How should I cast...

[elija]

Linux format have started a new programming academy which is a c primer. Very cool and a good way to get another perspective.

The first code is a basic shell!

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

int parse(char **args)
{

    static char line[100];
    printf("> ");
    if (gets(line) == NULL)
        return -1;


    *args++ = strtok(line, " \t");
    while (*args++ = strtok(NULL, " \t"));

    return 1;
}
void main()
{
    char *args[20];
    while(parse(args) > 0) {
        if (fork() == 0) {
            execvp(args[0], args);
            printf("%s not found\n", args[0]);
            exit(1);
        } else {
            wait(0);
        }
    }

}

I mostly understand this but... the lines in red generate warnings:

warning: assignment makes pointer from integer without a cast

So reading the strtok documentation I see that it returns a char * so I cast to char *

 *args++ = (char *)strtok(line, " \t");
    while (*args++ = (char *)strtok(NULL, " \t"));

They generate a different warning:

warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

What should I cast to so that I get rid of the warnings?

[Lakshmipathi]

With my gcc version It gave a bonus warning message

$ gcc elija.c
elija.c: In function ‘parse’:
elija.c:14: warning: assignment makes pointer from integer without a cast
elija.c:15: warning: assignment makes pointer from integer without a cast
/tmp/cchURIpq.o: In function `parse':
elija.c.text+0x24): warning: the `gets' function is dangerous and should not be used.

$ gcc --version
gcc (GCC) 4.4.4 20100630 (Red Hat 4.4.4-10)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Googling around a bit came across suggestion - Turn on more warning with "-Wall option"
gave this

[laks@space ~]$ gcc elija.c -Wall
elija.c: In function ‘parse’:
elija.c:14: warning: implicit declaration of function ‘strtok’
elija.c:14: warning: assignment makes pointer from integer without a cast
elija.c:15: warning: assignment makes pointer from integer without a cast
elija.c:15: warning: suggest parentheses around assignment used as truth value
elija.c: At top level:
elija.c:19: warning: return type of ‘main’ is not ‘int’
elija.c: In function ‘main’:
elija.c:28: warning: implicit declaration of function ‘wait’
/tmp/ccIHuq0E.o: In function `parse':
elija.c.text+0x24): warning: the `gets' function is dangerous and should not be used.

I did man strtok seems like it needs string.h which is missing with your code.
Added #include <string.h> then ran without -Wall now the warning disappeared

Thanks to your code - I learned about new method to get rid of warnings , previously I used to ignore them.

[elija]

Thanks Laks, I didn't even think about needing another header as the code works as printed. Loving the -Wall option as I was taught in college that warnings are basically errors that need to man up. So I'm having a go at fixing them up now.

Oh and

richard@andromeda shell $ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.6.1-4' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++,go --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-multiarch --with-multiarch-defaults=x86_64-linux-gnu --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib/x86_64-linux-gnu --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib/x86_64-linux-gnu --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin --enable-objc-gc --with-arch-32=i586 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.1 (Debian 4.6.1-4)

[Lakshmipathi]

I find it strange that gcc version 4.6.1 didn't show the "gets dangerous" warning message but gcc 4.4.4 shows it ?
Elija,you may be interested in reading more about it here - c - warning:gets function is dangerous - Stack Overflow

[elija]

That's an interesting read. Thanks.

I also read the fgets manpage and it says that gets is deprecated, so I wonder if gcc no longer issues a warning? I can't find anything on Google about it though.

Final code

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

//  Missing headers
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>

int parse(char **args)
{

	static char line[100];
	int l = 0;
	
	printf("> ");
	if (fgets(line, 100, stdin) == NULL)
		return -1;

	//  There is probably a simpler way of removing the carriage return
	l = strlen(line);

	if (10 == *(line + (l-1))) {
		*(line + (--l)) = '\0';
	}

	// cast strtok return to char *
	*args++ = (char *)strtok(line, " \t");

	// cast strtok return to char *
	// put paranthesis around assignment
	while ((*args++ = (char *)strtok(NULL, " \t")));

	return 1;
}

//  main should return int
int main()
{
	char *args[20];
	while(parse(args) > 0) {
		if (fork() == 0) {
			execvp(args[0], args);
			printf("%s not found\n", args[0]);
			exit(1);
		} else {
			wait(0);
		}
	}

	//  main should return int
	return 0;
}

[Lakshmipathi]

Okay. I checked above code ,Its fine with my gcc version too.