Declaration

extern int errno;

Description

This variable is hold a value that designates a system-level error. The perror() function prints out a message corresponding this the set error value in errno when it is called. A value of zero indicates no error. The other error values are given in the table below.

errno Value Table

Macro	Description	Value
EPERM	Operation not permitted	1
ENOENT	No such file or directory	2
ESRCH	No such process	3
EINTR	Interrupted function	4
EIO	I/O error	5
ENXIO	No such device or address	6
E2BIG	Argument list too long	7
ENOEXEC	Exec format error	8
EBADF	Bad file number	9
ECHILD	No spawned processes	10
EAGAIN	No more processes, not enough memory, or maximum nesting level reached	11
ENOMEM	Not enough memory	12
EACCES	Permission denied	13
EFAULT	Bad address	14
EBUSY	Device or resource busy	16
EEXIST	File exists	17
EXDEV	Cross-device link	18
ENODEV	No such device	19
ENOTDIR	Not a directory	20
EISDIR	Is a directory	21
EINVAL	Invalid argument	22
ENFILE	Too many files open in system	23
EMFILE	Too many open files	24
ENOTTY	Inappropriate I/O control operation	25
EFBIG	File too large	27
ENOSPC	No space left on device	28
ESPIPE	Invalid seek	29
EROFS	Read-only file system	30
EMLINK	Too many links	31
EPIPE	Broken pipe	32
EDOM	Math argument	33
ERANGE	Result too large	34
EDEADLK	Resource deadlock would occur	36
EDEADLOCK	Same as EDEADLK for backward compatibility	36
ENAMETOOLONG	Filename too long	38
ENOLCK	No locks available	39
ENOSYS	Function not supported	40
ENOTEMPTY	Directory not empty	41
EILSEQ	Illegal byte sequence	42
STRUNCATE	String was truncated	80

Example

#include <iostream>
#include <cerrno>

int main() {
    using namespace std;

    // Print the message before we set the error value.
    cout << "errno = " << errno << endl;
    perror(NULL);
    // Set errno to an error value.
    errno = 3;
    cout << "errno = " << errno << endl;
    perror(NULL);
    // Use the same error value with an appended message.
    cout << "errno = " << errno << endl;
    perror("XoaX.net");

    return 0;
}

Output