I'm trying to find out my DNS server address by reading it from resolv.h's _res struct. According to man 3 resolver the setup code should be.
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
extern struct state _res;
and then I just read out whatever I need. My problem is that trying to compile this I get
resolver.c:5:21: error: conflicting types for '__res_state'
extern struct state _res;
^
/usr/include/resolv.h:251:16: note: expanded from macro '_res'
#define _res (*__res_state())
^
/usr/include/resolv.h:249:28: note: previous declaration is here
extern struct __res_state *__res_state(void) __attribute__ ((__const__));
^
1 error generated.
by clang.
What am I doing wrong?
You shouldn't declare _res yourself - resolv.h includes the right declaration (despite what the man page implies).
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
int main() {
// call this first
res_init();
// do something with this list it contains list of dns servers
_res.nsaddr_list[0];
}
Related
My compiler (GCC) is giving me the warning:
warning: implicit declaration of function
Why is it coming?
You are using a function for which the compiler has not seen a declaration ("prototype") yet.
For example:
int main()
{
fun(2, "21"); /* The compiler has not seen the declaration. */
return 0;
}
int fun(int x, char *p)
{
/* ... */
}
You need to declare your function before main, like this, either directly or in a header:
int fun(int x, char *p);
The right way is to declare function prototype in header.
Example
main.h
#ifndef MAIN_H
#define MAIN_H
int some_main(const char *name);
#endif
main.c
#include "main.h"
int main()
{
some_main("Hello, World\n");
}
int some_main(const char *name)
{
printf("%s", name);
}
Alternative with one file (main.c)
static int some_main(const char *name);
int some_main(const char *name)
{
// do something
}
When you do your #includes in main.c, put the #include reference to the file that contains the referenced function at the top of the include list.
e.g. Say this is main.c and your referenced function is in "SSD1306_LCD.h"
#include "SSD1306_LCD.h"
#include "system.h" #include <stdio.h>
#include <stdlib.h>
#include <xc.h>
#include <string.h>
#include <math.h>
#include <libpic30.h> // http://microchip.wikidot.com/faq:74
#include <stdint.h>
#include <stdbool.h>
#include "GenericTypeDefs.h" // This has the 'BYTE' type definition
The above will not generate the "implicit declaration of function" error, but below will-
#include "system.h"
#include <stdio.h>
#include <stdlib.h>
#include <xc.h>
#include <string.h>
#include <math.h>
#include <libpic30.h> // http://microchip.wikidot.com/faq:74
#include <stdint.h>
#include <stdbool.h>
#include "GenericTypeDefs.h" // This has the 'BYTE' type definition
#include "SSD1306_LCD.h"
Exactly the same #include list, just different order.
Well, it did for me.
You need to declare the desired function before your main function:
#include <stdio.h>
int yourfunc(void);
int main(void) {
yourfunc();
}
When you get the error: implicit declaration of function it should also list the offending function. Often this error happens because of a forgotten or missing header file, so at the shell prompt you can type man 2 functionname and look at the SYNOPSIS section at the top, as this section will list any header files that need to be included. Or try http://linux.die.net/man/ This is the online man pages they are hyperlinked and easy to search.
Functions are often defined in the header files, including any required header files is often the answer. Like cnicutar said,
You are using a function for which the compiler has not seen a
declaration ("prototype") yet.
If you have the correct headers defined & are using a non GlibC library (such as Musl C) gcc will also throw error: implicit declaration of function when GNU extensions such as malloc_trim are encountered.
The solution is to wrap the extension & the header:
#if defined (__GLIBC__)
malloc_trim(0);
#endif
This error occurs because you are trying to use a function that the compiler does not understand. If the function you are trying to use is predefined in C language, just include a header file associated with the implicit function.
If it's not a predefined function then it's always a good practice to declare the function before the main function.
Don't forget, if any functions are called in your function, their prototypes must be situated above your function in the code. Otherwise, the compiler might not find them before it attempts to compile your function. This will generate the error in question.
The GNU C compiler is telling you that it can find that particular function name in the program scope. Try defining it as a private prototype function in your header file, and then import it into your main file.
I think the question is not 100% answered. I was searching for issue with missing typeof(), which is compile time directive.
Following links will shine light on the situation:
https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Typeof.html
https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Alternate-Keywords.html#Alternate-Keywords
as of conculsion try to use __typeof__() instead. Also gcc ... -Dtypeof=__typeof__ ... can help.
Please find my code below.
I am getting the error "/usr/include/linux/sysctl.h:40:2: error: unknown type name ‘size_t’"
Searching online, the only suggestion is to make sure you have stddef.h included in your code, which I do as can be seem below. There does not appear to be a solution available outside of this fix, which I have tried, so I am currently at a loss as to how to move forward.
Also note, this code is not pretty, but that is not the main issue with this thread. The error I am getting does not look like it is being thrown from a mistake in my code, but I may be wrong.
#include <linux/netfilter_ipv4.h>
#include <linux/netfilter.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <sys/types.h>
#include <linux/module.h>
#include <stddef.h>
struct nf_hook_ops{
struct list_head *list;
nf_hookfn *hook;
struct module *owner;
u_int8_t pf;
unsigned int hooknum;
int priority; /* Hooks are ordered in ascending priority. */
};
int nf_register_hook(struct nf_hook_ops *reg);
void nf_unregister_hook(struct nf_hook_ops *reg);
struct nf_hook_ops nfho = {
nfho.hook = hook_func_in,
nfho.hooknum = NF_INET_LOCAL_IN,
nfho.pf = PF_INE,
nfho.priority = NF_IP_PRI_FIRST
};
nf_register_hook(&nfho); // Register the hook
C is parsed strictly top to bottom, and #include does plain old textual inclusion, not anything clever that would qualify for the name of "module import". Therefore, the order of #include directives can matter. In this case, you're getting complaints about a type defined by stddef.h, so you must make sure that stddef.h is included before whatever needs it, which could be (indeed, is) another header file.
I can reproduce the error you're getting with the following two-line source file:
#include <linux/sysctl.h>
#include <stddef.h>
→
$ gcc -fsyntax-only test.c
In file included from test.c:1:0:
/usr/include/linux/sysctl.h:39:2: error: unknown type name ‘size_t’
If I exchange the order of the #include lines,
#include <stddef.h>
#include <linux/sysctl.h>
then there is no error. This is a bug in linux/sysctl.h, but I would not hold my breath for it to be fixed. I recommend moving stddef.h to the very top of the include list.
I can not reproduce the problem with your actual list of includes,
#include <linux/netfilter_ipv4.h>
#include <linux/netfilter.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <sys/types.h>
#include <linux/module.h>
#include <stddef.h>
but a gcc -H dump does not show linux/sysctl.h getting pulled in transitively by that set of includes, so probably it's just that I have a different version of the kernel headers on my Linux box than you do.
My compiler (GCC) is giving me the warning:
warning: implicit declaration of function
Why is it coming?
You are using a function for which the compiler has not seen a declaration ("prototype") yet.
For example:
int main()
{
fun(2, "21"); /* The compiler has not seen the declaration. */
return 0;
}
int fun(int x, char *p)
{
/* ... */
}
You need to declare your function before main, like this, either directly or in a header:
int fun(int x, char *p);
The right way is to declare function prototype in header.
Example
main.h
#ifndef MAIN_H
#define MAIN_H
int some_main(const char *name);
#endif
main.c
#include "main.h"
int main()
{
some_main("Hello, World\n");
}
int some_main(const char *name)
{
printf("%s", name);
}
Alternative with one file (main.c)
static int some_main(const char *name);
int some_main(const char *name)
{
// do something
}
When you do your #includes in main.c, put the #include reference to the file that contains the referenced function at the top of the include list.
e.g. Say this is main.c and your referenced function is in "SSD1306_LCD.h"
#include "SSD1306_LCD.h"
#include "system.h" #include <stdio.h>
#include <stdlib.h>
#include <xc.h>
#include <string.h>
#include <math.h>
#include <libpic30.h> // http://microchip.wikidot.com/faq:74
#include <stdint.h>
#include <stdbool.h>
#include "GenericTypeDefs.h" // This has the 'BYTE' type definition
The above will not generate the "implicit declaration of function" error, but below will-
#include "system.h"
#include <stdio.h>
#include <stdlib.h>
#include <xc.h>
#include <string.h>
#include <math.h>
#include <libpic30.h> // http://microchip.wikidot.com/faq:74
#include <stdint.h>
#include <stdbool.h>
#include "GenericTypeDefs.h" // This has the 'BYTE' type definition
#include "SSD1306_LCD.h"
Exactly the same #include list, just different order.
Well, it did for me.
You need to declare the desired function before your main function:
#include <stdio.h>
int yourfunc(void);
int main(void) {
yourfunc();
}
When you get the error: implicit declaration of function it should also list the offending function. Often this error happens because of a forgotten or missing header file, so at the shell prompt you can type man 2 functionname and look at the SYNOPSIS section at the top, as this section will list any header files that need to be included. Or try http://linux.die.net/man/ This is the online man pages they are hyperlinked and easy to search.
Functions are often defined in the header files, including any required header files is often the answer. Like cnicutar said,
You are using a function for which the compiler has not seen a
declaration ("prototype") yet.
If you have the correct headers defined & are using a non GlibC library (such as Musl C) gcc will also throw error: implicit declaration of function when GNU extensions such as malloc_trim are encountered.
The solution is to wrap the extension & the header:
#if defined (__GLIBC__)
malloc_trim(0);
#endif
This error occurs because you are trying to use a function that the compiler does not understand. If the function you are trying to use is predefined in C language, just include a header file associated with the implicit function.
If it's not a predefined function then it's always a good practice to declare the function before the main function.
Don't forget, if any functions are called in your function, their prototypes must be situated above your function in the code. Otherwise, the compiler might not find them before it attempts to compile your function. This will generate the error in question.
The GNU C compiler is telling you that it can find that particular function name in the program scope. Try defining it as a private prototype function in your header file, and then import it into your main file.
I think the question is not 100% answered. I was searching for issue with missing typeof(), which is compile time directive.
Following links will shine light on the situation:
https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Typeof.html
https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Alternate-Keywords.html#Alternate-Keywords
as of conculsion try to use __typeof__() instead. Also gcc ... -Dtypeof=__typeof__ ... can help.
I have three files, say A.c , B.c and C.c, all of which #include common.h
In common.h, I include "sys/socket.h" and I protect the common.h by macros:
#ifndef __COMMON_H
#define __COMMON_H
// body of file goes here
#endif
When i compile the code, I get several errors such as below
In file included from /usr/include/sys/socket.h:40,
from tcpperf.h:4,
from wrapunix.c:1:
/usr/include/bits/socket.h:425: error: conflicting types for 'recvmmsg'
/usr/include/bits/socket.h:425: note: previous declaration of 'recvmmsg' was here
In file included from /usr/include/sys/socket.h:40,
from tcpperf.h:4,
from wrapsock.c:1:
As you can see wrapunix.c and wrapsock.c, they both include tcpperf.h, but tcpperf.h is guarded with macros,yet gcc complains that recvmsg was declared multiple times. How do I resolve this issue?
Update:
Here is the header of tcpperf.h, that is causing issues
#ifndef _TCPPERF_H
#define _TCPPERF_H
#include <sys/types.h>
#include <sys/socket.h>
#include <time.h>
#include <regex.h>
#include <errno.h>
#include <sched.h>
#include <pthread.h>
#include <argp.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <linux/tcp.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>
#include <sys/prctl.h>
#include <unistd.h>
#include <sys/wait.h>
#endif
The above error can be reproduced by providing "-combine -fwhole-program" flags to gcc such as
gcc -std=gnu99 -Wall -combine -fwhole-program -I. error.c wrapunix.c wrapsock.c file1.c file2.c -o file2 -lrt
The error is "conflicting types for 'recvmmsg'" rather than just duplicate definition (which would be tolerated if equal). That means your .c source receives two different version of recvmmsg: one by your direct tcpperf.h inclusion and another one by inclusion it via sys/socket.h. I believe you have another version of tcpperf.h elsewhere in inclusion path with different (perhaps older version) recvmmsg.
The problem is almost certainly related to -combine. This is a bit of a guess, but in looking at the definition of recvmmsg:
extern int recvmmsg (int __fd, struct mmsghdr *__vmessages,
unsigned int __vlen, int __flags,
__const struct timespec *__tmo);
note that it takes a struct mmsghdr as an argument. However, while this prototype is unconditional, struct mmsghdr is only defined if __USE_GNU is set:
#ifdef __USE_GNU
/* For `recvmmsg'. */
struct mmsghdr
{
struct msghdr msg_hdr; /* Actual message header. */
unsigned int msg_len; /* Number of received bytes for the entry. */
};
#endif
-combine is basically equivalent to concatenating all your files together and then compiling them. Is there any chance that between the text of wrapunix.c and wrapsock.c that GNU_SOURCE is being defined? If that happened, then the first definition of recvmmsg would use a definition of struct mmsghdr that was local to just the prototype, while the second definition would use the real struct. Those two definitions would then be incompatible, which would result in the error message that you got.
I have following simple piece of code, which is a part of ipv6 handling module in a big project.
#include <ctype.h>
#include <sys/select.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <net/if.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <sys/stat.h>
int main(){
sockaddr_in6_t* pSadrIn6 = (sockaddr_in6_t*) malloc(sizeof sockaddr_in6_t);
return 0;
}
It gives me following not error:
error: ‘sockaddr_in6_t’ undeclared (first use in this function)
Is there any special library installation or linking that I need to access the library?
It looks like you copied this code from the Linux IPv6 HOWTO but didn't copy the additional typedefs:
/*
** Type definitions (for convenience).
*/
typedef enum { false = 0, true } boolean;
typedef struct sockaddr_in sockaddr_in_t;
typedef struct sockaddr_in6 sockaddr_in6_t;
Personally I would just use the types as they are (instead of extra typedefs to avoid typing struct), but whatever