Question

Assume that there are flag definitions such as:

SHF_WRITE     0x1
SHF_ALLOC     0x2
SHF_EXECINSTR 0x4
SHF_MASKPROC  0xf0000000

Given a flag, I need to output SHF_WRITE|SHF_ALLOC if the bits 0x1 and 0x2 is on.

How to do the trick in C?

Answer 1

Just create a character buffer with enough space to hold all possible combinations of strings and add to it the appropriate strings for each applicable bit set. (or you could ditch the buffer and write straight to stdout, your choice) Here’s a naive implementation of how you could do such a thing:

void print_flags(int flag)
{
#define BUFLEN (9+9+13+12+3+1)
                  /* for the text, pipes and null terminator*/
#define PAIRLEN 4
static struct { int value; const char *string; } pair[] =
{
    { SHF_WRITE,     "SHF_WRITE" },
    { SHF_ALLOC,     "SHF_ALLOC" },
    { SHF_EXECINSTR, "SHF_EXECINSTR" },
    { SHF_MASKPROC,  "SHF_MASKPROC" },
};

    char buf[BUFLEN];  /* declare the buffer */
    char *write = buf;    /* and a "write" pointer */
    int i;
    for (i = 0; i < PAIRLEN; i++)
    {
        if ((flag & pair[i].value) == pair[i].value) /* if flag is set... */
        {
            size_t written = write - buf;
            write += _snprintf(write, BUFLEN-written, "%s%s",
                written > 0 ? "|" : "",
                pair[i].string); /* write to the buffer */
        }
    }
    if (write != buf) /* if any of the flags were set... */
    {
        *write = '\0'; /* null terminate (just in case) */
        printf("(%s)", buf); /* print out the buffer */
    }
#undef PAIRLEN
#undef BUFLEN
}