Programming Tutorials

ERRNO.H Header File in C

By: Kamini in C Tutorials on 2007-10-03  

The header file ERRNO.H defines several macros used to define and document runtime errors. These macros are used in conjunction with the perror() function.

The ERRNO.H definitions include an external integer named errno. Many of the C library functions assign a value to this variable if an error occurs during function execution. The file ERRNO.H also defines a group of symbolic constants for these errors, listed in Table below.


The symbolic error constants defined in ERRNO.H.


Name Value Message and Meaning
E2BIG 1000 Argument list too long (list length exceeds 128 bytes).
EACCES 5 Permission denied (for example, trying to write to a file opened for read only).
EBADF 6 Bad file descriptor.
EDOM 1002 Math argument out of domain (an argument passed to a math function was outside the allowable range).
EEXIST 80 File exists.
EMFILE 4 Too many open files.
ENOENT 2 No such file or directory.
ENOEXEC 1001 Exec format error.
ENOMEM 8 Not enough core (for example, not enough memory to execute the exec() function).
ENOPATH 3 Path not found.
ERANGE 1003 Result out of range (for example, result returned by a math function is too large or too small for the return data type).

You can use errno two ways. Some functions signal, by means of their return value, that an error has occurred. If this happens, you can test the value of errno to determine the nature of the error and take appropriate action. Otherwise, when you have no specific indication that an error occurred, you can test errno. If it's nonzero, an error has occurred, and the specific value of errno indicates the nature of the error. Be sure to reset errno to zero after handling the error.

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