What is the proper #include for the function 'sleep()'? - c

I am using the Big Nerd Ranch book Objective-C Programming, and it starts out by having us write in C in the first few chapters. In one of my programs it has me create, I use the sleep function. In the book it told me to put #include <stdlib.h> under the #include <stdio.h> part. This is supposed to get rid of the warning that says "Implicit declaration of function 'sleep' is invalid in C99". But for some reason after I put #include <stdlib.h>, the warning does not go away.. This problem does not stop the program from running fine, but I was just curious on which #include I needed to use!

The sleep man page says it is declared in <unistd.h>.
Synopsis:
#include <unistd.h>
unsigned int sleep(unsigned int seconds);

sleep is a non-standard function.
On UNIX, you shall include <unistd.h>.
On MS-Windows, Sleep is rather from <windows.h>.
In every case, check the documentation.

this is what I use for a cross-platform code:
#ifdef _WIN32
#include <Windows.h>
#else
#include <unistd.h>
#endif
int main()
{
pollingDelay = 100
//do stuff
//sleep:
#ifdef _WIN32
Sleep(pollingDelay);
#else
usleep(pollingDelay*1000); /* sleep for 100 milliSeconds */
#endif
//do stuff again
return 0;
}

What is the proper #include for the function 'sleep()'?
sleep() isn't Standard C, but POSIX so it should be:
#include <unistd.h>

sleep(3) is in unistd.h, not stdlib.h. Type man 3 sleep on your command line to confirm for your machine, but I presume you're on a Mac since you're learning Objective-C, and on a Mac, you need unistd.h.

Given that sleep is a non-standard function, I created a sleep function with the standard library time.h
#include <time.h>
void sleep(double s) {
time_t cur_time = time(NULL);
while ((difftime(time(NULL), cur_time)) < s);
}

Related

Sleep() function in C not working on hp non-stop

I am trying something in C on hp-nonstop(tandem),
As part my task is to wait for sometime.
I try to use the
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
int main()
{
int i;
for(i=0;i<10;i++)
{
printf("Something");
sleep(5);
printf("Something");
fflush(stdout);
}
}
It's compiling without any problem,
While running it is giving ABENDED: each time different no.
The result calling sleep() from guardian environment is undefined. That might be leading to ABEND that you mentioned. If you want to wait for some time in guardian hp-nonstop environment, you should call DELAY(). It takes centi-seconds as arguments. So if you want to add delay of 5 seconds, you should call it as DELAY (500). You also need to include the header #include<cextdecs(DELAY)>

Sleep | warning implicit declaration of function `sleep'?

I am learning C.
In this program
I use sleep function to slowdown a count down.
My text book doesn't specify a library I should include to use the sleep function.
So I use it without including any special library for it and it works.
But it gives me this warning message in codeblocks.
I tried to include <windows.h> but still the same warning message appears.
warning D:\Project\C language\trial8\trial8.c|19|warning: implicit
declaration of function `sleep'|
And here is my code.
#include <stdio.h>
int main()
{
int start;
do
{
printf("Please enter the number to start\n");
printf("the countdown (1 to 100):");
scanf("%d",&start);
}
while(start<1 || start>100);
do
{
printf("T-minus %d\n",start);
start--;
sleep(3000);
}
while(start>0);
printf("Zero!\n Go!\n");
return(0);
}
I want to know what does the warning message mean? How important is it? Is there anything that I should do about it? Note that the program works anyway.
The issue is in the libraries (header files):
on Windows:
#include <windows.h> and Sleep(1000); => 1000 milliseconds
on Linux:
#include <unistd.h> and sleep(1); => 1 second
The function sleep is not part of C programming language. So, C compiler needs a declaration/prototype of it so that it can get to know about about number of arguments and their data types and return data type of the function. When it doesn't find it, it creates an Implicit Declaration of that function.
In Linux, sleep has a prototype in <unistd.h> and in windows, there is another function Sleep which has a prototype in <windows.h> or <synchapi.h>.
You can always get away with including header, if you explicitly supply the prototype of the function before using it. It is useful when you need only few functions from a header file.
The prototype of Sleep function in C on windows is:
VOID WINAPI Sleep(_In_ DWORD dwMilliseconds);
Remember, it is always a good practice to supply the prototype of the function being used either by including the appropriate header file or by explicitly writing it. Even, if you don't supply it, compiler will just throw a warning most of the time and it will make an assumption which in most cases will be something that you don't want. It is better to include the header file as API might change in future versions of the Library.
Windows doesn't have the sleep function. Instead, it has Sleep, which takes the number of milliseconds to sleep:
VOID WINAPI Sleep(
_In_ DWORD dwMilliseconds
);
You'll need to either #include <windows.h> or #include <synchapi.h>, depending on the version of Windows you're running. See MSDN for more details.
Update in 2022:
As it is stated on the Linux man page here we need to include unistd.h and should do fine for all OS.
#include <stdio.h>
#include <unistd.h>
int main()
{
sleep(1); /* sleep for 1 second*/
printf("END\n");
return 0;
}
To make it more cross-platform, try this:
#ifdef _WIN32
#include <Windows.h>
#else
#include <unistd.h>
#endif

implicit declaration of function ‘usleep’

So i've been looking stuff about usleep() and all I have found to get rid of this is #define which i have done... Any ohter suggestion? I need to get rid of this warning... Or any ideas on how to use sleep with miliseconds.
#define _BSB_SOURCE
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <unistd.h>
#include <time.h>
int r = rand() % 1000 +1;
usleep(r*1000);
pthread_mutex_lock (&count_mutex);
You need to remove -std=c99 from your compiler command or use the _XOPEN_SOURCE macro before including unistd.h.
If you want you can use -std=gnu99 instead of -std=c99.
nanosleep() worked for me:
On a relevant note: usleep() has been removed since POSIX-2008 and
recommends to use nanosleep() instead.

implicit declaration of function 'time' [-Wimplicit-function-declaration]|

Whenever I try to use srand function I get this warning
"implicit declaration of function 'time' [-Wimplicit-function-declaration]|"
and a windows error report appears when running the compiled file,
I'm a novice to c programming, I found this on a text book, but it doesn't work for me.
srand (time());
int x= (rand()%10) +1;
int y= (rand()%10) +1;
printf("\nx=%d,y=%d", x,y);
What do I need to correct this?
You need to make sure that you #include the right headers, in this case:
#include <stdlib.h> // rand(), srand()
#include <time.h> // time()
When in doubt, check the man pages:
$ man rand
$ man time
One further problem: time() requires an argument, which can be NULL, so your call to srand() should be:
srand(time(NULL));
Note that time() function uses current time (expressed in seconds since 1970) both in its return value and in its address argument.
I had this issue, and the problem was that in windows you need to include sys/time.h, but in linux you need time.h and I didn't notice.
I fixed this by adding a simple platform check:
#ifdef _WIN32
#include <sys/time.h>
#else
#include <time.h>
#endif
Note that this is for windows and linux, because that's what I needed for my program.

C89: getaddrinfo() on Windows?

I'm new to C89, and trying to do some socket programming:
void get(char *url) {
struct addrinfo *result;
char *hostname;
int error;
hostname = getHostname(url);
error = getaddrinfo(hostname, NULL, NULL, &result);
}
I am developing on Windows. Visual Studio complains that there is no such file if I use these include statements:
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
What should I do? Does this mean that I won't have portability to Linux?
On Windows, instead of the includes you have mentioned, the following should suffice:
#include <winsock2.h>
#include <windows.h>
You'll also have to link to ws2_32.lib. It's kind of ugly to do it this way, but for VC++ you can do this via: #pragma comment(lib, "ws2_32.lib")
Some other differences between Winsock and POSIX include:
You will have to call WSAStartup() before using any socket functions.
close() is now called closesocket().
Instead of passing sockets as int, there is a typedef SOCKET equal to the size of a pointer. You can still use comparisons with -1 for error, though Microsoft has a macro called INVALID_SOCKET to hide this.
For things like setting non-blocking flags, you'll use ioctlsocket() instead of fcntl().
You'll have to use send() and recv() instead of write() and read().
As for whether or not you will lose portability with Linux code if you start coding for Winsock... If you are not careful, then yes. But you can write code that tries to bridge the gaps using #ifdefs..
For example:
#ifdef _WINDOWS
/* Headers for Windows */
#include <winsock2.h>
#include <windows.h>
#else
/* Headers for POSIX */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
/* Mimic some of the Windows functions and types with the
* POSIX ones. This is just an illustrative example; maybe
* it'd be more elegant to do it some other way, like with
* a proper abstraction for the non-portable parts. */
typedef int SOCKET;
#define INVALID_SOCKET ((SOCKET)-1)
/* OK, "inline" is a C99 feature, not C89, but you get the idea... */
static inline int closesocket(int fd) { return close(fd); }
#endif
Then once you do something like this, you can code against the functions which appear in both OS's, using these wrappers where appropriate.

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