STRUCT_TM.cpp

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// checking for struct tm in <time.h>
 
 
#include <stddef.h>   // for offsetof, size_t
#include <stdio.h>    // for printf
#include <string.h>   // for strlen
#include <time.h>     // for struct tm
 
#include "config.h"
 
#ifndef _RWSTD_NO_OFFSETOF
#  define OFF(T, m)   offsetof (T, m)
#else
   // provide own equivalent when offsetof() macro doesn't work
   // (e.g., when using the EDG eccp front end with an incompatible
   // version of gcc)
#  define OFF(T, m)   (((const char*)&((T*)0)->m) - ((const char*)(T*)0))
#endif   // _RWSTD_NO_OFFSETOF
 
// prevent IBM xlC 5.0 errors caused by using /usr/include/string.h
// which #defines these (and other) names to undeclared symbols
#undef strlen
 
int main ()
{
    typedef struct tm TimeT;
 
    struct {
        size_t      offset;
        const char *name;
        const char *comment;
    } tm_info[] = {
 
#define ENTRY(member, comment)   \
        { OFF (struct tm, member), # member ";", "   /* " comment " */" }
 
        ENTRY (tm_sec,   "seconds after the minute [O..61] "),
        ENTRY (tm_min,   "minutes after the hour   [0..59] "),
        ENTRY (tm_hour,  "hours since midnight     [0..23] "),
        ENTRY (tm_mday,  "day of the month         [1..31] "),
        ENTRY (tm_mon,   "months since January     [0..11] "),
        ENTRY (tm_year,  "years since 1900                 "),
        ENTRY (tm_wday,  "days since Sunday        [0..6]  "),
        ENTRY (tm_yday,  "days since January 1     [0..365]"),
        ENTRY (tm_isdst, "Daylight Saving Time             "),
    };
 
    const size_t N = sizeof tm_info / sizeof *tm_info;
 
    char def [1024] = "";
 
    size_t curoff = 0;   // current offset
 
    // assume contiguous standard members
    // optionally followed by non-standard ones
    for (size_t i = 0; i != N; ++i) {
        for (size_t j = 0; j != N; ++j) {
            if (curoff == tm_info [j].offset) {
                sprintf (def + strlen (def),
                         "    int %-10s        %s \\\n",
                         tm_info [j].name, tm_info [j].comment);
                tm_info [j].offset = size_t (-1);
                curoff += sizeof (int);
                break;
            }
        }
        if (!curoff) {
            curoff = sizeof (struct tm) - N * sizeof (int);
        }
    }
 
    printf ("#define _RWSTD_STRUCT_TM {   "
            "/* sizeof (struct tm) == %u          */ \\\n%s",
            sizeof (struct tm), def);
 
 
    if (N * sizeof (int) != sizeof (struct tm)) {
        printf ("    char _C_fill [%u];        "
                "/* fill to size                      */ \\\n",
                sizeof (struct tm) - N * sizeof (int));
    }
 
    printf ("  }\n");
 
    return 0;
}