I am trying to compile a very simple C code in VS2013. I did a bit of Googling beforehand and I realised I need to follow certain steps for that e.g. change the compiler from Default to Compile As C Code in my project properties.
The problem is that when I compile following code:
#include <stdio.h>
main()
{
printf("Hello World! \n");
sleep(5);
}
I get these errors:
Error 1 error LNK2019: unresolved external symbol _sleep referenced in function _main
Error 2 error LNK1120: 1 unresolved externals
And if I change the code to:
#include <stdio.h>
#include <Windows.h>
main()
{
printf("Hello World! \n");
Sleep(5000);
}
Then it works fine.
How can I get my first code compiled successfully?
I have some tested codes in C and I need to use them. If I couldn't resolve above issue, then I have to update all syntaxes.
How can I get my first code compiled successfully?
You can't (at least exactly with this code as you stated). The point is that <stdio.h> header does not provide declaration for sleep() and C standard requires, that every function needs to be declared before its call. It's as simple as that.
The second thing is that sleep() function, that you want is likely from POSIX (specifically from <unistd.h> header). However, Visual Studio 2013 is not POSIX-compliant. Thus, the solution is to use Sleep(), that is declared in <Windows.h> header or possibly some other alternative.
Sleep is not a C standard function.
If you are on a UNIX system, then include <unistd.h>. However, if you are on a windows system, it should be in <windows.h> (which is why your second version of the code compiles successfully.
If you want to use in both systems, try something like this (from here):
#ifdef __linux__
//linux include code goes here
#elif _WIN32
// windows include code goes here
#else
#endif
The sleep function is not declared in <stdio.h>. If you're on unix, use sleep() from unistd.h, or for windows, Sleep() from Winbase.h (windows.h). If you want to see how Sleep() works, you can see how is sleep implemented at OS level?.
If you want to do it without having to change sleep() to Sleep(), you could use a macro:
#define sleep(s) Sleep((s) * 1000)
Macros are bad, so you may not want to do this in production code, but the advantage of the macro is that it will be guaranteed to be inlined, which will result in faster code.
sleep is a non-standard function:
On UNIX use <unistd.h>
On MS-Windows use <windows.h>
There is no way work around this besides writing your own code:
void sleep (int seconds)
{
Sleep (seconds * 1000); // Converting to milliseconds
}
Related
I'm fairly new to C but writing a small multithreaded application. I want to introduce a delay to a thread. I'd been using 'usleep' and the behavior is what I desire - but it generates warnings in C99.
implicit declaration of function ‘usleep’
It's only a warning, but it bothers me. I've Googled for an answer but all I could find was a while-loop / timer approach that seemed like it would be CPU intensive.
EDIT:
My includes are:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <time.h>
And I'm invoking the compiler with:
c99 program.c -Wall -pedantic -W -lpthread
EDIT #2:
I've created a new file that contains:
#include <unistd.h>
int main(void) {
usleep(10);
}
And I still get the warning.
EDIT #3:
As suggested, I've updated the text of the question.
The problem is that you are using a standard C99 compiler, but you're trying to access POSIX extensions. To expose the POSIX extensions you should for example define _POSIX_C_SOURCE to 200809L (for the current standard). For example this program:
#include <time.h>
int main(void) {
struct timespec reqtime;
reqtime.tv_sec = 1;
reqtime.tv_nsec = 500000000;
nanosleep(&reqtime, NULL);
}
will compile correctly and wait for 1.5 seconds (1 second + 500000000 nanoseconds) with the following compilation command:
c99 main.c -D _POSIX_C_SOURCE=200809L
The _POSIX_C_SOURCE macro must be defined with an appropriate value for the POSIX extensions to be available.
Also the options -Wall, -pedantic and -W are not defined for the POSIX c99 command, those look more like gcc commands to me (if they work on your system then that's fine, just be aware that they are not portable to other POSIX systems).
You are probably on a modern POSIX system:
POSIX.1-2008 removes the specification of usleep().
On my system (linux) there is a detailed explanation of the macros that must be set to get that function back. But you should just follow the advice that zvrba already gave, use nanosleep instead.
From the manual page: This function is obsolete. Use nanosleep instead.
implicit declaration of function ‘usleep’
This warning usually means that you didn't #include the right header file, in this case unistd.h.
Did you have the #include <unistd.h> ?.
And you can use some of the similar methods instead: nanosleep() for nanoseconds and sleep() for seconds. Or another way could be using clock(), but i think this is more CPU wasting.
In my current project I'm initializing a window to draw on. I'm using a function pointer to deal with initialization from multiple platforms. So I really only want one instance of the window_init function pointer.
I noticed that I had forgotten to mark this function pointer as extern in my header file and defined it a second time in my C file to give it storage.
The header is included in multiple files so should, to my understanding, generate a linker error. I tested this on my laptop and it did in fact generate a linker error. Why is my desktop not generating errors/how can I find out? In both instances I used mingw64 with cmake in Clion to build, using -Wall -Wextra -pedantic -Werror.
So for clarity: my desktop builds fine, my laptop gives me linker errors. I expect linker errors for both machines and I want to find out why that doesn't happen.
The solution is of course to use extern and then it builds on both machines, however I want to know what is going on.
Here is a minimal example which also builds fine on my desktop while I'd expect it to fail:
window.h
#ifndef CEXTERNTETS_WINDOW_H
#define CEXTERNTETS_WINDOW_H
void window_functions_init();
void(*window_init)(); //This is the first definition and should be marked as extern to my understanding if I want to give it storage in my c file
#endif //CEXTERNTETS_WINDOW_H
window.c
#include "window.h"
#include <stdio.h>
void(*window_init)(); //This is the second definition
void window_init_implementation()
{
printf("window_init_implementation\n");
}
void window_functions_init()
{
window_init = &window_functions_init;
}
second include (I need to include window.h in multiple files so I added it here and added a function so that the compiler can't optimize it away)
#ifndef CEXTERNTETS_SECOND_INCLUDE_H
#define CEXTERNTETS_SECOND_INCLUDE_H
#include "window.h"
void doit()
{
printf("doit");
}
#endif //CEXTERNTETS_SECOND_INCLUDE_H
main.c
#include <stdio.h>
#include "window.h"
#include "second_include.h"
int main() {
window_functions_init();
window_init();
doit();
printf("Hello, World!\n");
return 0;
}
edit:
Was able to find this about -fno-common.
This is a common compiler extension popular in the Unix world. To disable the extension, use -fno-common.
-fno-common is not enabled in GCC before version 10 by default with any warning or standard option. This technically does not constitute a violation of the standard, because the standard list this as undefined behaviour. Undefined behaviour does not generally require diagnostic messages. An implementation is allowed to define behaviour that the standard leaves undefined.
This is however undesirable, and GCC 10 finally defaults to -fno-common.
Despite the fact that i included '#include ' to my code, when i use built-in qsort function, clang gives me the error:
schedule.o: In function `chooseTicket':
schedule.c:(.text+0x16d): undefined reference to `qsort'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
start of the file (schedule.c) is like that:
#include "sched.h"
#include "schedproc.h"
#include <assert.h>
#include <minix/com.h>
#include <machine/archtypes.h>
#include <stdlib.h>
#include <lib.h>
#include <string.h>
#include <time.h>
and here is the function in which i used qsort built-in function
int chooseTicket(int* ticketList,int length,int totalTicket){
int randomValue;
int temp=0,prevTemp=0,selectedTicket=0,selectedIndex = 0;
time_t t;
struct schedproc *rmp;
int* sortedTicketList = malloc(length*sizeof(int));
memcpy(sortedTicketList,ticketList,length);
srandom((unsigned)time(&t));
randomValue = (random() % totalTicket);
qsort(sortedTicketList,length,sizeof(int),cmpFunc);//this line
note: Same errors also occured for 'rand()' and 'srand()' function and instead i have used 'random()' and 'srandom()', then the problem was solved. I don't understand despite the fact that 'rand()' and 'srand()' is generally accepted functions and header file contains these functions, why clang gives me linking errors while i am using 'rand()' and 'srand().
First, qsort is not a built-in, but part of the C standard library (formally, for hosted environments.)
Second, you need to learn that #include only allows access to the declarations of the functions in any given library. You need to link with the library, for your program to actually perform the call to the functionnality. Since you are getting a linker error here, no #include are going to help.
I guess you are writing a MINIX service, hence linking with libminc rather than with the full standard library ("libc"); in other words, this is a freestanding environment. And it happens qsort() is not in the restricted set of C functions included in libminc.
Either link with qsort.(c|o) specifically; or expand your own local version of libminc to include qsort(); or eat the whole cake and link with full libc, perhaps by adding DPADD+= ${LIBC}; LDADD+= -lc to the Makefile (I never tried to do that but it was supposed to work at some point, according to the code; it is not usual practice, so expect problems down the road.)
I'm fairly new to C but writing a small multithreaded application. I want to introduce a delay to a thread. I'd been using 'usleep' and the behavior is what I desire - but it generates warnings in C99.
implicit declaration of function ‘usleep’
It's only a warning, but it bothers me. I've Googled for an answer but all I could find was a while-loop / timer approach that seemed like it would be CPU intensive.
EDIT:
My includes are:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <time.h>
And I'm invoking the compiler with:
c99 program.c -Wall -pedantic -W -lpthread
EDIT #2:
I've created a new file that contains:
#include <unistd.h>
int main(void) {
usleep(10);
}
And I still get the warning.
EDIT #3:
As suggested, I've updated the text of the question.
The problem is that you are using a standard C99 compiler, but you're trying to access POSIX extensions. To expose the POSIX extensions you should for example define _POSIX_C_SOURCE to 200809L (for the current standard). For example this program:
#include <time.h>
int main(void) {
struct timespec reqtime;
reqtime.tv_sec = 1;
reqtime.tv_nsec = 500000000;
nanosleep(&reqtime, NULL);
}
will compile correctly and wait for 1.5 seconds (1 second + 500000000 nanoseconds) with the following compilation command:
c99 main.c -D _POSIX_C_SOURCE=200809L
The _POSIX_C_SOURCE macro must be defined with an appropriate value for the POSIX extensions to be available.
Also the options -Wall, -pedantic and -W are not defined for the POSIX c99 command, those look more like gcc commands to me (if they work on your system then that's fine, just be aware that they are not portable to other POSIX systems).
You are probably on a modern POSIX system:
POSIX.1-2008 removes the specification of usleep().
On my system (linux) there is a detailed explanation of the macros that must be set to get that function back. But you should just follow the advice that zvrba already gave, use nanosleep instead.
From the manual page: This function is obsolete. Use nanosleep instead.
implicit declaration of function ‘usleep’
This warning usually means that you didn't #include the right header file, in this case unistd.h.
Did you have the #include <unistd.h> ?.
And you can use some of the similar methods instead: nanosleep() for nanoseconds and sleep() for seconds. Or another way could be using clock(), but i think this is more CPU wasting.
I'm pretty sure there's such question, but I can't find it :\ Anyway, here's the issue:
What is the difference between wait in stdlib.h and sys/wait.h o.O ?
In details - I just encountered this problem and I could't compile a simple C program. I isolated the problem and here's what I got:
#include <stdlib.h>
//#include <sys/wait.h>
int main()
{
int status;
wait( &status );
return 0;
}
If stdlib.h is included, I got:
$ gcc asd.cpp
asd.cpp: In function ‘int main()’:
asd.cpp:9:16: error: conflicting declaration ‘wait& status’
asd.cpp:8:6: error: ‘status’ has a previous declaration as ‘int status’
What declaration ? O.o What is wait here, that conflicts with int status?
I found a thread in the net, where replacing stdlib.h with sys/wait.h solves the problem, but why is that and what is the difference?
EDIT: Thanks to sidyll's comment, I changed the file extention - from .cpp to .c and it worked! I'm shocked :) How is this so different? And still the same question - what is the different between those two wait-s ?
The difference is that the wait() in <sys/wait.h> is the one you should use.
From the wait(3) man page:
SYNOPSIS
#include <sys/types.h>
#include <sys/wait.h>
pid_t wait(int *status);
The wait function isn't defined by the ISO C standard, so a conforming C implementation isn't allowed to declare it in <stdlib.h> (because it's legal for a program to use the name wait for its own purposes). gcc with glibc apparently does so in its default non-conforming mode, but if you invoke it with gcc -ansi -pedantic or gcc -std=c99 -pedantic, it doesn't recognize the function name wait or the type pid_t.
I did gcc -E wait.cpp to dump the actual preprocessor expansions that take place. What I found was that on linux, the header /usr/include/bits/waitstatus.h is included which pulls in a union wait { ... } but the function wait() from sys/wait.h is never pulled in. The same thing happens with the c compilation, but the for whatever reason the compiler does not complain in that case.
To prove this to yourself, you can change your main to declare the wait as a variable rather than a function call, and the compiler will not complain:
int main() {
int status;
wait w;
return 0;
}
Note that GCC stands for GNU Compiler Collection, not GNU C Compiler (as many
other tools which were prefixed with a g). It's not a C-only compiler. And
many languages are detected by file extensions. Adam Rosenfield is partialy
correct in his comment. Yes, g++ will add the C++ library in the linker phase,
but that's not the unique difference (more on this later).
To explain how changing the extension solved it, please take a look in this text
straight from GCC's manual:
Compiling C++ Programs
C++ source files conventionally use one of the suffixes.C, .cc, .cpp,
.CPP, .c++, .cp,or.cxx;C++ header files often use.hhor.H;and
preprocessed C++ files use the suffix .ii. GCC recognizes files with
these names and compiles them as C++ programs even if you call the
compiler the same way as for compiling C programs (usually with the
namegcc).
So, "GCC regocnizes files with these names" and your program was being compiled
as C++ source. I guess that C++ has some special use of &, which I can't tell
exactly (I don't know C++). Hence the error.
Now, regarding the difference between g++ and gcc, continue with the next
paragraph:
However, the use ofgccdoes not add the C++ library.g++is a program
that calls GCC and treats.c, .hand.ifiles as C++ source files
instead of C source files unless-xis used, and automatically
specifies linking against the C++ library. This program is also useful
when precompiling a C header file with a.hextension for use in C++
compilations. On many systems,g++is also installed with the name
c++.
On the real question: there aren't two waits here in my system (Darwin 11), only
the standard syscall. Check if what Kevin said isn't happening. It's the same,
stdlib.h includes sys/wait.h:
#include <_types.h>
#if !defined(_ANSI_SOURCE)
#include <sys/wait.h>
#if (!defined(_POSIX_C_SOURCE) || defined(_DARWIN_C_SOURCE))
#include <alloca.h>
#endif /* (!_POSIX_C_SOURCE || _DARWIN_C_SOURCE) */
#endif /* !_ANSI_SOURCE */
Check your header.