errno.texi

一个C源代码分析器

@node Error Reporting, Memory Allocation, Introduction, Top
@chapter Error Reporting
@cindex error reporting
@cindex reporting errors
@cindex error codes
@cindex status codes

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.

This chapter describes how the error reporting facility works.  Your
program should include the header file @file{errno.h} to use this
facility.
@pindex errno.h

@menu
* Checking for Errors::         How errors are reported by library functions.
* Error Codes::                 Error code macros; all of these expand 
                                 into integer constant values.
* Error Messages::              Mapping error codes onto error messages.
@end menu

@node Checking for Errors, Error Codes,  , Error Reporting
@section Checking for Errors

Most library functions return a special value to indicate that they have
failed.  The special value is typically @code{-1}, a null pointer, or a
constant such as @code{EOF} 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}.  This variable is declared in the header file
@file{errno.h}.
@pindex errno.h

@comment errno.h
@comment ANSI
@deftypevr {Variable} {volatile int} errno
The variable @code{errno} contains the system error number.  You can
change the value of @code{errno}.

Since @code{errno} is declared @code{volatile}, it might be changed
asynchronously by a signal handler; see @ref{Defining Handlers}.
However, a properly written signal handler saves and restores the value
of @code{errno}, so you generally do not need to worry about this
possibility except when writing signal handlers.

The initial value of @code{errno} 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}
when they succeed; thus, the value of @code{errno} after a successful
call is not necessarily zero, and you should not use @code{errno} to
determine @emph{whether} a call failed.  The proper way to do that is
documented for each function.  @emph{If} the call the failed, you can
examine @code{errno}.

Many library functions can set @code{errno} 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} when the
function returns an error.

@strong{Portability Note:} ANSI C specifies @code{errno} 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 ()}.  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.

There are a few library functions, like @code{sqrt} and @code{atan},
that return a perfectly legitimate value in case of an error, but also
set @code{errno}.  For these functions, if you want to check to see
whether an error occurred, the recommended method is to set @code{errno}
to zero before calling the function, and then check its value afterward.
@end deftypevr

@pindex errno.h
All the error codes have symbolic names; they are macros defined in
@file{errno.h}.  The names start with @samp{E} and an upper-case
letter or digit; you should consider names of this form to be
reserved names.  @xref{Reserved Names}.

The error code values are all positive integers and are all distinct,
with one exception: @code{EWOULDBLOCK} and @code{EAGAIN} are the same.
Since the values are distinct, you can use them as labels in a
@code{switch} statement; just don't use both @code{EWOULDBLOCK} and
@code{EAGAIN}.  Your program should not make any other assumptions about
the specific values of these symbolic constants.

The value of @code{errno} 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.

On non-GNU systems, almost any system call can return @code{EFAULT} 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} in the descriptions of individual functions.

In some Unix systems, many system calls can also return @code{EFAULT} if
given as an argument a pointer into the stack, and the kernel for some
obscure reason fails in its attempt to extend the stack.  If this ever
happens, you should probably try using statically or dynamically
allocated memory instead of stack memory on that system.

@node Error Codes, Error Messages, Checking for Errors, Error Reporting
@section Error Codes

@pindex errno.h
The error code macros are defined in the header file @file{errno.h}.
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.

@comment errno.h
@comment POSIX.1: Operation not permitted
@deftypevr Macro int EPERM
@comment errno 1 @c DO NOT REMOVE
Operation not permitted; only the owner of the file (or other resource)
or processes with special privileges can perform the operation.
@end deftypevr

@comment errno.h
@comment POSIX.1: No such file or directory
@deftypevr Macro int ENOENT
@comment errno 2 @c DO NOT REMOVE
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.
@end deftypevr

@comment errno.h
@comment POSIX.1: No such process
@deftypevr Macro int ESRCH
@comment errno 3 @c DO NOT REMOVE
No process matches the specified process ID.
@end deftypevr

@comment errno.h
@comment POSIX.1: Interrupted system call
@deftypevr Macro int EINTR
@comment errno 4 @c DO NOT REMOVE
Interrupted function call; an asynchronous signal occured and prevented
completion of the call.  When this happens, you should try the call
again.

You can choose to have functions resume after a signal that is handled,
rather than failing with @code{EINTR}; see @ref{Interrupted
Primitives}.
@end deftypevr

@comment errno.h
@comment POSIX.1: Input/output error
@deftypevr Macro int EIO
@comment errno 5 @c DO NOT REMOVE
Input/output error; usually used for physical read or write errors.
@end deftypevr

@comment errno.h
@comment POSIX.1: Device not configured
@deftypevr Macro int ENXIO
@comment errno 6 @c DO NOT REMOVE
No such device or address.  The system tried to use the device
represented by a file you specified, and it couldn't find the device.
This can mean that the device file was installed incorrectly, or that
the physical device is missing or not correctly attached to the
computer.
@end deftypevr

@comment errno.h
@comment POSIX.1: Argument list too long
@deftypevr Macro int E2BIG
@comment errno 7 @c DO NOT REMOVE
Argument list too long; used when the arguments passed to a new program
being executed with one of the @code{exec} functions (@pxref{Executing a
File}) occupy too much memory space.  This condition never arises in the
GNU system.
@end deftypevr

@comment errno.h
@comment POSIX.1: Exec format error
@deftypevr Macro int ENOEXEC
@comment errno 8 @c DO NOT REMOVE
Invalid executable file format.  This condition is detected by the
@code{exec} functions; see @ref{Executing a File}.
@end deftypevr

@comment errno.h
@comment POSIX.1: Bad file descriptor
@deftypevr Macro int EBADF
@comment errno 9 @c DO NOT REMOVE
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).
@end deftypevr

@comment errno.h
@comment POSIX.1: No child processes
@deftypevr Macro int ECHILD
@comment errno 10 @c DO NOT REMOVE
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.
@end deftypevr

@comment errno.h
@comment POSIX.1: Resource deadlock avoided
@deftypevr Macro int EDEADLK
@comment errno 11 @c DO NOT REMOVE
Deadlock avoided; allocating a system resource would have resulted in a
deadlock situation.  The system does not guarantee that it will notice
all such situations.  This error means you got lucky and the system
noticed; it might just hang.  @xref{File Locks}, for an example.
@end deftypevr

@comment errno.h
@comment POSIX.1: Cannot allocate memory
@deftypevr Macro int ENOMEM
@comment errno 12 @c DO NOT REMOVE
No memory available.  The system cannot allocate more virtual memory
because its capacity is full.
@end deftypevr

@comment errno.h
@comment POSIX.1: Permission denied
@deftypevr Macro int EACCES
@comment errno 13 @c DO NOT REMOVE
Permission denied; the file permissions do not allow the attempted operation.
@end deftypevr

@comment errno.h
@comment POSIX.1: Bad address
@deftypevr Macro int EFAULT
@comment errno 14 @c DO NOT REMOVE
Bad address; an invalid pointer was detected.
@end deftypevr

@comment errno.h
@comment BSD: Block device required
@deftypevr Macro int ENOTBLK
@comment errno 15 @c DO NOT REMOVE
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.
@end deftypevr

@comment errno.h
@comment POSIX.1: Device busy
@deftypevr Macro int EBUSY
@comment errno 16 @c DO NOT REMOVE
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.
@end deftypevr

@comment errno.h
@comment POSIX.1: File exists
@deftypevr Macro int EEXIST
@comment errno 17 @c DO NOT REMOVE
File exists; an existing file was specified in a context where it only
makes sense to specify a new file.
@end deftypevr

@comment errno.h
@comment POSIX.1: Invalid cross-device link
@deftypevr Macro int EXDEV
@comment errno 18 @c DO NOT REMOVE
An attempt to make an improper link across file systems was detected.
This happens not only when you use @code{link} (@pxref{Hard Links}) but
also when you rename a file with @code{rename} (@pxref{Renaming Files}).
@end deftypevr

@comment errno.h
@comment POSIX.1: Operation not supported by device
@deftypevr Macro int ENODEV
@comment errno 19 @c DO NOT REMOVE
The wrong type of device was given to a function that expects a
particular sort of device.
@end deftypevr

@comment errno.h
@comment POSIX.1: Not a directory
@deftypevr Macro int ENOTDIR
@comment errno 20 @c DO NOT REMOVE
A file that isn't a directory was specified when a directory is required.
@end deftypevr

@comment errno.h
@comment POSIX.1: Is a directory
@deftypevr Macro int EISDIR
@comment errno 21 @c DO NOT REMOVE
File is a directory; attempting to open a directory for writing gives
this error.
@end deftypevr

@comment errno.h
@comment POSIX.1: Invalid argument
@deftypevr Macro int EINVAL
@comment errno 22 @c DO NOT REMOVE
Invalid argument.  This is used to indicate various kinds of problems
with passing the wrong argument to a library function.
@end deftypevr

@comment errno.h
@comment POSIX.1: Too many open files
@deftypevr Macro int EMFILE
@comment errno 24 @c DO NOT REMOVE
The current process has too many files open and can't open any more.
Duplicate descriptors do count toward this limit.

In BSD and GNU, the number of open files is controlled by a resource
limit that can usually be increased.  If you get this error, you might
want to increase the @code{RLIMIT_NOFILE} limit or make it unlimited;
@pxref{Limits on Resources}.
@end deftypevr

@comment errno.h
@comment POSIX.1: Too many open files in system
@deftypevr Macro int ENFILE
@comment errno 23 @c DO NOT REMOVE
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
@ref{Linked Channels}.  This error never occurs in the GNU system.
@end deftypevr

@comment errno.h
@comment POSIX.1: Inappropriate ioctl for device
@deftypevr Macro int ENOTTY
@comment errno 25 @c DO NOT REMOVE
Inappropriate I/O control operation, such as trying to set terminal
modes on an ordinary file.
@end deftypevr