Will SEEK_SET SEEK_CUR SEEK_END not be 0,1,2 on any C lib?

Question

For whence parameter of fseek (or lseek in POSIX), some languages that calls C lib assume SEEK_SET, SEEK_CUR, SEEK_END are 0,1,2, respectively, such as gfortran, Nim [^py].

[^py]: while Python's seek specializes concrete values of SEEK_*, CPython does map 0,1,2 to C lib's values internally.

I've looked up C and POSIX specialization, neither specializes concrete values of SEEK_*.

In code, I wonder in which OS and C compiler, the following code's output isn't 012:

#include <stdio.h>

int main(){
    printf("%d%d%d\n", SEEK_SET, SEEK_CUR, SEEK_END);
}

I've tested on some OSes and C compilers, where the output is just 012:

Windows: gcc(mingw), MSVC
Debian: clang, gcc
FreeBSD: clang
Solaris(SunOS5): Oracle Solaris Studio C Compiler
Android(Termux): clang

And if there're none, then it's less meaningful to add fix for the existing implementations using fixed values.

To clearify, I wonder if there is any C libraries/OSes whose SEEK_* values differ — lit
– lit, Commented Aug 1, 2024 at 3:31
Why does that matter? If you use the SEEK_* constants, the actual values should be irrelevant to you regardless of the library or OS. That's the entire reason those identifiers exist. — Ken White
– Ken White, Commented Aug 1, 2024 at 3:37
Because, as mentioned above, some langauges' implementation used fixed values instead of SEEK_*. I wonder if they'll fail on some libraries/OSes — lit
– lit, Commented Aug 1, 2024 at 3:45
And as I said, the values assigned to SEEK_* should not matter to you. If the SEEK_* defines don't exist, then clearly you'll have to use that language's fixed values instead. Asking us to provide a list of those languages isn't an appropriate use of this site. — Ken White
– Ken White, Commented Aug 1, 2024 at 3:49
"I wonder if they'll fail on some libraries/OSes", I'm just thinking if any C lib/OSes uses other values for SEEK_ macros — lit
– lit, Commented Aug 1, 2024 at 3:54

chux · Accepted Answer · 2024-08-01 04:21:11Z

1

Will SEEK_SET SEEK_CUR SEEK_END aren't 0,1,2 on any C lib?

Maybe. Yet if not today, maybe tomorrow.

If you must use 0, 1, 2 in calling fseek(), use the number as an array index.
I doubt there is a real case where one must do so.

#include <stdio.h>
const int whence [] = { SEEK_SET, SEEK_CUR, SEEK_END };

int result = fseek(stream, 0, whence[2]);  // Go to the end

answered Aug 1, 2024 at 4:21

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Brad Lanam · Accepted Answer · 2024-08-01 14:31:42Z

1

In the ideal instance, you would want to write a C program to create an include file for the constants for the language that does not have those constants defined. e.g.

  #include <stdio.h> 

  int 
  main (int argc, char *argv []) 
  {
    printf ("INTEGER, PARAMETER :: SEEK_SET = %d, SEEK_CUR = %d, SEEK_END = %d\n", 
        SEEK_SET, SEEK_CUR, SEEK_END);
  }

I have no idea what fortran include files look like, this would need to be fixed.

In this particular instance, SEEK_SET, SEEK_CUR and SEEK_END are well defined as 0, 1, and 2, and you aren't going to have a problem with them.

answered Aug 1, 2024 at 14:31

Brad Lanam

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