Introduction to C Programming - Programming Errors

It is quite likely that your attempt to write a hello world program will be completely successful and will work first time. It will probably be the only C program you ever write that works first time. There are many different types of mistake you might make when writing C programs. These can result in messages from the compiler, or sometimes the linker, programs that bring the computer grinding to a halt or programs that simply produce the wrong answer to whatever problem they were trying to solve or programs that get stuck in a never-ending loop. As a beginner you are more likely to encounter compiler or linker error messages, particularly if you have already done some programming in another programming language. The rest of this section illustrates some possible errors and their consequences.

In the first example the programmer has forgotten that the C programming language is case sensitive.

MAIN()
{
	printf("hello, world\n");
}

ld: Undefined -
	.main
	_main
	_end

ld: Undefined symbol 
   _main 
Compilation failed

Linker Error: Undefined symbol _main in function main

hw.c

Microsoft (R) Overlay Linker  Version 3.65   
Copyright (C) Microsoft Corp 1983-1988.  All rights reserved.

Object Modules [.OBJ]: HW.OBJ 
Run File [HW.EXE]: HW.EXE /NOI
List File [NUL.MAP]: NUL
Libraries [.LIB]: SLIBCEC.LIB 

LINK : error L2029: Unresolved externals:



_main in file(s):
 C:\MSC\LIB\SLIBCEC.LIB(dos\crt0.asm)

There was 1 error detected

All these errors reflect the fact that the linker cannot put the program together properly. On Unix based systems the linker is usually called "ld". Along with the user supplied main() function all C programs include something often called the run-time support package which is actually the code that the operating system kicks into life when starting up your program. The run-time support package then expects to call the user supplied function main(), if there is no user supplied main() then the linker cannot finish the installation of the run-time support package. In this case the user had supplied MAIN() rather than main(). "MAIN" is a perfectly valid C function name but it isn't "main".

The rather extensive set of messages from Microsoft C 5.1 were partly generated by the compiler, which translated the program without difficulty, and partly generated by the linker. The reference to "crt0" is, in fact, a reference to the C Run Time package which tries to call main() to start the program running.

In the second example the programmer, probably confusing C with another programming language, had used single quotes rather than double quotes to enclose the string passed to printf().

main()
{
	printf('hello, world\n');
}

"hw.c", line 3: too many characters in character constant
"hw.c", line 3: warning: Character constant contains more than one byte

"hw.c", line 3: too many characters in character constant
Compilation failed

Error C:\ISPTESTS\HW.C 3:
Character constant too long in function main

hw.c
hw.c(3) : error C2015: too many chars in constant

In the third example the programmer, again possibly confused by another programming language, had missed out the semi-colon on line 3.

main()
{
	printf("hello, world\n")
}

"hw.c", line 4: syntax error

"hw.c", line 4: syntax error at or near symbol }
Compilation failed

Error C:\ISPTESTS\HW.C 4:
Statement missing ; in function main

hw.c
hw.c(4) : error C2143: syntax error : missing ';' before '}'

In the fourth example the programmer wrote print() rather than printf().

main()
{
	print("hello, world\n");
}

ld: Undefined -
	.print
	_print

ld: Undefined symbol 
   _print 
Compilation failed

Linking C:\ISPTESTS\HW.C 4:
Linker Error: Undefined symbol _print in module HW.C

hw.c

Microsoft (R) Overlay Linker  Version 3.65   
Copyright (C) Microsoft Corp 1983-1988.  All rights reserved.

Object Modules [.OBJ]: HW.OBJ 
Run File [HW.EXE]: HW.EXE /NOI
List File [NUL.MAP]: NUL
Libraries [.LIB]: SLIBCEC.LIB 

LINK : error L2029: Unresolved externals:



_print in file(s):
 HW.OBJ(hw.c)

There was 1 error detected

In the final example the programmer left out the parentheses immediately after main.

main
{
	printf("hello, world\n");
}

"hw.c", line 2: syntax error
"hw.c", line 3: illegal character: 134 (octal)
"hw.c", line 3: cannot recover from earlier errors: goodbye!

"hw.c", line 2: syntax error at or near symbol {
"hw.c", line 2: unknown size
Compilation failed

Error C:\ISPTESTS\HW.C 2: Declaration syntax error

hw.c
hw.c(2) : error C2054: expected '(' to follow 'main'

You may find it interesting to try the various erroneous versions of the "hello world" program on your particular system. Do you think your compiler generates more helpful error messages?

If you are using Turbo C you will see the following message, even when compiling a correct version of the hello world program

Warning C:\ISPTESTS\HW.C 4:
   Function should return a value in function main

The message means that the user supplied function main() should return a value to the run-time support package. There is no requirement to do so and most compilers recognise main() as a special case in this respect. Returning a value to the run-time support package should not be confused with returning a value, sometimes known as an exit code, to the host operating system.