I was able to run code that uses the randomize function without including the time.h library. is it automatically included with some other libraries i might have already included in my code? Below is a list of the libraries I included:
#include <graphics.h>
#include <stdlib.h>
#include <stdio.h>
#include <conio.h>
#include <dos.h>
#include <string.h>
#include <io.h>
This is very very specific to the version and implementation of your library. The standard doesn't force any header to include time.h 1 so you cannot rely on that.
In your case, it could be that one of dos.h, io.h, conio.h for example has included time.h (or any other of the headers there for all it's worth).
1 At least not the ones there and not likely in your seemingly ancient library. C11 says threads.h should include time.h
What does <compiler with high warning level> yourcode.c say? My guess would be:
either one of the non-standard DOS-specific headers (conio.h, dos.h, io.h, ...) includes it,
or there's no declaration at all, i. e. it's not included, in which case your compiler silently and implicitly assumes a function signature (specifically, it assumes a return value of int and whatever type of argument you call it with for the first time).
Note that the latter case is wrong, and you should pay attention not to do it (since it may lead your program invoking undefined behavior). Always compile with all warnings enabled so you can track down such an error.
When C compiler can't find a prototype to a function it assumes it is a function that returns int. It also prints a warning function if you didn't change the default settings.
So. In your case perhaps time.h was included, but be aware that it can cause a lot of problems if it wasn't.
Related
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 have a program written in c which uses the execvpe(3) function, and I've got a line set to include the requisite header file:
#include <unistd.h>
I compile this file with the following command...
gcc foo.c -o foo
...only to get the following warning:
warning: implicit declaration of function ‘execvpe’ [-Wimplicit-function-declaration]
I've encountered similar behavior with files that reference the pthread_create(3) function. The difference is obviously that whereas the pthread_create(3) man page clearly states that one should "Compile and link with -pthread", the man page for the exec(3) family of functions does not have any such instructions. Furthermore, I cannot find any reference in the manual or online to an analogous compiler flag for the exec(3) family.
I'd appreciate any information you have on this matter. If there is some flag I should be using at compile time, or if I am looking in entirely the wrong place for a solution, please let me know.
The man page here states that it is necessary to define the _GNU_SOURCE feature test macro to enable the function declaration:
#define _GNU_SOURCE
#include <unistd.h>
Interestingly however the link to unistd.h on the same man page takes you to an implementation that does not declare execvpe at all. You could check your system's unistd.h file to check that it is declared and is dependent on _GNU_SOURCE - that is to solve this an similar problems in the future - check the header content to see if it is even there and what macros it may depend on.
If it is not in the header file, then it is most probably also not in the library, but you could check as follows:
#include <unistd.h>
extern int execvpe(const char *file, char *const argv[], char *const envp[]);
which will satisfy the compiler, but if you then get a linker error, then the function is simply not included in the library in any case.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Why #include <stdio.h> is not required to use printf()?
//#include <stdio.h>
//#include <conio.h>
main(){
printf("Hi");
getch();
}
As I was programming this, it shocked me that it worked without actually importing any c libraries such as stdio that contains the printf function. Why is that so? (Used Dev-C++ 4.9.9.2, saved as .c, not .cpp)
C allows you to call functions without first defining the prototypes. (C++ does not do this.) An implicit prototype for printf will be defined as follows:
int printf();
Coincidentally, the calling conventions for this implicit prototype matched the actual calling conventions for printf on your platform.
In general, you cannot count on this working, and there are a large number of cases where it won't work. I recommend enabling compiler warnings to detect implicit prototype declarations so you can fix them (by including the correct header).
Footnote: #include does not import libraries, it merely pastes files into your source code at compile time. The <stdio.h> header contains (directly or indirectly) certain prototypes, but the library has to be linked in separately. Since printf is usually in a library that is linked to programs by default, you usually don't have to do anything to use printf.
The #include preprocessor directive does not import any library; there is no such notion of import in the definition of the C language.
You just happen to call a function named printf and the standard C library (e.g. libc.so on Linux, I don't know how Windows call it) happens to be linked in by default. Since you call a function by a name known to that library, it gets linked, and is being called at runtime.
However, you should enable all warnings in you compiler, and it would warn that you call an undeclared function.
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.
If I use malloc in my code:
int *x = malloc(sizeof(int));
I get this warning from gcc:
new.c:7: warning: implicit declaration of function ‘malloc’
new.c:7: warning: incompatible implicit declaration of built-in function ‘malloc’
You need to add:
#include <stdlib.h>
This file includes the declaration for the built-in function malloc. If you don't do that, the compiler thinks you want to define your own function named malloc and it warns you because:
You don't explicitly declare it and
There already is a built-in function by that name which has a different signature than the one that was implicitly declared (when a function is declared implicitly, its return and argument types are assumed to be int, which isn't compatible with the built-in malloc, which takes a size_t and returns a void*).
You haven't done #include <stdlib.h>.
You need to include the header file that declares the function, for example:
#include <stdlib.h>
If you don't include this header file, the function is not known to the compiler. So it sees it as undeclared.
Make a habit of looking your functions up in help.
Most help for C is modelled on the unix manual pages.
Using :
man malloc
gives pretty useful results.
Googling man malloc will show you what I mean.
In unix you also get apropos for things that are related.
Beside the other very good answers, I would like to do a little nitpick and cover something what is not discussed yet in the other answers.
When you are at Linux, To use malloc() in your code,
You don´t actually have to #include <stdlib.h>.
(Although the use of stdlib.h is very common and probably every non-toy-program should include it either way because it provides a wide range of useful C standard library functions and macros)
You could also #include <malloc.h> instead.
But please note that the use of malloc.h is deprecated and it makes your code non-portable. If you want to use malloc() you should always and ever (except for explicit reasons to do otherwise) #include <stdlib.h>.
The reasons why, are best explained in the answers to this question:
difference between <stdlib.h> and <malloc.h>