part of the Standard C Library (libc). Some functions are part of other libraries (e.g., the math library, libm, or the real-time library, librt) in which case the manual page will indicate the linker option needed to link against the required library (e.g., -lm and -lrt, respectively, for the aforementioned libraries). In some cases, the programmer must define a feature test macro in order to obtain the declaration of a function from the header file speci- fied 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 feature_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
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
intro(2), errno(3), feature_test_macros(7), libc(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 http://www.kernel.org/doc/man-pages/. Linux 2010-09-10 INTRO(3)

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

NAME
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. In such cases, the required macro is described in the man page. For further information on feature test macros, see feature_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 3.25 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2010-02-03 INTRO(2)

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