C while loop condition

Question

I'm reading Computer Systems for my CS class and I've come across a while loop condition that's puzzling me, here's the code:

int parseline(char *buf, char **argv)
{
    char *delim; /* Points to first space delimiter */
    int argc; /* Number of args */
    int bg; /* Background job? */

    buf[strlen(buf)-1] = ’ ’; /* Replace trailing ’\n’ with space */
    while (*buf && (*buf == ’ ’)) /* Ignore leading spaces */
        buf++;

     /* Build the argv list */
     argc = 0;
     while ((delim = strchr(buf, ’ ’))) {
         argv[argc++] = buf;
         *delim = ’\0’;
          buf = delim + 1;
          while (*buf && (*buf == ’ ’)) /* Ignore spaces */
              buf++;
     }

In

while (*buf && (*buf == ’ ’)) /* Ignore spaces */

the while loop has two operands to logical && but I don't understand what is the purpose of the first operand (*buf). The second operand is checking for empty space, but I would think that the second operand by itself would suffice for the purpose of this loop.

What would happen if buf == NULL? I tried running a simplified version of code with buf == NULL and got a seg fault because, from what I'm guessing, the loop condition tried to deference a NULL pointer. Is this bad programming practice to have a pointer in the loop condition? Should a loop with a flag condition be utilized instead? — David
– David, Commented Mar 11, 2018 at 22:52

HolyBlackCat · Accepted Answer · 2018-03-11 21:49:19Z

3

Yes, the *buf && is superfluous.

*buf is false for '\0' and true for everything else.

*buf == ' ' is true for ' ' and false for everything else, including '\0'.

answered Mar 11, 2018 at 21:49

HolyBlackCat

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1 Comment

Jean-François Fabre Over a year ago

another approach: (*buf && (*buf == ’ ’)): when *buf==' ' the first condition is true as well so it's redundant all right

chux · Accepted Answer · 2018-03-11 22:09:56Z

The following is functionally the same as while (buf == ' ') if the quote marks ’ are changed to '.

//                      v-v--- not standard quote marks.
while (*buf && (*buf == ’ ’))

With a good compiler, neither is faster as an optimizing compiler with emit the same code.

To me it is simply pedantic code insuring the loop is not taken with a null character.

What is bad about the code includes:

buf[strlen(buf)-1] = ’ ’; is a UB if buf[0] == 0.

buf[strlen(buf)-1] = ’ ’; /* Replace trailing ’\n’ with space */ may lop off a non-'\n'.

A better alternative that address the 2 preceding concerns: buf[strcspn(buf, "\n")] = '\0';

Instead of "Ignoring spaces", it is more C-like to ignore white-spaces.

"Build the argv list" usually requires a final argv[argc] == NULL.

Of course these are sides issues to the main question and without the larger context may/may not apply.

sg7 · Accepted Answer · 2018-03-11 22:15:57Z

1

The second operand is checking for empty space, but I would think that the second operand by itself would suffice for the purpose of this loop.

  while (*buf && (*buf == ’ ’)) /* Ignore leading spaces */
        buf++;

It would suffice. The loop would break for *buf == '\0'.

answered Mar 11, 2018 at 22:15

sg7

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