Error Reporting
***************

Many functions in theglibc 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 errno.h to use this facility.

Checking for Errors
===================

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

 -- Variable: volatile int errno
     ISO, errno.h The variable errno contains the system error number.  You can change the value of errno.

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

     The initial value of errno at program startup is zero.  In many cases, when a library function encounters an error, it will set errno to a non-zero value to indicate what specific error condition occurred.  The documentation for each function lists the error conditions that are possible for that function.  Not all library functions use this mechanism; some return an error code directly, instead.

     Warning: Many library functions may set errno to some meaningless non-zero value even if they did not encounter any errors, and even if they return error codes directly.  Therefore, it is usually incorrect to check whether an error occurred by inspecting the value of errno.  The proper way to check for error is documented for each function.

     Portability Note: ISO C specifies 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 *__errno_location ().  In fact, that is what it is on gnulinuxhurdsystems.  Theglibc, on each system, does whatever is right for the particular system.

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

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

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

The value of 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.

Except on gnuhurdsystems, almost any system call can return 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 gnuhurdsystems, we have saved space by not mentioning EFAULT in the descriptions of individual functions.

In some Unix systems, many system calls can also return 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.

Error Codes
===========

The error code macros are defined in the header file errno.h. All of them expand into integer constant values.  Some of these error codes can't occur on gnusystems, but they can occur using theglibc on other systems.

 -- Macro: int EPERM
     POSIX.1, errno.h EPERM, 1, Operation not permitted Only the owner of the file (or other resource) or processes with special privileges can perform the operation.

 -- Macro: int ENOENT
     POSIX.1, errno.h ENOENT, 2, 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.

 -- Macro: int ESRCH
     POSIX.1, errno.h ESRCH, 3, No such process No process matches the specified process ID.

 -- Macro: int EINTR
     POSIX.1, errno.h EINTR, 4, Interrupted system call An asynchronous signal occurred 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 EINTR; see Interrupted Primitives.

 -- Macro: int EIO
     POSIX.1, errno.h EIO, 5, Input/output error Usually used for physical read or write errors.

 -- Macro: int ENXIO
     POSIX.1, errno.h ENXIO, 6, 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.

 -- Macro: int E2BIG
     POSIX.1, errno.h E2BIG, 7, Argument list too long Used when the arguments passed to a new program being executed with one of the exec functions (see Executing a File) occupy too much memory space.  This condition never arises on gnuhurdsystems.

 -- Macro: int ENOEXEC
     POSIX.1, errno.h ENOEXEC, 8, Exec format error Invalid executable file format.  This condition is detected by the exec functions; see Executing a File.

 -- Macro: int EBADF
     POSIX.1, errno.h EBADF, 9, 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).

 -- Macro: int ECHILD
     POSIX.1, errno.h ECHILD, 10, No child processes This error happens on operations that are supposed to manipulate child processes, when there aren't any processes to manipulate.

 -- Macro: int EDEADLK
     POSIX.1, errno.h EDEADLK, 11, Resource 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.  See File Locks, for an example.

 -- Macro: int ENOMEM
     POSIX.1, errno.h ENOMEM, 12, Cannot allocate memory The system cannot allocate more virtual memory because its capacity is full.

 -- Macro: int EACCES
     POSIX.1, errno.h EACCES, 13, Permission denied The file permissions do not allow the attempted operation.

 -- Macro: int EFAULT
     POSIX.1, errno.h EFAULT, 14, Bad address An invalid pointer was detected. On gnuhurdsystems, this error never happens; you get a signal instead.

 -- Macro: int ENOTBLK
     BSD, errno.h ENOTBLK, 15, Block device required 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.

 -- Macro: int EBUSY
     POSIX.1, errno.h EBUSY, 16, Device or 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.

 -- Macro: int EEXIST
     POSIX.1, errno.h EEXIST, 17, File exists An existing file was specified in a context where it only makes sense to specify a new file.

 -- Macro: int EXDEV
     POSIX.1, errno.h EXDEV, 18, Invalid cross-device link An attempt to make an improper link across file systems was detected. This happens not only when you use link (see Hard Links) but also when you rename a file with rename (see Renaming Files).

 -- Macro: int ENODEV
     POSIX.1, errno.h ENODEV, 19, No such device The wrong type of device was given to a function that expects a particular sort of device.

 -- Macro: int ENOTDIR
     POSIX.1, errno.h ENOTDIR, 20, Not a directory A file that isn't a directory was specified when a directory is required.

 -- Macro: int EISDIR
     POSIX.1, errno.h EISDIR, 21, Is a directory You cannot open a directory for writing, or create or remove hard links to it.

 -- Macro: int EINVAL
     POSIX.1, errno.h EINVAL, 22, Invalid argument This is used to indicate various kinds of problems with passing the wrong argument to a library function.

 -- Macro: int EMFILE
     POSIX.1, errno.h EMFILE, 24, Too many open files 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 RLIMIT_NOFILE limit or make it unlimited; see Limits on Resources.

 -- Macro: int ENFILE
     POSIX.1, errno.h ENFILE, 23, Too many open files in system 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 Linked Channels.  This error never occurs on gnuhurdsystems.

 -- Macro: int ENOTTY
     POSIX.1, errno.h ENOTTY, 25, Inappropriate ioctl for device Inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file.

 -- Macro: int ETXTBSY
     BSD, errno.h ETXTBSY, 26, Text file busy An attempt to execute a file that is currently open for writing, or write to a file that is currently being executed.  Often using a debugger to run a program is considered having it open for writing and will cause this error.  (The name stands for ``text file busy''.)  This is not an error on gnuhurdsystems; the text is copied as necessary.

 -- Macro: int EFBIG
     POSIX.1, errno.h EFBIG, 27, File too large The size of a file would be larger than allowed by the system.

 -- Macro: int ENOSPC
     POSIX.1, errno.h ENOSPC, 28, No space left on device Write operation on a file failed because the disk is full.

 -- Macro: int ESPIPE
     POSIX.1, errno.h ESPIPE, 29, Illegal seek Invalid seek operation (such as on a pipe).

 -- Macro: int EROFS
     POSIX.1, errno.h EROFS, 30, Read-only file system An attempt was made to modify something on a read-only file system.

 -- Macro: int EMLINK
     POSIX.1, errno.h EMLINK, 31, Too many links The link count of a single file would become too large. rename can cause this error if the file being renamed already has as many links as it can take (see Renaming Files).

 -- Macro: int EPIPE
     POSIX.1, errno.h EPIPE, 32, 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 SIGPIPE signal; this signal terminates the program if not handled or blocked.  Thus, your program will never actually see EPIPE unless it has handled or blocked SIGPIPE.

 -- Macro: int EDOM
     ISO, errno.h EDOM, 33, Numerical argument out of domain Used by mathematical functions when an argument value does not fall into the domain over which the function is defined.

 -- Macro: int ERANGE
     ISO, errno.h ERANGE, 34, Numerical result out of range Used by mathematical functions when the result value is not representable because of overflow or underflow.

 -- Macro: int EAGAIN
     POSIX.1, errno.h EAGAIN, 35, Resource temporarily unavailable The call might work if you try again later.  The macro EWOULDBLOCK is another name for EAGAIN; they are always the same in theglibc.

     This error can happen in a few different situations:

       * An operation that would block was attempted on an object that has non-blocking mode selected.  Trying the same operation again will block until some external condition makes it possible to read, write, or connect (whatever the operation).  You can use select to find out when the operation will be possible; see Waiting for I/O.

          Portability Note: In many older Unix systems, this condition was indicated by EWOULDBLOCK, which was a distinct error code different from EAGAIN.  To make your program portable, you should check for both codes and treat them the same.

       * A temporary resource shortage made an operation impossible.  fork can return this error.  It indicates that the shortage is expected to pass, so your program can try the call again later and it may succeed. It is probably a good idea to delay for a few seconds before trying it again, to allow time for other processes to release scarce resources. Such shortages are usually fairly serious and affect the whole system, so usually an interactive program should report the error to the user and return to its command loop.

 -- Macro: int EWOULDBLOCK
     BSD, errno.h EWOULDBLOCK, EAGAIN, Operation would block In theglibc, this is another name for EAGAIN (above). The values are always the same, on every operating system.

     C libraries in many older Unix systems have EWOULDBLOCK as a separate error code.

 -- Macro: int EINPROGRESS
     BSD, errno.h EINPROGRESS, 36, Operation now in progress An operation that cannot complete immediately was initiated on an object that has non-blocking mode selected.  Some functions that must always block (such as connect; see Connecting) never return EAGAIN.  Instead, they return EINPROGRESS to indicate that the operation has begun and will take some time.  Attempts to manipulate the object before the call completes return EALREADY.  You can use the select function to find out when the pending operation has completed; see Waiting for I/O.

 -- Macro: int EALREADY
     BSD, errno.h EALREADY, 37, Operation already in progress An operation is already in progress on an object that has non-blocking mode selected.

 -- Macro: int ENOTSOCK
     BSD, errno.h ENOTSOCK, 38, Socket operation on non-socket A file that isn't a socket was specified when a socket is required.

 -- Macro: int EMSGSIZE
     BSD, errno.h EMSGSIZE, 40, Message too long The size of a message sent on a socket was larger than the supported maximum size.

 -- Macro: int EPROTOTYPE
     BSD, errno.h EPROTOTYPE, 41, Protocol wrong type for socket The socket type does not support the requested communications protocol.

 -- Macro: int ENOPROTOOPT
     BSD, errno.h ENOPROTOOPT, 42, Protocol not available You specified a socket option that doesn't make sense for the particular protocol being used by the socket.  See Socket Options.

 -- Macro: int EPROTONOSUPPORT
     BSD, errno.h EPROTONOSUPPORT, 43, Protocol not supported The socket domain does not support the requested communications protocol (perhaps because the requested protocol is completely invalid). See Creating a Socket.

 -- Macro: int ESOCKTNOSUPPORT
     BSD, errno.h ESOCKTNOSUPPORT, 44, Socket type not supported The socket type is not supported.

 -- Macro: int EOPNOTSUPP
     BSD, errno.h EOPNOTSUPP, 45, Operation not supported The operation you requested is not supported.  Some socket functions don't make sense for all types of sockets, and others may not be implemented for all communications protocols.  On gnuhurdsystems, this error can happen for many calls when the object does not support the particular operation; it is a generic indication that the server knows nothing to do for that call.

 -- Macro: int EPFNOSUPPORT
     BSD, errno.h EPFNOSUPPORT, 46, Protocol family not supported The socket communications protocol family you requested is not supported.

 -- Macro: int EAFNOSUPPORT
     BSD, errno.h EAFNOSUPPORT, 47, Address family not supported by protocol The address family specified for a socket is not supported; it is inconsistent with the protocol being used on the socket.  See Sockets.

 -- Macro: int EADDRINUSE
     BSD, errno.h EADDRINUSE, 48, Address already in use The requested socket address is already in use.  See Socket Addresses.

 -- Macro: int EADDRNOTAVAIL
     BSD, errno.h EADDRNOTAVAIL, 49, Cannot assign requested address The requested socket address is not available; for example, you tried to give a socket a name that doesn't match the local host name. See Socket Addresses.

 -- Macro: int ENETDOWN
     BSD, errno.h ENETDOWN, 50, Network is down A socket operation failed because the network was down.

 -- Macro: int ENETUNREACH
     BSD, errno.h ENETUNREACH, 51, Network is unreachable A socket operation failed because the subnet containing the remote host was unreachable.

 -- Macro: int ENETRESET
     BSD, errno.h ENETRESET, 52, Network dropped connection on reset A network connection was reset because the remote host crashed.

 -- Macro: int ECONNABORTED
     BSD, errno.h ECONNABORTED, 53, Software caused connection abort A network connection was aborted locally.

 -- Macro: int ECONNRESET
     BSD, errno.h ECONNRESET, 54, Connection reset by peer A network connection was closed for reasons outside the control of the local host, such as by the remote machine rebooting or an unrecoverable protocol violation.

 -- Macro: int ENOBUFS
     BSD, errno.h ENOBUFS, 55, No buffer space available The kernel's buffers for I/O operations are all in use.  In GNU, this error is always synonymous with ENOMEM; you may get one or the other from network operations.

 -- Macro: int EISCONN
     BSD, errno.h EISCONN, 56, Transport endpoint is already connected You tried to connect a socket that is already connected. See Connecting.

 -- Macro: int ENOTCONN
     BSD, errno.h ENOTCONN, 57, Transport endpoint is not connected The socket is not connected to anything.  You get this error when you try to transmit data over a socket, without first specifying a destination for the data.  For a connectionless socket (for datagram protocols, such as UDP), you get EDESTADDRREQ instead.

 -- Macro: int EDESTADDRREQ
     BSD, errno.h EDESTADDRREQ, 39, Destination address required No default destination address was set for the socket.  You get this error when you try to transmit data over a connectionless socket, without first specifying a destination for the data with connect.

 -- Macro: int ESHUTDOWN
     BSD, errno.h ESHUTDOWN, 58, Cannot send after transport endpoint shutdown The socket has already been shut down.

 -- Macro: int ETOOMANYREFS
     BSD, errno.h ETOOMANYREFS, 59, Too many references: cannot splice

 -- Macro: int ETIMEDOUT
     BSD, errno.h ETIMEDOUT, 60, Connection timed out A socket operation with a specified timeout received no response during the timeout period.

 -- Macro: int ECONNREFUSED
     BSD, errno.h ECONNREFUSED, 61, Connection refused A remote host refused to allow the network connection (typically because it is not running the requested service).

 -- Macro: int ELOOP
     BSD, errno.h ELOOP, 62, Too many levels of symbolic links Too many levels of symbolic links were encountered in looking up a file name. This often indicates a cycle of symbolic links.

 -- Macro: int ENAMETOOLONG
     POSIX.1, errno.h ENAMETOOLONG, 63, File name too long Filename too long (longer than PATH_MAX; see Limits for Files) or host name too long (in gethostname or sethostname; see Host Identification).

 -- Macro: int EHOSTDOWN
     BSD, errno.h EHOSTDOWN, 64, Host is down The remote host for a requested network connection is down.

 -- Macro: int EHOSTUNREACH
     BSD, errno.h EHOSTUNREACH, 65, No route to host The remote host for a requested network connection is not reachable.

 -- Macro: int ENOTEMPTY
     POSIX.1, errno.h ENOTEMPTY, 66, Directory not empty Directory not empty, where an empty directory was expected.  Typically, this error occurs when you are trying to delete a directory.

 -- Macro: int EPROCLIM
     BSD, errno.h EPROCLIM, 67, Too many processes This means that the per-user limit on new process would be exceeded by an attempted fork.  See Limits on Resources, for details on the RLIMIT_NPROC limit.

 -- Macro: int EUSERS
     BSD, errno.h EUSERS, 68, Too many users The file quota system is confused because there are too many users.

 -- Macro: int EDQUOT
     BSD, errno.h EDQUOT, 69, Disk quota exceeded The user's disk quota was exceeded.

 -- Macro: int ESTALE
     BSD, errno.h ESTALE, 70, Stale file handle This indicates an internal confusion in the file system which is due to file system rearrangements on the server host for NFS file systems or corruption in other file systems. Repairing this condition usually requires unmounting, possibly repairing and remounting the file system.

 -- Macro: int EREMOTE
     BSD, errno.h EREMOTE, 71, Object is remote An attempt was made to NFS-mount a remote file system with a file name that already specifies an NFS-mounted file. (This is an error on some operating systems, but we expect it to work properly on gnuhurdsystems, making this error code impossible.)

 -- Macro: int EBADRPC
     BSD, errno.h EBADRPC, 72, RPC struct is bad

 -- Macro: int ERPCMISMATCH
     BSD, errno.h ERPCMISMATCH, 73, RPC version wrong

 -- Macro: int EPROGUNAVAIL
     BSD, errno.h EPROGUNAVAIL, 74, RPC program not available

 -- Macro: int EPROGMISMATCH
     BSD, errno.h EPROGMISMATCH, 75, RPC program version wrong

 -- Macro: int EPROCUNAVAIL
     BSD, errno.h EPROCUNAVAIL, 76, RPC bad procedure for program

 -- Macro: int ENOLCK
     POSIX.1, errno.h ENOLCK, 77, No locks available This is used by the file locking facilities; see File Locks.  This error is never generated by gnuhurdsystems, but it can result from an operation to an NFS server running another operating system.

 -- Macro: int EFTYPE
     BSD, errno.h EFTYPE, 79, Inappropriate file type or format The file was the wrong type for the operation, or a data file had the wrong format.

     On some systems chmod returns this error if you try to set the sticky bit on a non-directory file; see Setting Permissions.

 -- Macro: int EAUTH
     BSD, errno.h EAUTH, 80, Authentication error

 -- Macro: int ENEEDAUTH
     BSD, errno.h ENEEDAUTH, 81, Need authenticator

 -- Macro: int ENOSYS
     POSIX.1, errno.h ENOSYS, 78, Function not implemented This indicates that the function called is not implemented at all, either in the C library itself or in the operating system.  When you get this error, you can be sure that this particular function will always fail with ENOSYS unless you install a new version of the C library or the operating system.

 -- Macro: int ELIBEXEC
     GNU, errno.h ELIBEXEC, 83, Cannot exec a shared library directly

 -- Macro: int ENOTSUP
     POSIX.1, errno.h ENOTSUP, 118, Not supported A function returns this error when certain parameter values are valid, but the functionality they request is not available. This can mean that the function does not implement a particular command or option value or flag bit at all.  For functions that operate on some object given in a parameter, such as a file descriptor or a port, it might instead mean that only that specific object (file descriptor, port, etc.) is unable to support the other parameters given; different file descriptors might support different ranges of parameter values.

     If the entire function is not available at all in the implementation, it returns ENOSYS instead.

 -- Macro: int EILSEQ
     ISO, errno.h EILSEQ, 106, Invalid or incomplete multibyte or wide character While decoding a multibyte character the function came along an invalid or an incomplete sequence of bytes or the given wide character is invalid.

 -- Macro: int EBACKGROUND
     GNU, errno.h EBACKGROUND, 100, Inappropriate operation for background process On gnuhurdsystems, servers supporting the term protocol return this error for certain operations when the caller is not in the foreground process group of the terminal.  Users do not usually see this error because functions such as read and write translate it into a SIGTTIN or SIGTTOU signal.  See Job Control, for information on process groups and these signals.

 -- Macro: int EDIED
     GNU, errno.h EDIED, 101, Translator died On gnuhurdsystems, opening a file returns this error when the file is translated by a program and the translator program dies while starting up, before it has connected to the file.

 -- Macro: int ED
     GNU, errno.h ED, 102, ? The experienced user will know what is wrong.

 -- Macro: int EGREGIOUS
     GNU, errno.h EGREGIOUS, 103, You really blew it this time You did what?

 -- Macro: int EIEIO
     GNU, errno.h EIEIO, 104, Computer bought the farm Go home and have a glass of warm, dairy-fresh milk.

 -- Macro: int EGRATUITOUS
     GNU, errno.h EGRATUITOUS, 105, Gratuitous error This error code has no purpose.

 -- Macro: int EBADMSG
     XOPEN, errno.h EBADMSG, 107, Bad message

 -- Macro: int EIDRM
     XOPEN, errno.h EIDRM, 108, Identifier removed

 -- Macro: int EMULTIHOP
     XOPEN, errno.h EMULTIHOP, 109, Multihop attempted

 -- Macro: int ENODATA
     XOPEN, errno.h ENODATA, 110, No data available

 -- Macro: int ENOLINK
     XOPEN, errno.h ENOLINK, 111, Link has been severed

 -- Macro: int ENOMSG
     XOPEN, errno.h ENOMSG, 112, No message of desired type

 -- Macro: int ENOSR
     XOPEN, errno.h ENOSR, 113, Out of streams resources

 -- Macro: int ENOSTR
     XOPEN, errno.h ENOSTR, 114, Device not a stream

 -- Macro: int EOVERFLOW
     XOPEN, errno.h EOVERFLOW, 115, Value too large for defined data type

 -- Macro: int EPROTO
     XOPEN, errno.h EPROTO, 116, Protocol error

 -- Macro: int ETIME
     XOPEN, errno.h ETIME, 117, Timer expired

 -- Macro: int ECANCELED
     POSIX.1, errno.h ECANCELED, 119, Operation canceled An asynchronous operation was canceled before it completed.  See Asynchronous I/O.  When you call aio_cancel, the normal result is for the operations affected to complete with this error; see Cancel AIO Operations.

 -- Macro: int EOWNERDEAD
     GNU, errno.h EOWNERDEAD, 120, Owner died

 -- Macro: int ENOTRECOVERABLE
     GNU, errno.h ENOTRECOVERABLE, 121, State not recoverable

The following error codes are defined by the Linux/i386 kernel. They are not yet documented.

 -- Macro: int ERESTART
     Linux???, errno.h ERESTART, ???/85, Interrupted system call should be restarted

 -- Macro: int ECHRNG
     Linux???, errno.h ECHRNG, ???/44, Channel number out of range

 -- Macro: int EL2NSYNC
     Obsolete, errno.h EL2NSYNC, ???/45, Level 2 not synchronized

 -- Macro: int EL3HLT
     Obsolete, errno.h EL3HLT, ???/46, Level 3 halted

 -- Macro: int EL3RST
     Obsolete, errno.h EL3RST, ???/47, Level 3 reset

 -- Macro: int ELNRNG
     Linux???, errno.h ELNRNG, ???/48, Link number out of range

 -- Macro: int EUNATCH
     Linux???, errno.h EUNATCH, ???/49, Protocol driver not attached

 -- Macro: int ENOCSI
     Linux???, errno.h ENOCSI, ???/50, No CSI structure available

 -- Macro: int EL2HLT
     Obsolete, errno.h EL2HLT, ???/51, Level 2 halted

 -- Macro: int EBADE
     Linux???, errno.h EBADE, ???/52, Invalid exchange

 -- Macro: int EBADR
     Linux???, errno.h EBADR, ???/53, Invalid request descriptor

 -- Macro: int EXFULL
     Linux???, errno.h EXFULL, ???/54, Exchange full

 -- Macro: int ENOANO
     Linux???, errno.h ENOANO, ???/55, No anode

 -- Macro: int EBADRQC
     Linux???, errno.h EBADRQC, ???/56, Invalid request code

 -- Macro: int EBADSLT
     Linux???, errno.h EBADSLT, ???/57, Invalid slot

 -- Macro: int EDEADLOCK
     Linux???, errno.h EDEADLOCK, ???/58, File locking deadlock error

 -- Macro: int EBFONT
     Linux???, errno.h EBFONT, ???/59, Bad font file format

 -- Macro: int ENONET
     Linux???, errno.h ENONET, ???/64, Machine is not on the network

 -- Macro: int ENOPKG
     Linux???, errno.h ENOPKG, ???/65, Package not installed

 -- Macro: int EADV
     Linux???, errno.h EADV, ???/68, Advertise error

 -- Macro: int ESRMNT
     Linux???, errno.h ESRMNT, ???/69, Srmount error

 -- Macro: int ECOMM
     Linux???, errno.h ECOMM, ???/70, Communication error on send

 -- Macro: int EDOTDOT
     Linux???, errno.h EDOTDOT, ???/73, RFS specific error

 -- Macro: int ENOTUNIQ
     Linux???, errno.h ENOTUNIQ, ???/76, Name not unique on network

 -- Macro: int EBADFD
     Linux???, errno.h EBADFD, ???/77, File descriptor in bad state

 -- Macro: int EREMCHG
     Linux???, errno.h EREMCHG, ???/78, Remote address changed

 -- Macro: int ELIBACC
     Linux???, errno.h ELIBACC, ???/79, Can not access a needed shared library

 -- Macro: int ELIBBAD
     Linux???, errno.h ELIBBAD, ???/80, Accessing a corrupted shared library

 -- Macro: int ELIBSCN
     Linux???, errno.h ELIBSCN, ???/81, .lib section in a.out corrupted

 -- Macro: int ELIBMAX
     Linux???, errno.h ELIBMAX, ???/82, Attempting to link in too many shared libraries

 -- Macro: int ESTRPIPE
     Linux???, errno.h ESTRPIPE, ???/86, Streams pipe error

 -- Macro: int EUCLEAN
     Linux???, errno.h EUCLEAN, ???/117, Structure needs cleaning

 -- Macro: int ENOTNAM
     Linux???, errno.h ENOTNAM, ???/118, Not a XENIX named type file

 -- Macro: int ENAVAIL
     Linux???, errno.h ENAVAIL, ???/119, No XENIX semaphores available

 -- Macro: int EISNAM
     Linux???, errno.h EISNAM, ???/120, Is a named type file

 -- Macro: int EREMOTEIO
     Linux???, errno.h EREMOTEIO, ???/121, Remote I/O error

 -- Macro: int ENOMEDIUM
     Linux???, errno.h ENOMEDIUM, ???/???, No medium found

 -- Macro: int EMEDIUMTYPE
     Linux???, errno.h EMEDIUMTYPE, ???/???, Wrong medium type

 -- Macro: int ENOKEY
     Linux, errno.h ENOKEY, ???/???, Required key not available

 -- Macro: int EKEYEXPIRED
     Linux, errno.h EKEYEXPIRED, ???/???, Key has expired

 -- Macro: int EKEYREVOKED
     Linux, errno.h EKEYREVOKED, ???/???, Key has been revoked

 -- Macro: int EKEYREJECTED
     Linux, errno.h EKEYREJECTED, ???/???, Key was rejected by service

 -- Macro: int ERFKILL
     Linux, errno.h ERFKILL, ???/???, Operation not possible due to RF-kill

 -- Macro: int EHWPOISON
     Linux, errno.h EHWPOISON, ???/???, Memory page has hardware error

Error Messages
==============

The library has functions and variables designed to make it easy for your program to report informative error messages in the customary format about the failure of a library call.  The functions strerror and perror give you the standard error message for a given error code; the variable program_invocation_short_name gives you convenient access to the name of the program that encountered the error.

 -- Function: char * strerror (int ERRNUM)
     ISO, string.h The strerror function maps the error code (see Checking for Errors) specified by the ERRNUM argument to a descriptive error message string.  The string is translated according to the current locale.  The return value is a pointer to this string.

     The value ERRNUM normally comes from the variable errno.

     You should not modify the string returned by strerror.  Also, if you make subsequent calls to strerror or strerror_l, or the thread that obtained the string exits, the returned pointer will be invalidated.

     As there is no way to restore the previous state after calling strerror, library code should not call this function because it may interfere with application use of strerror, invalidating the string pointer before the application is done using it.  Instead, strerror_r, snprintf with the %m or %#m specifiers, strerrorname_np, or strerrordesc_np can be used instead.

     The strerror function preserves the value of errno and cannot fail.

     The function strerror is declared in string.h.

 -- Function: char * strerror_l (int ERRNUM, locale_t LOCALE)
     POSIX, string.h This function is like strerror, except that the returned string is translated according to LOCALE (instead of the current locale used by strerror).  Note that calling strerror_l invalidates the pointer returned by strerror and vice versa.

     The function strerror_l is defined by POSIX and is declared in string.h.

 -- Function: char * strerror_r (int ERRNUM, char *BUF, size_t N)
     GNU, string.h The following description is for the GNU variant of the function, used if _GNU_SOURCE is defined.  See Feature Test Macros.

     The strerror_r function works like strerror but instead of returning a pointer to a string that is managed by theglibc, it can use the user supplied buffer starting at BUF for storing the string.

     At most N characters are written (including the NUL byte) to BUF, so it is up to the user to select a buffer large enough. Whether returned pointer points to the BUF array or not depends on the ERRNUM argument.  If the result string is not stored in BUF, the string will not change for the remaining execution of the program.

     The function strerror_r as described above is a GNU extension and it is declared in string.h.  There is a POSIX variant of this function, described next.

 -- Function: int strerror_r (int ERRNUM, char *BUF, size_t N)
     POSIX, string.h

     This variant of the strerror_r function is used if a standard is selected that includes strerror_r, but _GNU_SOURCE is not defined.  This POSIX variant of the function always writes the error message to the specified buffer BUF of size N bytes.

     Upon success, strerror_r returns 0.  Two more return values are used to indicate failure.

     EINVAL
          The ERRNUM argument does not correspond to a known error constant.

     ERANGE
          The buffer size N is not large enough to store the entire error message.

     Even if an error is reported, strerror_r still writes as much of the error message to the output buffer as possible.  After a call to strerror_r, the value of errno is unspecified.

     If you want to use the always-copying POSIX semantics of strerror_r in a program that is potentially compiled with _GNU_SOURCE defined, you can use snprintf with the %m conversion specifier, like this:

          int saved_errno = errno;
          errno = errnum;
          int ret = snprintf (buf, n, "%m");
          errno = saved_errno;
          if (strerrorname_np (errnum) == NULL)
            return EINVAL;
          if (ret >= n)
            return ERANGE:
          return 0;

     This function is declared in string.h if it is declared at all. It is a POSIX extension.

 -- Function: void perror (const char *MESSAGE)
     ISO, stdio.h prelim:stderr This function prints an error message to the stream stderr; see Standard Streams.  The orientation of stderr is not changed.

     If you call perror with a MESSAGE that is either a null pointer or an empty string, perror just prints the error message corresponding to errno, adding a trailing newline.

     If you supply a non-null MESSAGE argument, then perror prefixes its output with this string.  It adds a colon and a space character to separate the MESSAGE from the error string corresponding to errno.

     The function perror is declared in stdio.h.

 -- Function: const char * strerrorname_np (int ERRNUM)
     GNU, string.h This function returns the name describing the error ERRNUM or NULL if there is no known constant with this value (e.g "EINVAL" for EINVAL).  The returned string does not change for the remaining execution of the program.

     This function is a GNU extension, declared in the header file string.h.

 -- Function: const char * strerrordesc_np (int ERRNUM)
     GNU, string.h This function returns the message describing the error ERRNUM or NULL if there is no known constant with this value (e.g "Invalid argument" for EINVAL).  Different than strerror the returned description is not translated, and the returned string does not change for the remaining execution of the program.

     This function is a GNU extension, declared in the header file string.h.

strerror and perror produce the exact same message for any given error code under the same locale; the precise text varies from system to system.  With theglibc, the messages are fairly short; there are no multi-line messages or embedded newlines.  Each error message begins with a capital letter and does not include any terminating punctuation.

Many programs that don't read input from the terminal are designed to exit if any system call fails.  By convention, the error message from such a program should start with the program's name, sans directories. You can find that name in the variable program_invocation_short_name; the full file name is stored the variable program_invocation_name.

 -- Variable: char * program_invocation_name
     GNU, errno.h This variable's value is the name that was used to invoke the program running in the current process.  It is the same as argv[0].  Note that this is not necessarily a useful file name; often it contains no directory names.  See Program Arguments.

     This variable is a GNU extension and is declared in errno.h.

 -- Variable: char * program_invocation_short_name
     GNU, errno.h This variable's value is the name that was used to invoke the program running in the current process, with directory names removed.  (That is to say, it is the same as program_invocation_name minus everything up to the last slash, if any.)

     This variable is a GNU extension and is declared in errno.h.

The library initialization code sets up both of these variables before calling main.

Portability Note: If you want your program to work with non-GNU libraries, you must save the value of argv[0] in main, and then strip off the directory names yourself.  We added these extensions to make it possible to write self-contained error-reporting subroutines that require no explicit cooperation from main.

Here is an example showing how to handle failure to open a file correctly.  The function open_sesame tries to open the named file for reading and returns a stream if successful.  The fopen library function returns a null pointer if it couldn't open the file for some reason.  In that situation, open_sesame constructs an appropriate error message using the strerror function, and terminates the program.  If we were going to make some other library calls before passing the error code to strerror, we'd have to save it in a local variable instead, because those other library functions might overwrite errno in the meantime.

     #define _GNU_SOURCE

     #include <errno.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>

     FILE *
     open_sesame (char *name)
     {
       FILE *stream;

       errno = 0;
       stream = fopen (name, "r");
       if (stream == NULL)
         {
           fprintf (stderr, "%s: Couldn't open file %s; %s\n",
                    program_invocation_short_name, name, strerror (errno));
           exit (EXIT_FAILURE);
         }
       else
         return stream;
     }

Using perror has the advantage that the function is portable and available on all systems implementing ISO C.  But often the text perror generates is not what is wanted and there is no way to extend or change what perror does.  The GNU coding standard, for instance, requires error messages to be preceded by the program name and programs which read some input files should provide information about the input file name and the line number in case an error is encountered while reading the file.  For these occasions there are two functions available which are widely used throughout the GNU project. These functions are declared in error.h.

 -- Function: void error (int STATUS, int ERRNUM, const char *FORMAT, ...)
     GNU, error.h The error function can be used to report general problems during program execution.  The FORMAT argument is a format string just like those given to the printf family of functions.  The arguments required for the format can follow the FORMAT parameter. Just like perror, error also can report an error code in textual form.  But unlike perror the error value is explicitly passed to the function in the ERRNUM parameter.  This eliminates the problem mentioned above that the error reporting function must be called immediately after the function causing the error since otherwise errno might have a different value.

     error prints first the program name.  If the application defined a global variable error_print_progname and points it to a function this function will be called to print the program name. Otherwise the string from the global variable program_name is used.  The program name is followed by a colon and a space which in turn is followed by the output produced by the format string.  If the ERRNUM parameter is non-zero the format string output is followed by a colon and a space, followed by the error message for the error code ERRNUM.  In any case is the output terminated with a newline.

     The output is directed to the stderr stream.  If the stderr wasn't oriented before the call it will be narrow-oriented afterwards.

     The function will return unless the STATUS parameter has a non-zero value.  In this case the function will call exit with the STATUS value for its parameter and therefore never return.  If error returns, the global variable error_message_count is incremented by one to keep track of the number of errors reported.

 -- Function: void error_at_line (int STATUS, int ERRNUM, const char *FNAME, unsigned int LINENO, const char *FORMAT, ...)
     GNU, error.h prelim:error_at_line/error_one_per_line mtslocale/error_one_per_line

     The error_at_line function is very similar to the error function.  The only differences are the additional parameters FNAME and LINENO.  The handling of the other parameters is identical to that of error except that between the program name and the string generated by the format string additional text is inserted.

     Directly following the program name a colon, followed by the file name pointed to by FNAME, another colon, and the value of LINENO is printed.

     This additional output of course is meant to be used to locate an error in an input file (like a programming language source code file etc).

     If the global variable error_one_per_line is set to a non-zero value error_at_line will avoid printing consecutive messages for the same file and line.  Repetition which are not directly following each other are not caught.

     Just like error this function only returns if STATUS is zero.  Otherwise exit is called with the non-zero value.  If error returns, the global variable error_message_count is incremented by one to keep track of the number of errors reported.

As mentioned above, the error and error_at_line functions can be customized by defining a variable named error_print_progname.

 -- Variable: void (*error_print_progname) (void)
     GNU, error.h If the error_print_progname variable is defined to a non-zero value the function pointed to is called by error or error_at_line.  It is expected to print the program name or do something similarly useful.

     The function is expected to print to the stderr stream and must be able to handle whatever orientation the stream has.

     The variable is global and shared by all threads.

 -- Variable: unsigned int error_message_count
     GNU, error.h The error_message_count variable is incremented whenever one of the functions error or error_at_line returns.  The variable is global and shared by all threads.

 -- Variable: int error_one_per_line
     GNU, error.h The error_one_per_line variable influences only error_at_line.  Normally the error_at_line function creates output for every invocation.  If error_one_per_line is set to a non-zero value error_at_line keeps track of the last file name and line number for which an error was reported and avoids directly following messages for the same file and line.  This variable is global and shared by all threads.

A program which read some input file and reports errors in it could look like this:

     {
       char *line = NULL;
       size_t len = 0;
       unsigned int lineno = 0;

       error_message_count = 0;
       while (! feof_unlocked (fp))
         {
           ssize_t n = getline (&line, &len, fp);
           if (n <= 0)
             /* End of file or error.  */
             break;
           ++lineno;

           /* Process the line.  */
           ...

           if (Detect error in line)
             error_at_line (0, errval, filename, lineno,
                            "some error text %s", some_variable);
         }

       if (error_message_count != 0)
         error (EXIT_FAILURE, 0, "%u errors found", error_message_count);
     }

error and error_at_line are clearly the functions of choice and enable the programmer to write applications which follow the GNU coding standard.  Theglibc additionally contains functions which are used in BSD for the same purpose.  These functions are declared in err.h.  It is generally advised to not use these functions.  They are included only for compatibility.

 -- Function: void warn (const char *FORMAT, ...)
     BSD, err.h The warn function is roughly equivalent to a call like

            error (0, errno, format, the parameters)

     except that the global variables error respects and modifies are not used.

 -- Function: void vwarn (const char *FORMAT, va_list AP)
     BSD, err.h The vwarn function is just like warn except that the parameters for the handling of the format string FORMAT are passed in as a value of type va_list.

 -- Function: void warnx (const char *FORMAT, ...)
     BSD, err.h The warnx function is roughly equivalent to a call like

            error (0, 0, format, the parameters)

     except that the global variables error respects and modifies are not used.  The difference to warn is that no error number string is printed.

 -- Function: void vwarnx (const char *FORMAT, va_list AP)
     BSD, err.h The vwarnx function is just like warnx except that the parameters for the handling of the format string FORMAT are passed in as a value of type va_list.

 -- Function: void err (int STATUS, const char *FORMAT, ...)
     BSD, err.h The err function is roughly equivalent to a call like

            error (status, errno, format, the parameters)

     except that the global variables error respects and modifies are not used and that the program is exited even if STATUS is zero.

 -- Function: void verr (int STATUS, const char *FORMAT, va_list AP)
     BSD, err.h The verr function is just like err except that the parameters for the handling of the format string FORMAT are passed in as a value of type va_list.

 -- Function: void errx (int STATUS, const char *FORMAT, ...)
     BSD, err.h The errx function is roughly equivalent to a call like

            error (status, 0, format, the parameters)

     except that the global variables error respects and modifies are not used and that the program is exited even if STATUS is zero.  The difference to err is that no error number string is printed.

 -- Function: void verrx (int STATUS, const char *FORMAT, va_list AP)
     BSD, err.h The verrx function is just like errx except that the parameters for the handling of the format string FORMAT are passed in as a value of type va_list.