errno详解

NAME         top

       errno - number of last error

SYNOPSIS         top

       #include <errno.h>

DESCRIPTION         top

       The <errno.h> header file defines the integer variable errno, which       is set by system calls and some library functions in the event of an       error to indicate what went wrong.  Its value is significant only       when the return value of the call indicated an error (i.e., -1 from       most system calls; -1 or NULL from most library functions); a       function that succeeds is allowed to change errno.       Valid error numbers are all nonzero; errno is never set to zero by       any system call or library function.       For some system calls and library functions (e.g., getpriority(2)),       -1 is a valid return on success.  In such cases, a successful return       can be distinguished from an error return by setting errno to zero       before the call, and then, if the call returns a status that       indicates that an error may have occurred, checking to see if errno       has a nonzero value.       errno is defined by the ISO C standard to be a modifiable lvalue of       type int, and must not be explicitly declared; errno may be a macro.       errno is thread-local; setting it in one thread does not affect its       value in any other thread.       All the error names specified by POSIX.1 must have distinct values,       with the exception of EAGAIN and EWOULDBLOCK, which may be the same.       Below is a list of the symbolic error names that are defined on       Linux.  Some of these are marked POSIX.1, indicating that the name is       defined by POSIX.1-2001, or C99, indicating that the name is defined       by C99.       E2BIG           Argument list too long (POSIX.1)       EACCES          Permission denied (POSIX.1)       EADDRINUSE      Address already in use (POSIX.1)       EADDRNOTAVAIL   Address not available (POSIX.1)       EAFNOSUPPORT    Address family not supported (POSIX.1)       EAGAIN          Resource temporarily unavailable (may be the same                       value as EWOULDBLOCK) (POSIX.1)       EALREADY        Connection already in progress (POSIX.1)       EBADE           Invalid exchange       EBADF           Bad file descriptor (POSIX.1)       EBADFD          File descriptor in bad state       EBADMSG         Bad message (POSIX.1)       EBADR           Invalid request descriptor       EBADRQC         Invalid request code       EBADSLT         Invalid slot       EBUSY           Device or resource busy (POSIX.1)       ECANCELED       Operation canceled (POSIX.1)       ECHILD          No child processes (POSIX.1)       ECHRNG          Channel number out of range       ECOMM           Communication error on send       ECONNABORTED    Connection aborted (POSIX.1)       ECONNREFUSED    Connection refused (POSIX.1)       ECONNRESET      Connection reset (POSIX.1)       EDEADLK         Resource deadlock avoided (POSIX.1)       EDEADLOCK       Synonym for EDEADLK       EDESTADDRREQ    Destination address required (POSIX.1)       EDOM            Mathematics argument out of domain of function                       (POSIX.1, C99)       EDQUOT          Disk quota exceeded (POSIX.1)       EEXIST          File exists (POSIX.1)       EFAULT          Bad address (POSIX.1)       EFBIG           File too large (POSIX.1)       EHOSTDOWN       Host is down       EHOSTUNREACH    Host is unreachable (POSIX.1)       EIDRM           Identifier removed (POSIX.1)       EILSEQ          Illegal byte sequence (POSIX.1, C99)       EINPROGRESS     Operation in progress (POSIX.1)       EINTR           Interrupted function call (POSIX.1); see signal(7).       EINVAL          Invalid argument (POSIX.1)       EIO             Input/output error (POSIX.1)       EISCONN         Socket is connected (POSIX.1)       EISDIR          Is a directory (POSIX.1)       EISNAM          Is a named type file       EKEYEXPIRED     Key has expired       EKEYREJECTED    Key was rejected by service       EKEYREVOKED     Key has been revoked       EL2HLT          Level 2 halted       EL2NSYNC        Level 2 not synchronized       EL3HLT          Level 3 halted       EL3RST          Level 3 halted       ELIBACC         Cannot access a needed shared library       ELIBBAD         Accessing a corrupted shared library       ELIBMAX         Attempting to link in too many shared libraries       ELIBSCN         lib section in a.out corrupted       ELIBEXEC        Cannot exec a shared library directly       ELOOP           Too many levels of symbolic links (POSIX.1)       EMEDIUMTYPE     Wrong medium type       EMFILE          Too many open files (POSIX.1)       EMLINK          Too many links (POSIX.1)       EMSGSIZE        Message too long (POSIX.1)       EMULTIHOP       Multihop attempted (POSIX.1)       ENAMETOOLONG    Filename too long (POSIX.1)       ENETDOWN        Network is down (POSIX.1)       ENETRESET       Connection aborted by network (POSIX.1)       ENETUNREACH     Network unreachable (POSIX.1)       ENFILE          Too many open files in system (POSIX.1)       ENOBUFS         No buffer space available (POSIX.1 (XSI STREAMS                       option))       ENODATA         No message is available on the STREAM head read queue                       (POSIX.1)       ENODEV          No such device (POSIX.1)       ENOENT          No such file or directory (POSIX.1)       ENOEXEC         Exec format error (POSIX.1)       ENOKEY          Required key not available       ENOLCK          No locks available (POSIX.1)       ENOLINK         Link has been severed (POSIX.1)       ENOMEDIUM       No medium found       ENOMEM          Not enough space (POSIX.1)       ENOMSG          No message of the desired type (POSIX.1)       ENONET          Machine is not on the network       ENOPKG          Package not installed       ENOPROTOOPT     Protocol not available (POSIX.1)       ENOSPC          No space left on device (POSIX.1)       ENOSR           No STREAM resources (POSIX.1 (XSI STREAMS option))       ENOSTR          Not a STREAM (POSIX.1 (XSI STREAMS option))       ENOSYS          Function not implemented (POSIX.1)       ENOTBLK         Block device required       ENOTCONN        The socket is not connected (POSIX.1)       ENOTDIR         Not a directory (POSIX.1)       ENOTEMPTY       Directory not empty (POSIX.1)       ENOTSOCK        Not a socket (POSIX.1)       ENOTSUP         Operation not supported (POSIX.1)       ENOTTY          Inappropriate I/O control operation (POSIX.1)       ENOTUNIQ        Name not unique on network       ENXIO           No such device or address (POSIX.1)       EOPNOTSUPP      Operation not supported on socket (POSIX.1)                       (ENOTSUP and EOPNOTSUPP have the same value on Linux,                       but according to POSIX.1 these error values should be                       distinct.)       EOVERFLOW       Value too large to be stored in data type (POSIX.1)       EPERM           Operation not permitted (POSIX.1)       EPFNOSUPPORT    Protocol family not supported       EPIPE           Broken pipe (POSIX.1)       EPROTO          Protocol error (POSIX.1)       EPROTONOSUPPORT Protocol not supported (POSIX.1)       EPROTOTYPE      Protocol wrong type for socket (POSIX.1)       ERANGE          Result too large (POSIX.1, C99)       EREMCHG         Remote address changed       EREMOTE         Object is remote       EREMOTEIO       Remote I/O error       ERESTART        Interrupted system call should be restarted       EROFS           Read-only filesystem (POSIX.1)       ESHUTDOWN       Cannot send after transport endpoint shutdown       ESPIPE          Invalid seek (POSIX.1)       ESOCKTNOSUPPORT Socket type not supported       ESRCH           No such process (POSIX.1)       ESTALE          Stale file handle (POSIX.1)                       This error can occur for NFS and for other                       filesystems       ESTRPIPE        Streams pipe error       ETIME           Timer expired (POSIX.1 (XSI STREAMS option))                       (POSIX.1 says "STREAM ioctl(2) timeout")       ETIMEDOUT       Connection timed out (POSIX.1)       ETXTBSY         Text file busy (POSIX.1)       EUCLEAN         Structure needs cleaning       EUNATCH         Protocol driver not attached       EUSERS          Too many users       EWOULDBLOCK     Operation would block (may be same value as EAGAIN)                       (POSIX.1)       EXDEV           Improper link (POSIX.1)       EXFULL          Exchange full

NOTES         top

       A common mistake is to do           if (somecall() == -1) {               printf("somecall() failed\n");               if (errno == ...) { ... }           }       where errno no longer needs to have the value it had upon return from       somecall() (i.e., it may have been changed by the printf(3)).  If the       value of errno should be preserved across a library call, it must be       saved:           if (somecall() == -1) {               int errsv = errno;               printf("somecall() failed\n");               if (errsv == ...) { ... }           }       It was common in traditional C to declare errno manually (i.e.,       extern int errno) instead of including <errno.h>.  Do not do this.       It will not work with modern versions of the C library.  However, on       (very) old UNIX systems, there may be no <errno.h> and the       declaration is needed.

原帖地址：http://man7.org/linux/man-pages/man3/errno.3.html