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<TITLE>The GNU C Library - Error Reporting</TITLE>
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<H1><A NAME="SEC14" HREF="library_toc.html#SEC14" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC14">Error Reporting</A></H1>
<P>
Many functions in the GNU C library detect and report error conditions,
and sometimes your programs need to check for these error conditions.
For example, when you open an input file, you should verify that the
file was actually opened correctly, and print an error message or take
other appropriate action if the call to the library function failed.
<P>
This chapter describes how the error reporting facility works.  Your
program should include the header file <TT>`errno.h'</TT> to use this
facility.
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<P>
<H2><A NAME="SEC15" HREF="library_toc.html#SEC15" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC15">Checking for Errors</A></H2>
<P>
Most library functions return a special value to indicate that they have
failed.  The special value is typically <CODE>-1</CODE>, a null pointer, or a
constant such as <CODE>EOF</CODE> that is defined for that purpose.  But this
return value tells you only that an error has occurred.  To find out
what kind of error it was, you need to look at the error code stored in the
variable <CODE>errno</CODE>.  This variable is declared in the header file
<TT>`errno.h'</TT>.
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<P>
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<U>Variable:</U> volatile int <B>errno</B><P>
The variable <CODE>errno</CODE> contains the system error number.  You can
change the value of <CODE>errno</CODE>.
<P>
Since <CODE>errno</CODE> is declared <CODE>volatile</CODE>, it might be changed
asynchronously by a signal handler; see section <A HREF="library_21.html#SEC351" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_21.html#SEC351">Defining Signal Handlers</A>.
However, a properly written signal handler saves and restores the value
of <CODE>errno</CODE>, so you generally do not need to worry about this
possibility except when writing signal handlers.
<P>
The initial value of <CODE>errno</CODE> at program startup is zero.  Many
library functions are guaranteed to set it to certain nonzero values
when they encounter certain kinds of errors.  These error conditions are
listed for each function.  These functions do not change <CODE>errno</CODE>
when they succeed; thus, the value of <CODE>errno</CODE> after a successful
call is not necessarily zero, and you should not use <CODE>errno</CODE> to
determine <EM>whether</EM> a call failed.  The proper way to do that is
documented for each function.  <EM>If</EM> the call the failed, you can
examine <CODE>errno</CODE>.
<P>
Many library functions can set <CODE>errno</CODE> to a nonzero value as a
result of calling other library functions which might fail.  You should
assume that any library function might alter <CODE>errno</CODE>.
<P>
<STRONG>Portability Note:</STRONG> ANSI C specifies <CODE>errno</CODE> as a
"modifiable lvalue" rather than as a variable, permitting it to be
implemented as a macro.  For example, its expansion might involve a
function call, like <CODE>*_errno ()</CODE>.  In fact, that is what it is
on the GNU system itself.  The GNU library, on non-GNU systems, does
whatever is right for the particular system.
<P>
There are a few library functions, like <CODE>sqrt</CODE> and <CODE>atan</CODE>,
that return a perfectly legitimate value in case of an error, but also
set <CODE>errno</CODE>.  For these functions, if you want to check to see
whether an error occurred, the recommended method is to set <CODE>errno</CODE>
to zero before calling the function, and then check its value afterward.
<P>
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<P>
All the error codes have symbolic names; they are macros defined in
<TT>`errno.h'</TT>.  The names start with <SAMP>`E'</SAMP> and an upper-case
letter or digit; you should consider names of this form to be
reserved names.  See section <A HREF="library_1.html#SEC11" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_1.html#SEC11">Reserved Names</A>.
<P>
The error code values are all positive integers and are all distinct.
(Since the values are distinct, you can use them as labels in a
<CODE>switch</CODE> statement, for example.)  Your program should not make any
other assumptions about the specific values of these symbolic constants.
<P>
The value of <CODE>errno</CODE> doesn't necessarily have to correspond to any
of these macros, since some library functions might return other error
codes of their own for other situations.  The only values that are
guaranteed to be meaningful for a particular library function are the
ones that this manual lists for that function.
<P>
On non-GNU systems, almost any system call can return <CODE>EFAULT</CODE> if
it is given an invalid pointer as an argument.  Since this could only
happen as a result of a bug in your program, and since it will not
happen on the GNU system, we have saved space by not mentioning
<CODE>EFAULT</CODE> in the descriptions of individual functions.
<P>
<H2><A NAME="SEC16" HREF="library_toc.html#SEC16" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC16">Error Codes</A></H2>
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<P>
The error code macros are defined in the header file <TT>`errno.h'</TT>.
All of them expand into integer constant values.  Some of these error
codes can't occur on the GNU system, but they can occur using the GNU
library on other systems.
<P>
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<U>Macro:</U> int <B>EPERM</B><P>
Operation not permitted; only the owner of the file (or other resource)
or processes with special privileges can perform the operation.
<P>
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<U>Macro:</U> int <B>ENOENT</B><P>
No such file or directory.  This is a "file doesn't exist" error
for ordinary files that are referenced in contexts where they are
expected to already exist.
<P>
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<U>Macro:</U> int <B>ESRCH</B><P>
No process matches the specified process ID.
<P>
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<U>Macro:</U> int <B>EINTR</B><P>
Interrupted function call; an asynchronous signal occured and prevented
completion of the call.  When this happens, you should try the call
again.
<P>
You can choose to have functions resume after a signal that is handled,
rather than failing with <CODE>EINTR</CODE>; see section <A HREF="library_21.html#SEC362" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_21.html#SEC362">Primitives Interrupted by Signals</A>.
<P>
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<U>Macro:</U> int <B>EIO</B><P>
Input/output error; usually used for physical read or write errors.
<P>
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<U>Macro:</U> int <B>ENXIO</B><P>
No such device or address.  Typically, this means that a file
representing a device has been installed incorrectly, and the
system can't find the right kind of device driver for it.
<P>
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<U>Macro:</U> int <B>E2BIG</B><P>
Argument list too long; used when the arguments passed to a new program
being executed with one of the <CODE>exec</CODE> functions (see section <A HREF="library_23.html#SEC406" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_23.html#SEC406">Executing a File</A>) occupy too much memory space.  This condition never arises in the
GNU system.
<P>
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<U>Macro:</U> int <B>ENOEXEC</B><P>
Invalid executable file format.  This condition is detected by the
<CODE>exec</CODE> functions; see section <A HREF="library_23.html#SEC406" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_23.html#SEC406">Executing a File</A>.
<P>
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<U>Macro:</U> int <B>EBADF</B><P>
Bad file descriptor; for example, I/O on a descriptor that has been
closed or reading from a descriptor open only for writing (or vice
versa).
<P>
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<U>Macro:</U> int <B>ECHILD</B><P>
There are no child processes.  This error happens on operations that are
supposed to manipulate child processes, when there aren't any processes
to manipulate.
<P>
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<U>Macro:</U> int <B>EDEADLK</B><P>
Deadlock avoided; allocating a system resource would have resulted in
a deadlock situation.  For an example, See section <A HREF="library_12.html#SEC185" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_12.html#SEC185">File Locks</A>.
<P>
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<U>Macro:</U> int <B>ENOMEM</B><P>
No memory available.  The system cannot allocate more virtual memory
because its capacity is full.
<P>
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<U>Macro:</U> int <B>EACCES</B><P>
Permission denied; the file permissions do not allow the attempted operation.
<P>
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<U>Macro:</U> int <B>EFAULT</B><P>
Bad address; an invalid pointer was detected.
<P>
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<U>Macro:</U> int <B>ENOTBLK</B><P>
A file that isn't a block special file was given in a situation that
requires one.  For example, trying to mount an ordinary file as a file
system in Unix gives this error.
<P>
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<U>Macro:</U> int <B>EBUSY</B><P>
Resource busy; a system resource that can't be shared is already in use.
For example, if you try to delete a file that is the root of a currently
mounted filesystem, you get this error.
<P>
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<U>Macro:</U> int <B>EEXIST</B><P>
File exists; an existing file was specified in a context where it only
makes sense to specify a new file.
<P>
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<U>Macro:</U> int <B>EXDEV</B><P>
An attempt to make an improper link across file systems was detected.
<P>
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<U>Macro:</U> int <B>ENODEV</B><P>
The wrong type of device was given to a function that expects a
particular sort of device.
<P>
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<U>Macro:</U> int <B>ENOTDIR</B><P>
A file that isn't a directory was specified when a directory is required.
<P>
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<U>Macro:</U> int <B>EISDIR</B><P>
File is a directory; attempting to open a directory for writing gives
this error.
<P>
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<U>Macro:</U> int <B>EINVAL</B><P>
Invalid argument.  This is used to indicate various kinds of problems
with passing the wrong argument to a library function.
<P>
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<U>Macro:</U> int <B>ENFILE</B><P>
There are too many distinct file openings in the entire system.  Note
that any number of linked channels count as just one file opening; see
section <A HREF="library_12.html#SEC177" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_12.html#SEC177">Linked Channels</A>.
<P>
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<U>Macro:</U> int <B>EMFILE</B><P>
The current process has too many files open and can't open any more.
Duplicate descriptors do count toward this limit.
<P>
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<U>Macro:</U> int <B>ENOTTY</B><P>
Inappropriate I/O control operation, such as trying to set terminal
modes on an ordinary file.
<P>
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<U>Macro:</U> int <B>ETXTBSY</B><P>
An attempt to execute a file that is currently open for writing, or
write to a file that is currently being executed.  (The name stands
for "text file busy".)  This is not an error in the GNU system; the
text is copied as necessary.
<P>
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<U>Macro:</U> int <B>EFBIG</B><P>
File too big; the size of a file would be larger than allowed by the system.
<P>
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<U>Macro:</U> int <B>ENOSPC</B><P>
No space left on device; write operation on a file failed because the
disk is full.
<P>
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<U>Macro:</U> int <B>ESPIPE</B><P>
Invalid seek operation (such as on a pipe).
<P>
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<U>Macro:</U> int <B>EROFS</B><P>
An attempt was made to modify a file on a read-only file system.
<P>
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<U>Macro:</U> int <B>EMLINK</B><P>
Too many links; the link count of a single file is too large.
<P>
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<U>Macro:</U> int <B>EPIPE</B><P>
Broken pipe; there is no process reading from the other end of a pipe.
Every library function that returns this error code also generates a
<CODE>SIGPIPE</CODE> signal; this signal terminates the program if not handled
or blocked.  Thus, your program will never actually see <CODE>EPIPE</CODE>
unless it has handled or blocked <CODE>SIGPIPE</CODE>.
<P>
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<U>Macro:</U> int <B>EDOM</B><P>
Domain error; used by mathematical functions when an argument value does
not fall into the domain over which the function is defined.
<P>
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<U>Macro:</U> int <B>ERANGE</B><P>
Range error; used by mathematical functions when the result value is
not representable because of overflow or underflow.
<P>
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<U>Macro:</U> int <B>EAGAIN</B><P>
Resource temporarily unavailable; the call might work if you try again
later.  Only <CODE>fork</CODE> returns error code <CODE>EAGAIN</CODE> for such a
reason.
<P>
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<U>Macro:</U> int <B>EWOULDBLOCK</B><P>
An operation that would block was attempted on an object that has
non-blocking mode selected.
<P>
<STRONG>Portability Note:</STRONG> In 4.4BSD and GNU, <CODE>EWOULDBLOCK</CODE> and
<CODE>EAGAIN</CODE> are the same.  Earlier versions of BSD (see section <A HREF="library_1.html#SEC6" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_1.html#SEC6">Berkeley Unix</A>) have two distinct codes, and use <CODE>EWOULDBLOCK</CODE> to indicate
an I/O operation that would block on an object with non-blocking mode
set, and <CODE>EAGAIN</CODE> for other kinds of errors.<P>
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<U>Macro:</U> int <B>EINPROGRESS</B><P>