1ERRNO(3P)                  POSIX Programmer's Manual                 ERRNO(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
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NAME

12       errno - error return value
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SYNOPSIS

15       #include <errno.h>
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DESCRIPTION

18       The lvalue errno is used by many functions to return error values.
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20       Many functions provide an error number in errno, which has type int and
21       is defined in <errno.h>. The value of errno shall be defined only after
22       a call to a function for which it is explicitly stated to  be  set  and
23       until  it  is  changed  by the next function call or if the application
24       assigns it a value. The value of errno should only be examined when  it
25       is  indicated  to  be valid by a function's return value.  Applications
26       shall obtain the definition of errno by the inclusion of <errno.h>.  No
27       function in this volume of IEEE Std 1003.1-2001 shall set errno to 0.
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29       It  is  unspecified  whether errno is a macro or an identifier declared
30       with external linkage. If a macro definition is suppressed in order  to
31       access  an  actual  object, or a program defines an identifier with the
32       name errno, the behavior is undefined.
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34       The symbolic values stored in errno are documented in the  ERRORS  sec‐
35       tions on all relevant pages.
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RETURN VALUE

38       None.
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ERRORS

41       None.
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43       The following sections are informative.
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EXAMPLES

46       None.
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APPLICATION USAGE

49       Previously  both  POSIX and X/Open documents were more restrictive than
50       the ISO C standard in that they required errno  to  be  defined  as  an
51       external  variable, whereas the ISO C standard required only that errno
52       be defined as a modifiable lvalue with type int.
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54       An application that needs to examine the value of  errno  to  determine
55       the  error  should  set it to 0 before a function call, then inspect it
56       before a subsequent function call.
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RATIONALE

59       None.
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FUTURE DIRECTIONS

62       None.
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SEE ALSO

65       Error Numbers, the Base  Definitions  volume  of  IEEE Std 1003.1-2001,
66       <errno.h>
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69       Portions  of  this text are reprinted and reproduced in electronic form
70       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
71       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
72       Specifications Issue 6, Copyright (C) 2001-2003  by  the  Institute  of
73       Electrical  and  Electronics  Engineers, Inc and The Open Group. In the
74       event of any discrepancy between this version and the original IEEE and
75       The  Open Group Standard, the original IEEE and The Open Group Standard
76       is the referee document. The original Standard can be  obtained  online
77       at http://www.opengroup.org/unix/online.html .
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81IEEE/The Open Group                  2003                            ERRNO(3P)
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