Should I declare system call functions in C?

Question

I read this answer: Should I declare a function prototype in C?

My question is more specific:

In a program using system calls like access() , open() , creat() , write() , read() ... Do I have to declare every function of the system call? So does C work? Because I get the following:

 hw1.c: In function 'main': hw1.c:50:9: warning: implicit declaration of function 'access' [-Wimplicit-function-declaration] hw1.c:131:9: warning: implicit declaration of function 'lseek' [-Wimplicit-function-declaration] hw1.c: In function 'writeFile': hw1.c:159:17: warning: implicit declaration of function 'write' [-Wimplicit-function-declaration] 

Basically, it seems that C is angry with every system call function that I use. I'm a little new to C, and this seems odd to me, although I know that I have to declare the functions I'm writing, I would think that C would know the system call functions and I would not need to explicitly declare them in the code.

I need to do something like this:

int access(const char *pathname, int mode);

If so, why does it make sense? I use other languages ​​and should never do this.

Answer 1

Yes, you must include the correct header for every call to your system function. You yourself do not write declarations - you are mistaken. Use the headline!

For the functions you are quoting, the corresponding headers are:

 #include <unistd.h> /* Many POSIX functions (but not all, by a large margin) */ #include <fcntl.h> /* open(), creat() - and fcntl() */ 

See POSIX 2008 for the right header for other POSIX features.

The C99 standard requires all functions to be declared or defined before they are used.

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For your own functions, you must emulate a "system." There will be a header that declares a function, a source file that implements it, and other source files that use this function. Other source files use the header to get the correct declaration. The implementation file includes a header to ensure that its implementation is consistent with what other source files expect. So, the headline is the glue that holds it all together.

Answer 2

You do not need to declare each as such. What you need to do is include the correct header files containing declarations of these functions. For instance:

 #include <stdio.h> #include <stdlib.h> #include <fcntl.h> #include <unistd.h> 

stdio.h , fcntl.h and unistd.h (assuming you encode * nix) are the ones you need for the functions mentioned. See the documentation (i.e. man lseek ) for details.

Answer 3

You should do this in almost all languages ​​except the set of "core" libraries. Java, C #, Perl, etc. They have import or use forms, or make links in assembly configurations, or ... for things outside the main library. C is no different, except that it has very small (that is, Non-existent) "core libraries" - none of the library headers are turned on by default.

#include is what you are looking for. Check the manual page for each of these features and you'll find out which headings you need to include.