Is errnum an external variable like errno?

The strerror library function definition is :

char *strerror (int errnum)

It will map the integer errnum argument (which can be the errno value) to an error rmessage and return a pointer to the message.

I don't know how to use errnum, I tried to use it an external variable like errno, but doesn't work.

In the following script,

if (!(df = fopen(data_file, "r")))
{
fprintf(stderr, "(%s) Cannot open %s: %s\n", argv[0], data_file, strerror(errnum));
exit(1);
}

I got the error message, it shows :
'errnum' : undeclared identifier

Who can give me any idea? Thanks.

You need to use errno itself. errno is set on an error in a system-level call. Because errno holds the value for the last call that set it, this value may be changed by succeeding calls. Always check errno immediately before and after a call that may set it.

Code:

---

fprintf(stderr, "(%s) Cannot open %s: %s\n", argv[0], data_file, strerror(errnum));

to

fprintf(stderr, "(%s) Cannot open %s: %s\n", argv[0], data_file, strerror( errno ));

---

On my system errno isn't a variable but a #defined
function call.