undefined reference to `main'

otacon 14112 · 03-18-2008, 02:24 AM

Hi guys, when I wrote the following program and ran it, I got the errors listed below.

Code:

#include <stdio.h>
#include <string.h>

int main(void)
{

  FILE* f_write;
  char buf[1001];

  f_write = fopen("hello.c", "w");

  strcpy(buf, "#include <stdio.h>\n\nint main(void)\n{\n  printf(\"Hello world!\");\n  return 0;\n}\n");
//  buf = "#include <stdio.h>\n\nint main(void)\n{\n  printf(\"Hello world!\";\n  return 0;\n}";

  fputs(buf, f_write);

  fflush(stdout);

  close();

  system("gcc -o hello hello.c");

  return 0;
}

/usr/lib/gcc/i486-linux-gnu/4.1.3/../../../../lib/crt1.o: In function `_start':
(.text+0x18): undefined reference to `main'
collect2: ld returned 1 exit status

Does anyone know why it's giving me these errors? I was just messing around and wanted to try and just see if it would work, and now I'm actually really wanting to get it to work just to do it. I tried the compile line that I put in the program, and it works on the command line. Why doesn't it work in the program?

otacon

ophirg · 03-18-2008, 05:18 AM

why fflush to stdout?
and you need to include stdlib.h

95se · 03-18-2008, 11:24 AM

First, add this to the top:

Code:

#include <unistd.h>

Try compiling the program again, you'll get an error.
I think close does not do what you think it's suppose to do. But I'll get back to that in a second... Your hello.c is empty when you compile it (try putting system("cat hello.c") right before the other call to system), so there is no main function defined in it (this is because you didn't actually close the file). _start is actually the entry point in an elf executable, the standard C library actually implements this, and does some set up/clean up before/after calling main. If it doesn't exist, then there will be an undefined reference to main in the C runtime library's _start function. Back to the actual problem; fclose is what you are looking for, and it takes a FILE pointer as an argument.

Code:

#include <stdio.h>
#include <string.h>

int main(void)
{

  FILE* f_write;
  char buf[1001];

  f_write = fopen("hello.c", "w");

  strcpy(buf, "#include <stdio.h>\n\nint main(void)\n{\n  printf(\"Hello world!\\n\");\n  return 0;\n}\n");
//  buf = "#include <stdio.h>\n\nint main(void)\n{\n  printf(\"Hello world!\";\n  return 0;\n}";

  fputs(buf, f_write);

  fclose(f_write);

  system("gcc -o hello hello.c");

  return 0;
}

otacon 14112 · 03-18-2008, 12:27 PM

Ok so my mistake was I should have put in fputs and stdlib. I should have remembered that stdlib solves alot of warnings. I never worked much with file handling in c. I just found an online example that made me want to try it, and I ended up modifying it to see what would happen. I should have known I would forget some stuff during my vacation from c. I've been messing with php and mysql a lot lately.

Thanks ophirg and 95se. But why would I need unistd.h? I skimmed over an online article about it and I didn't really see anything that would help my program unless I overlooked something.

95se · 03-18-2008, 04:23 PM

Quote:

Originally Posted by otacon 14112

Thanks ophirg and 95se. But why would I need unistd.h?

No problem. You don't need unistd.h, it was just my way of telling you why it would compile with the close() function call you had, but why it isn't working the way you thought (ie. the function exists, its just not the one you want).