c compilation error

kpachopoulos · 08-14-2006, 02:01 PM

Hi,
here's the code:

Code:

#include <stdlib.h>
#include <stdio.h>

FILE* source;
char c;

main(int argc, char *argv[])
{
	if (argc==2)
	{	
		source=fopen(argv[1],"r");
		if (source==NULL) perror("error opening file");
	
		print_output(source);
	}
	
}

print_output(FILE* f)
{		
	//char* buf_pointer;

	while (c!=EOF)
	{
		//buf_pointer=do_buffer(f);
		printf("%s", (char*) (do_buffer(f)));
	}

	printf("\n");
}

char* do_buffer(FILE* f)
{
	char buffer[80];
	int i=0;
	initialize_buffer(&buffer);

	while ((i<79) && (c!=EOF))
	{
		c=(char)fgetc(source);
		buffer[i]=c;
		i++;
	}

	return buffer;
}

initialize_buffer(char b[])
{
	int current_pos=0;
	for (;current_pos<sizeof(b); current_pos++)
	{
		b[current_pos]=NULL;
	}
}

And this is the output, when i try to compile it:

Code:

kostas@vakhos:~$ gcc -o cp cp.c
cp.c:33: error: conflicting types for ‘do_buffer’
cp.c:26: error: previous implicit declaration of ‘do_buffer’ was here
cp.c: In function ‘do_buffer’:
cp.c:45: warning: function returns address of local variable
cp.c: In function ‘initialize_buffer’:
cp.c:53: warning: assignment makes integer from pointer without a cast
cp.c:66:2: warning: no newline at end of file

What's the problem?
Thanks

a.malich · 08-14-2006, 02:18 PM

Hi,

Quote:

cp.c: In function ‘do_buffer’:
cp.c:45: warning: function returns address of local variable

You declared buffer inside the do_buffer function and then you try to return its address - while buffer is a local variable, it's gone when the function has finished, and the value do_buffer returned points to an invalid address.

Simply declare buffer globally (like you did with c).

Quote:

cp.c: In function ‘initialize_buffer’:
cp.c:53: warning: assignment makes integer from pointer without a cast

(This is a guess) NULL is an integer value, while b is an array of type char. Replace NULL with '\0' (backslash zero), which is a char.

Code:

b[current_pos]='\0';

That _should_ do it.

Quote:

cp.c:66:2: warning: no newline at end of file

Simply go to the very end of the file and hit Enter.

Don't know about the rest (yet).

Also, functions that do not return anything should be declared like this:

Code:

void i_dont_return_anything(int number);

a.malich

zhangmaike · 08-14-2006, 02:35 PM

For the implicit-declaration thing, you use do_buffer() in main() before you declare it. This is bad. Use a function prototype, like this:

Code:

char* do_buffer(FILE*);

The same should be done for your other functions. Place them in your global declarations, before main().

By the way, why is source declared globally, when you are passing it as a parameter? Generally, it is considered better to pass variables as parameters than to define global ones. Same thing with the char c. Although it isn't used as a parameter, you'll have to be very careful to avoid accidentally clobbering c by a calling a function which also uses c. A warning: c is being used uninitialized in print_output() when first called from main().

As far as declaring buffer as global to solve the returning-address-of-local-variable problem in do_buffer(), the usual way of doing this is to avoid global variables and pass a char* as a parameter to do_buffer(), and declare the buffer itself in the routine that calls do_buffer()... that is, declare char buffer[80] in print_output(), and pass it as a parameter to do_buffer(). If you do it this way, you'll have to add a char* parameter to both the function definition and the prototype.

Quote:

NULL is an integer value, while b is an array of type char. Replace NULL with '\0' (backslash zero), which is a char.

Sort of. NULL has the type of void*. Assigning a pointer type to any integer type always gives a warning. Using 0 or '\0' will solve this problem.

a.malich · 08-14-2006, 02:52 PM

Thanks for clearing some things up, zhangmaike. I only skimmed through the code, hence I thought that main() is the 'caller' of do_buffer. I never would've thought NULL is void*, though. :)

a.malich