warning: initializer element is not computable at load time

Question

I fail to understand gcc -pedantic output in the following scenario:

$ gcc -pedantic parse.c -lpopt
parse.c: In function ‘main’:
parse.c:19:7: warning: initializer element is not computable at load time
       { "bps", 'b', POPT_ARG_INT, &speed, 0,
       ^
parse.c:20:7: warning: initializer element is not computable at load time
       "signaling rate in bits-per-second", "BPS" },
       ^
parse.c:27:7: warning: initializer element is not computable at load time
       { "raw", 'r', 0, &raw, 0,
       ^
parse.c:28:7: warning: initializer element is not computable at load time
       "don't perform any character conversions" },
       ^

Using the following C code (taken from here):

$ cat parse.c
#include <popt.h>

int main(int argc, const char *argv[]) {
  char    c;            /* used for argument parsing */
  int     speed = 0;    /* used in argument parsing to set speed */
  int     raw = 0;      /* raw mode? */
  struct poptOption optionsTable[] = {
      { "bps", 'b', POPT_ARG_INT, &speed, 0,
      "signaling rate in bits-per-second", "BPS" },
      { "crnl", 'c', 0, 0, 'c',
      "expand cr characters to cr/lf sequences" },
      { "hwflow", 'h', 0, 0, 'h',
      "use hardware (RTS/CTS) flow control" },
      { "noflow", 'n', 0, 0, 'n',
      "use no flow control" },
      { "raw", 'r', 0, &raw, 0,
      "don't perform any character conversions" },
      { "swflow", 's', 0, 0, 's',
      "use software (XON/XOF) flow control" } ,
      POPT_AUTOHELP
        { NULL, 0, 0, NULL, 0 }
  };
}

The very same code does compile with either -ansi or -std=c89. Why is the -pedantic option failing ?

Using:

$ gcc --version
gcc (Debian 4.9.1-19) 4.9.1
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

and

$ apt-cache policy libpopt-dev
libpopt-dev:
  Installed: 1.16-10
  Candidate: 1.16-10
  Version table:
 *** 1.16-10 0
        500 http://ftp.fr.debian.org/debian/ jessie/main amd64 Packages
        100 /var/lib/dpkg/status

score 2 · Answer 1 · answered Feb 09 '15 at 13:16

Because variable speed is allocated on the stack, its address is not known at compile time. Only once main() is executing, will its address be known. Same for variable raw. As any good compiler, your compiler's error message does not point exactly at the element in error and gives you headaches understanding what it is complaining about.

score 1 · Accepted Answer · answered Feb 09 '15 at 11:40

In C89 standard, initializer list must be compile time constants expressions, hence the warnings. However in C99 it is supported.

In C89, you can do:

struct poptOption optionsTable[2];

optionsTable[0] = {"bps", 'b', POPT_ARG_INT, &speed, 0,
  "signaling rate in bits-per-second", "BPS" };
optionsTable[1] = {"crnl", 'c', 0, 0, 'c',
  "expand cr characters to cr/lf sequences" };

warning: initializer element is not computable at load time

2 Answers2