=============== Error Reporting =============== 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 ``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``. ``volatile int errno`` [Variable] 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 Section 24.4 [Defining Signal Handlers], page 681. 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. 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 ``errno`` when they succeed; thus, the value of ``errno`` after a successful call is not necessarily zero, and you should not use ``errno`` to determine whether a call failed. The proper way to do that is documented for each function. If the call failed, you can examine ``errno``. Many library functions can set ``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 ``errno`` when the function returns an error. **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 GNU/Linux and GNU/Hurd systems. The GNU C Library, 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 GNU/Hurd systems, 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 GNU/Hurd systems, 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 GNU systems, but they can occur using the GNU C Library on other systems. Macro:`int` **EPERM** Operation not permitted; only the owner of the file (or other resource) or processes with special privileges can perform the operation. ``int ENOENT`` 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. ``int ESRCH`` No process matches the specified process ID. ``int EINTR`` Interrupted function 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. ``int EIO`` Input/output error; usually used for physical read or write errors. ``int ENXIO`` 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. ``int E2BIG`` 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 GNU/Hurd systems. ``int ENOEXEC`` Invalid executable file format. This condition is detected by the ``exec`` functions; see Executing a File. ``int EBADF`` 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). ``int ECHILD`` 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. ``int EDEADLK`` 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. ``int ENOMEM`` No memory available. The system cannot allocate more virtual memory because its capacity is full. ``int EACCES`` Permission denied; the file permissions do not allow the attempted operation. ``int EFAULT`` Bad address; an invalid pointer was detected. On GNU/Hurd systems, this error never happens; you get a signal instead. ``int ENOTBLK`` 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. ``int EBUSY`` 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. ``int EEXIST`` File exists; an existing file was specified in a context where it only makes sense to specify a new file. ``int EXDEV`` 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). ``int ENODEV`` The wrong type of device was given to a function that expects a particular sort of device. ``int ENOTDIR`` A file that isn’t a directory was specified when a directory is required. ``int EISDIR`` File is a directory; you cannot open a directory for writing, or create or remove hard links to it. ``int EINVAL`` Invalid argument. This is used to indicate various kinds of problems with passing the wrong argument to a library function. ``int EMFILE`` 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. ``int ENFILE`` 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 GNU/Hurd systems. ``int ENOTTY`` Inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file. ``int ETXTBSY`` 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 GNU/Hurd systems; the text is copied as necessary. ``int EFBIG`` File too big; the size of a file would be larger than allowed by the system. ``int ENOSPC`` No space left on device; write operation on a file failed because the disk is full. ``int ESPIPE`` Invalid seek operation (such as on a pipe). ``int EROFS`` An attempt was made to modify something on a read-only file system. ``int EMLINK`` 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). ``int EPIPE`` 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. ``int EDOM`` Domain error; used by mathematical functions when an argument value does not fall into the domain over which the function is defined. ``int ERANGE`` Range error; used by mathematical functions when the result value is not representable because of overflow or underflow. ``int EAGAIN`` 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 the GNU C Library. 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. ``int EWOULDBLOCK`` In the GNU C Library, 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. ``int EINPROGRESS`` 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. ``int EALREADY`` An operation is already in progress on an object that has non-blocking mode selected. ``int ENOTSOCK`` A file that isn’t a socket was specified when a socket is required. ``int EMSGSIZE`` The size of a message sent on a socket was larger than the supported maximum size. ``int EPROTOTYPE`` The socket type does not support the requested communications protocol. ``int ENOPROTOOPT`` You specified a socket option that doesn’t make sense for the particular protocol being used by the socket. See Socket Options. ``int EPROTONOSUPPORT`` The socket domain does not support the requested communications protocol (perhaps because the requested protocol is completely invalid). See Creating a Socket. ``int ESOCKTNOSUPPORT`` The socket type is not supported. ``int EOPNOTSUPP`` 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 GNU/Hurd systems, 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. ``int EPFNOSUPPORT`` The socket communications protocol family you requested is not supported. ``int EAFNOSUPPORT`` The address family specified for a socket is not supported; it is inconsistent with the protocol being used on the socket. See Sockets. ``int EADDRINUSE`` The requested socket address is already in use. See Socket Addresses. ``int EADDRNOTAVAIL`` 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. ``int ENETDOWN`` A socket operation failed because the network was down. ``int ENETUNREACH`` A socket operation failed because the subnet containing the remote host was unreachable. ``int ENETRESET`` A network connection was reset because the remote host crashed. ``int ECONNABORTED`` A network connection was aborted locally. ``int ECONNRESET`` 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. ``int ENOBUFS`` 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. ``int EISCONN`` You tried to connect a socket that is already connected. See Connecting. ``int ENOTCONN`` 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. ``int EDESTADDRREQ`` 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``. ``int ESHUTDOWN`` The socket has already been shut down. ``int ETOOMANYREFS`` ??? ``int ETIMEDOUT`` A socket operation with a specified timeout received no response during the timeout period. ``int ECONNREFUSED`` A remote host refused to allow the network connection (typically because it is not running the requested service). ``int ELOOP`` Too many levels of symbolic links were encountered in looking up a file name. This often indicates a cycle of symbolic links. ``int ENAMETOOLONG`` Filename too long (longer than ``PATH_MAX``; see Limits for Files) or host name too long (in ``gethostname`` or ``sethostname``; see Host Identification). ``int EHOSTDOWN`` The remote host for a requested network connection is down. ``int EHOSTUNREACH`` The remote host for a requested network connection is not reachable. ``int ENOTEMPTY`` Directory not empty, where an empty directory was expected. Typically, this error occurs when you are trying to delete a directory. ``int EPROCLIM`` 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. ``int EUSERS`` The file quota system is confused because there are too many users. ``int EDQUOT`` The user’s disk quota was exceeded. ``int ESTALE`` 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. ``int EREMOTE`` 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 GNU/Hurd systems, making this error code impossible.) ``int EBADRPC`` ??? ``int ERPCMISMATCH`` ??? ``int EPROGUNAVAIL`` ??? ``int EPROGMISMATCH`` ??? ``int EPROCUNAVAIL`` ??? ``int ENOLCK`` No locks available. This is used by the file locking facilities; see File Locks. This error is never generated by GNU/Hurd systems, but it can result from an operation to an NFS server running another operating system. ``int EFTYPE`` 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. ``int EAUTH`` ??? ``int ENEEDAUTH`` ??? ``int ENOSYS`` 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. ``int ENOTSUP`` 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. ``int EILSEQ`` While decoding a multibyte character the function came along an invalid or an incomplete sequence of bytes or the given wide character is invalid. ``int EBACKGROUND`` On GNU/Hurd systems, 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. ``int EDIED`` On GNU/Hurd systems, 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. ``int ED`` The experienced user will know what is wrong. ``int EGREGIOUS`` You did **what**? ``int EIEIO`` Go home and have a glass of warm, dairy-fresh milk. ``int EGRATUITOUS`` This error code has no purpose. ``int EBADMSG`` ``int EIDRM`` ``int EMULTIHOP`` ``int ENODATA`` ``int ENOLINK`` ``int ENOMSG`` ``int ENOSR`` ``int ENOSTR`` ``int EOVERFLOW`` ``int EPROTO`` ``int ETIME`` ``int ECANCELED`` 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. *The following error codes are defined by the Linux/i386 kernel. They are not yet documented.* ``int ERESTART`` ``int ECHRNG`` ``int EL2NSYNC`` ``int EL3HLT`` ``int EL3RST`` ``int ELNRNG`` ``int EUNATCH`` ``int ENOCSI`` ``int EL2HLT`` ``int EBADE`` ``int EBADR`` ``int EXFULL`` ``int ENOANO`` ``int EBADRQC`` ``int EBADSLT`` ``int EDEADLOCK`` ``int EBFONT`` ``int ENONET`` ``int ENOPKG`` ``int EADV`` ``int ESRMNT`` ``int ECOMM`` ``int EDOTDOT`` ``int ENOTUNIQ`` ``int EBADFD`` ``int EREMCHG`` ``int ELIBACC`` ``int ELIBBAD`` ``int ELIBSCN`` ``int ELIBMAX`` ``int ELIBEXEC`` ``int ESTRPIPE`` ``int EUCLEAN`` ``int ENOTNAM`` ``int ENAVAIL`` ``int EISNAM`` ``int EREMOTEIO`` ``int ENOMEDIUM`` ``int EMEDIUMTYPE`` ``int ENOKEY`` ``int EKEYEXPIRED`` ``int EKEYREVOKED`` ``int EKEYREJECTED`` ``int EOWNERDEAD`` ``int ENOTRECOVERABLE`` ``int ERFKILL`` ``int EHWPOISON`` 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. char * strerror ( int errnum ) Preliminary: | MT-Unsafe race:strerror | AS-Unsafe heap i18n | AC-Unsafe mem | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. The strerror function maps the error code (see Section 2.1 [Checking for Errors], page 22) specified by the errnum argument to a descriptive error message string. 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, the string might be overwritten. (But it’s guaranteed that no library function ever calls strerror behind your back.) The function strerror is declared in string.h. char * strerror_r ( int errnum, char *buf, size t n ) Preliminary: | MT-Safe | AS-Unsafe i18n | AC-Unsafe | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. The strerror_r function works like strerror but instead of returning the error message in a statically allocated buffer shared by all threads in the process, it returns a private copy for the thread. This might be either some permanent global data or a message string in the user supplied buffer starting at buf with the length of n bytes. At most n characters are written (including the NUL byte) so it is up to the user to select a buffer large enough. This function should always be used in multi-threaded programs since there is no way to guarantee the string returned by strerror really belongs to the last call of the current thread. The function strerror_r is a GNU extension and it is declared in string.h. void perror ( const char *message ) Preliminary: | MT-Safe race:stderr | AS-Unsafe corrupt i18n heap lock | AC-Unsafe corrupt lock mem fd | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. This function prints an error message to the stream stderr; see Section 12.2 [Standard Streams], page 250. 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 new- line. 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. strerror and perror produce the exact same message for any given error code; the precise text varies from system to system. With the GNU C Library, 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. char * program_invocation_name 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 Section 25.1 [Program Arguments], page 708. This variable is a GNU extension and is declared in errno.h. char * program_invocation_short_name 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. .. code-block::c #define _GNU_SOURCE #include #include #include #include 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. void error ( int status, int errnum, const char *format, . . . ) Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap i18n | AC-Safe | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. 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. void error_at_line ( int status, int errnum, const char *fname, unsigned int lineno, const char *format, . . . ) Preliminary: | MT-Unsafe race:error at line/error one per line locale | AS-Unsafe corrupt heap i18n | AC-Unsafe corrupt/error one per line | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. The error_at_line function is very similar to the error function. The only dif- ferences 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. void (*error_print_progname) (void) 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. unsigned int error_message_count 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. int error_one_per_line 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: .. code-block:: c { 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. The GNU C Library addi- tionally 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. void warn ( const char *format, . . . ) Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap i18n | AC-Unsafe corrupt lock mem | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. 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. void vwarn ( const char *format, va list ap ) Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap i18n | AC-Unsafe corrupt lock mem | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. 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. void warnx ( const char *format, . . . ) Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe corrupt lock mem | See Section 1.2.2.1 [POSIX Safety Concepts], page 2 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 dif- ference to warn is that no error number string is printed. void vwarnx ( const char *format, va list ap ) Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe corrupt lock mem | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. 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. void err ( int status, const char *format, . . . ) Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap i18n | AC-Unsafe corrupt lock mem | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. 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. void verr ( int status, const char *format, va list ap ) Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap i18n | AC-Unsafe corrupt lock mem | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. 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. void errx ( int status, const char *format, . . . ) Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe corrupt lock mem | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. 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. void verrx ( int status, const char *format, va list ap ) Preliminary: | MT-Safe locale | AS-Unsafe corrupt heap | AC-Unsafe corrupt lock mem | See Section 1.2.2.1 [POSIX Safety Concepts], page 2. 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.