a problem about "incompatible implicit declaration "

kingliang123 · 01-16-2010, 08:49 PM

/**main.c**/
/**no head file**/
int main()
{
printf("hello\n");
return 0;
}

build,say,warning:incompatible implicit declaration of built-in function ‘printf’

/*main.c*/
#include<stdio.h>
int main()
{
lnux();
return 0;
}

int lnux(void)
{
return 0;
}
build,say,warning: implicit declaration of function ‘lnux’.

printf, lnux, there are both function, and didn't declaration,why build, them, appear different warning?

due to printf is lib function? lnux not

smeezekitty · 01-16-2010, 09:44 PM

i assume because printf is not printf(char *) like you passed to it but rather printf(char *, ...)

konsolebox · 01-16-2010, 11:41 PM

you must declare the function prototype first or define the function before main.

Code:

int lnux(...)
{
   ...
}

main() ...

nadroj · 01-17-2010, 12:10 AM

Im quite confused about what you're actually asking. I was going to explain about what konsolebox said, but I dont know if thats what you're asking. If it isnt, you can turn up your warning levels and you will notice that your first example has a similar warning message as the second one.

Code:

jordan@jordan-laptop:~$ gcc test.c
test.c: In function ‘main’:
test.c:3: warning: incompatible implicit declaration of built-in function ‘printf’

Code:

jordan@jordan-laptop:~$ gcc -Wall test.c
test.c: In function ‘main’:
test.c:3: warning: implicit declaration of function ‘printf’
test.c:3: warning: incompatible implicit declaration of built-in function ‘printf’

Because of this, I think you only see that warning message unique to your first example because its specific to standard C functions. I have read that 'implicit functions' are basically prototyped as

Code:

int foo()

So that it's return type is int, but nothing is said about its arguments. Note that "foo()" does not mean the function does not accept any arguments--"foo(void)" is used for that case. So I would try some other "simpler" standard functions, i.e. one that matches the implicit function prototype of "int foo()". Maybe in this case you will only get the 1 warning message that you get with your "lnux" function", and not the "incompatible" one.

raju.mopidevi · 01-17-2010, 01:15 AM

printf() already declared in stdio.h
you have to declare your lnux().
i.e.

/*main.c*/
int lnux(void);
#include<stdio.h>
int main()
{
lnux();
return 0;
}

int lnux(void)
{
return 0;
}

konsolebox · 01-17-2010, 01:22 AM

@nadroj There were actually 2 warnings that's why it's confusing. I only commented on the 2nd one.