Missing main() prevents executable creation while linking

Question

I am using Microsoft Visual C++ on Windows. I compiled it fine without any errors. However, when I tried to execute it, I got two errors. I read the debugging errors, and I could not understand them. I am somewhat a newbie in C programming.

This code is from Kernighan and Ritchie's textbook on page 61:

#include <ctype.h>

/* atoi: convert s to integer; version 2 */
int atoi(char s[])
{
    int i, n, sign;

    for (i=0; isspace(s[i]); i++) /* skip white space */
        ;
    sign = (s[i] == '-') ? -1: 1;
    if (s[i] == '+' || s[i] == '-') /* skip sign */
        i++;
    for (n=0; isdigit(s[i]); i++)
        n = 10 * n + (s[i] - '0');
    return sign*n;
}

The error:

--------------------Configuration: 3.5 - Win32 Debug--------------------
Linking... LIBCD.lib(crt0.obj) : error LNK2001: unresolved external symbol _main Debug/3.5.exe : fatal error LNK1120: 1 unresolved externals Error executing link.exe. 3.5.exe - 2 error(s), 0 warning(s)

`However, when I tried to execute it, I got two errors` Can you please post the full error message for the errors? — Mark Byers, Dec 17 '12 at 11:33
--------------------Configuration: 3.5 - Win32 Debug-------------------- Linking... LIBCD.lib(crt0.obj) : error LNK2001: unresolved external symbol _main Debug/3.5.exe : fatal error LNK1120: 1 unresolved externals Error executing link.exe. 3.5.exe - 2 error(s), 0 warning(s) — Joseph Lee, Dec 17 '12 at 11:35
I expect it to run counting the number of spaces, digits, and characters. — Joseph Lee, Dec 17 '12 at 11:41
Perhaps you should start by reading chapter one of the K&R book ... ;-) — alk, Dec 17 '12 at 11:42
@JosephLee: Number of spaces, digits, and characters ... *of* *what*?! — David Schwartz, Dec 17 '12 at 12:03

score 5 · Answer 1 · answered Dec 17 '12 at 11:35

5

That's not a complete program. It's just a function. You can't execute it without writing some code to call it.

answered Dec 17 '12 at 11:35

David Schwartz

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Which involves writing a `main` – Pubby Dec 17 '12 at 11:39
Thank you, David. Exactly, how should I write inside the main? – Joseph Lee Dec 17 '12 at 11:57
1

@JosephLee: You should write code that does whatever you want to do. – David Schwartz Dec 17 '12 at 12:04

score 3 · Answer 2 · edited Dec 17 '12 at 12:21

3

Every C program needs an entry point and main() provides that.

Please refer to the following:

edited Dec 17 '12 at 12:21

alk

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answered Dec 17 '12 at 11:42

Jay

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score 1 · Answer 3 · answered Dec 17 '12 at 13:00

I wonder how you wrote the entire atoi function, without knowing about importance of main(). :) You should write a main() function and call your atoi like this:

#include <stdio.h>//Required for printf used in main()
#include <ctype.h>

/* atoi: convert s to integer; version 2 */
int atoi(char s[])
{
    int i, n, sign;

    for (i=0; isspace(s[i]); i++) /* skip white space */
        ;
    sign = (s[i] == '-') ? -1: 1;
    if (s[i] == '+' || s[i] == '-') /* skip sign */
        i++;
    for (n=0; isdigit(s[i]); i++)
        n = 10 * n + (s[i] - '0');
    return sign*n;
}


/* This is the part you've been missing. */
int main(int argc, char **argv)
{
   printf("%d\n",atoi("-100"));
   return 0;
}

I would also suggest you to: First try to write a hello world program in C++, then move to writing advanced stuff like this.

Missing main() prevents executable creation while linking

3 Answers3