Undefined Reference to Main Error

This is a discussion on Undefined Reference to Main Error within the C Programming forums, part of the General Programming Boards category; I've been studying "The C Programming Language" text and checking my answers here: The C Programming Language Answers To Exercises ...

  1. #1
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    Undefined Reference to Main Error

    I've been studying "The C Programming Language" text and checking my answers here: The C Programming Language Answers To Exercises. I'm on exercise 4-2 and when I try to compile the answer given on the website, I get this error:
    [T3256@GURY Exercise.4-2]$ cc answer.c
    /usr/lib/gcc/i386-redhat-linux/4.3.2/../../../crt1.o: In function `_start':
    (.text+0x18): undefined reference to `main'
    collect2: ld returned 1 exit status
    I'm confused because the main function is defined at the very bottom of the answer. Here is the code:
    Code:
    /*
    **  Written by Dann Corbit as K&R 2, Exercise 4-2 (Page 73).
    **  Keep in mind that this is *JUST* a student exercise, and is
    **  light years away from being robust.
    **
    **  Actually, it's kind of embarassing, but I'm too lazy to fix it.
    **
    **  Caveat Emptor, not my fault if demons fly out of your nose,
    **  and all of that.
    */
    #include <ctype.h>
    #include <limits.h>
    #include <float.h>
    #include <signal.h>
    #include <stdio.h>
    
    int my_atof(char *string, double *pnumber)
    {
        /* Convert char string to double data type. */
        double retval;
        double one_tenth = 0.1;
        double ten = 10.0;
        double zero = 0.0;
        int found_digits = 0;
        int is_negative = 0;
        char *num;
    
        /* Check pointers. */
        if (pnumber == 0) {
            return 0;
        }
        if (string == 0) {
            *pnumber = zero;
            return 0;
        }
        retval = zero;
    
        num = string;
    
        /* Advance past white space. */
        while (isspace(*num))
            num++;
    
        /* Check for sign. */
        if (*num == '+')
            num++;
        else if (*num == '-') {
            is_negative = 1;
            num++;
        }
        /* Calculate the integer part. */
        while (isdigit(*num)) {
            found_digits = 1;
            retval *= ten;
            retval += *num - '0';
            num++;
        }
    
        /* Calculate the fractional part. */
        if (*num == '.') {
            double scale = one_tenth;
            num++;
            while (isdigit(*num)) {
                found_digits = 1;
                retval += scale * (*num - '0');
                num++;
                scale *= one_tenth;
            }
        }
        /* If this is not a number, return error condition. */
        if (!found_digits) {
            *pnumber = zero;
            return 0;
        }
        /* If all digits of integer & fractional part are 0, return 0.0 */
        if (retval == zero) {
            *pnumber = zero;
            return 1; /* Not an error condition, and no need to
                                     * continue. */
        }
        /* Process the exponent (if any) */
        if ((*num == 'e') || (*num == 'E')) {
            int neg_exponent = 0;
            int get_out = 0;
            long index;
            long exponent = 0;
            double getting_too_big = DBL_MAX * one_tenth;
            double getting_too_small = DBL_MIN * ten;
    
            num++;
            if (*num == '+')
                num++;
            else if (*num == '-') {
                num++;
                neg_exponent = 1;
            }
            /* What if the exponent is empty?  Return the current result. */
            if (!isdigit(*num)) {
                if (is_negative)
                    retval = -retval;
    
                *pnumber = retval;
    
                return (1);
            }
            /* Convert char exponent to number <= 2 billion. */
            while (isdigit(*num) && (exponent < LONG_MAX / 10)) {
                exponent *= 10;
                exponent += *num - '0';
                num++;
            }
    
            /* Compensate for the exponent. */
            if (neg_exponent) {
                for (index = 1; index <= exponent && !get_out; index++)
                    if (retval < getting_too_small) {
                        get_out = 1;
                        retval = DBL_MIN;
                    } else
                        retval *= one_tenth;
            } else
                for (index = 1; index <= exponent && !get_out; index++) {
                    if (retval > getting_too_big) {
                        get_out = 1;
                        retval = DBL_MAX;
                    } else
                        retval *= ten;
                }
        }
        if (is_negative)
            retval = -retval;
    
        *pnumber = retval;
    
        return (1);
    }
    /*
    ** Lame and evil wrapper function to give the exercise the requested
    ** interface.  Dann Corbit will plead innocent to the end.
    ** It's very existence means that the code is not conforming.
    ** Pretend you are a C library implementer, OK?  But you would fix
    ** all those bleeding gaps, I am sure.
    */
    double atof(char *s)
    {
        double d = 0.0;
        if (!my_atof(s, &d))
        {
    #ifdef DEBUG
            fputs("Error converting string in [sic] atof()", stderr);
    #endif
            raise(SIGFPE);
        }
        return d;
    }
    
    #ifdef UNIT_TEST
    char *strings[] = {
        "1.0e43",
        "999.999",
        "123.456e-9",
        "-1.2e-3",
        "1.2e-3",
        "-1.2E3",
        "-1.2e03",
        "cat",
        "",
        0
    };
    int main(void)
    {
        int i = 0;
        for (; *strings[i]; i++)
            printf("atof(%s) = %g\n", strings[i], atof(strings[i]));
        return 0;
    }
    #endif
    I'd appreciate some help. Thanks.

  2. #2
    Registered User slingerland3g's Avatar
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    At first glance place #endif above main.

  3. #3
    Registered User slingerland3g's Avatar
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    Also are you sure you are needing to place char *strings within your #ifdef UNIT_TEST? As this may fail your main code as this references strings[].

  4. #4
    C++まいる!Cをこわせ! Elysia's Avatar
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    If UNIT_TEST isn't defined, then main will never be in the code either.
    Not what you want, I suspect.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    I didn't write that code btw. It's just the answer I'm trying to use to check my work. I'm only on chapter 4.

  6. #6
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    I concur with the above diagnosis. I'd say the #endif should go just prior to the 0 entry for the string[] initilizer list. That way, the string is legally defined but may be empty list depending on existence of UNIT_TEST.

  7. #7
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    I changed the code like this:
    Code:
    #ifdef UNIT_TEST
    char  *strings[] = {
        "1.0e43",
        "999.999",
        "123.456e-9",
        "-1.2e-3",
        "1.2e-3",
        "-1.2E3",
        "-1.2e03",
        "cat",
        "",
        0
    };
    #endif
    
    int  main(void)
    {
        int             i = 0;
        for (; *strings[i]; i++)
            printf("atof(%s) = %g\n", strings[i], atof(strings[i]));
        return 0;
    }
    Now I get this error:
    [T3256@GURY Exercise.4-2]$ cc temp.c
    temp.c: In function ‘main’:
    temp.c:175: error: ‘strings’ undeclared (first use in this function)
    temp.c:175: error: (Each undeclared identifier is reported only once
    temp.c:175: error: for each function it appears in.)
    Thanks.

  8. #8
    C++まいる!Cをこわせ! Elysia's Avatar
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    Again, it's the same ordeal.
    You never create your strings array in the code because UNIT_TEST isn't defined.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  9. #9
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    I betcha the first source code posted was correct.

    If UNIT_TEST is defined than that code includes main(). Else it doesn't.

    This .c source is meant to be linked with other source code - where main() is usually during "production". Whereas, the UNIT_TEST switch is meant to have this piece of code compiled and executed in isolation.

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    Well all of this is beyond my level of C knowledge. I'll just have to do without this one test for now. Thanks for the help anyway guys.

  11. #11
    C++まいる!Cをこわせ! Elysia's Avatar
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    Well, if it helps, all you need to do is add:
    #define UNIT_TEST
    At the top of the source file. Then it should compile.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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