errno - number of last error
Synopsis
Description
Note
#include <errno.h>extern int errno;
The integer errno is set by system calls (and some library functions) to indicate what went wrong. Its value is significant only when the call returned an error (usually -1), and a library function that does succeed is allowed to change errno.Sometimes, when -1 is also a legal return value one has to zero errno before the call in order to detect possible errors.
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.
Valid error numbers are all non-zero; errno is never set to zero by any library function. All the error names specified by POSIX.1 must have distinct values.
POSIX.1 (2001 edition) lists the following symbolic error names. Of these, EDOM and ERANGE are in the ISO C standard. ISO C Amendment 1 defines the additional error number EILSEQ for coding errors in multibyte or wide characters.
E2BIG Arg list too long EACCES Permission denied EADDRINUSE Address in use EADDRNOTAVAIL Address not available EAFNOSUPPORT Address family not supported EAGAIN Resource temporarily unavailable EALREADY Connection already in progress EBADF Bad file descriptor EBADMSG Bad message EBUSY Resource busy ECANCELED Operation canceled ECHILD No child processes ECONNABORTED Connection aborted ECONNREFUSED Connection refused ECONNRESET Connection reset EDEADLK Resource deadlock avoided EDESTADDRREQ Destination address required EDOM Domain error EDQUOT Reserved EEXIST File exists EFAULT Bad address EFBIG File too large EHOSTUNREACH Host is unreachable EIDRM Identifier removed EILSEQ Illegal byte sequence EINPROGRESS Operation in progress EINTR Interrupted function call EINVAL Invalid argument EIO Input/output error EISCONN Socket is connected EISDIR Is a directory ELOOP Too many levels of symbolic links EMFILE Too many open files EMLINK Too many links EMSGSIZE Inappropriate message buffer length EMULTIHOP Reserved ENAMETOOLONG Filename too long ENETDOWN Network is down ENETRESET Connection aborted by network ENETUNREACH Network unreachable ENFILE Too many open files in system ENOBUFS No buffer space available ENODATA No message is available on the STREAM head read queue ENODEV No such device ENOENT No such file or directory ENOEXEC Exec format error ENOLCK No locks available ENOLINK Reserved ENOMEM Not enough space ENOMSG No message of the desired type ENOPROTOOPT Protocol not available ENOSPC No space left on device ENOSR No STREAM resources ENOSTR Not a STREAM ENOSYS Function not implemented ENOTCONN The socket is not connected ENOTDIR Not a directory ENOTEMPTY Directory not empty ENOTSOCK Not a socket ENOTSUP Not supported ENOTTY Inappropriate I/O control operation ENXIO No such device or address EOPNOTSUPP Operation not supported on socket EOVERFLOW Value too large to be stored in data type EPERM Operation not permitted EPIPE Broken pipe EPROTO Protocol error EPROTONOSUPPORT Protocol not supported EPROTOTYPE Protocol wrong type for socket ERANGE Result too large EROFS Read-only file system ESPIPE Invalid seek ESRCH No such process ESTALE Reserved ETIME STREAM ioctl() timeout ETIMEDOUT Operation timed out ETXTBSY Test file busy EWOULDBLOCK Operation would block (may be same value as EAGAIN) EXDEV Improper link
A common mistake is to dowhere errno no longer needs to have the value it had upon return from somecall(). If the value of errno should be preserved across a library call, it must be saved:if (somecall() == -1) { printf("somecall() failed\n"); if (errno == ...) { ... } }
if (somecall() == -1) { int errsv = errno; printf("somecall() failed\n"); if (errsv == ...) { ... } }
perror(3), strerror(3)
| ERRNO (3) | 2002-10-05 |