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Decent C

I've been writing c for over 30 years, but it can still be a nightmare - the problems are well documented elsewhere. The discipline of writing device drivers for Linux/Solaris/HP-UX/AIX etc etc and crypto code for RSA (thanks, Michael for the excellent code-reviews - I learned so much) have led to a few points that help make code clearer, easier to maintain and less likely to contain memory leaks and other faults.

Single return point

Every routine should have a single return point. All possible execution paths go through this point and therefore have the chance to have resources released. The main thing, of course, is to free resources that are no longer needed eg memory that has been malloc()'ed, file/socket descriptors and in device drivers, to release DMA and interrupt resources. See the following example.

Error responses should be close to the failing statement

and not dangling at the end of some huge }}}} block ten pages away!!! Deal with it _now_, to hell with making the Happy Path clearer - the Happy Path is easy, it's the errors that make the thing beastly.

Use goto

Don't be overly scared to use 'goto', it's actually quite useful when used appropriately. The evils of 'goto' were all too well propagated when “structured programming” got started in the 70-80's. But it still has a vital role in simplifying code. For example, until modern languages catch up with google go's 'defer' statement (which stacks up clauses to be run on routine exit) this technique is essential - even in C++!!!

The best place for the return logic block is at the end of the routine (the single exit point, remember?) under a label. When errors are detected and the routine needs to bail, a simple “goto end” (or “goto hell” if you prefer) allows cleanup to be done. This also eliminates tortuous if-else spaghetti code and ties the remedial action close to the failing statement - not dangling at the end of some nested }}}} block 10 pages down the code.

Do it even when it looks stupidly simple - later, when the routine is made more elaborate, then the logic is in place and you will remember to cleanup:

/* @return 0 if file is foobarable. Otherwise 1. 
 */
int is_not_foobar_able(
    char *filename, /** filename IN name of file to test */
    int verbose,    /** verbose IN set to non-0 to make it verbose */
    int bufsize     /** malloc this much heap */
    ) 
{
    struct stat buf;
    int retval = 1; /* default return is 'error' */
    char *bigbuf = NULL; /* just to demonstrate an error return with cleanup */
 
    if (verbose) printf("Checking %s\n", filename);
 
    if (stat(filename, &buf)) {
        goto end; /* error return */
    }
 
    if ((buf.st_mode & S_IFMT) == S_IFREG) { 
        /* it's a regular file */
        if ( !(buf.st_mode & 0111)) {
            goto end; /* error return */
        }
    }
 
    if ((bigbuf = malloc(bufsize)) == NULL) {
        fprintf(stderr, "%s: malloc failed\n", progname);
        goto end; /* error return */
    }
 
    ...
 
    retval = 0; /* non-error return */
 
end:
    /* cleanup */
    if (bigbuf) free(bigbuf);
    return(retval);
}

Initialise all variables

Especially pointers which should be initialised to NULL. Then, the return logic in the single exit point can clean them up if non-NULL. See the previous example.

Declare a maximum of one variable per statement

The dangers are things like:

int* a, b;

Is 'b' a pointer to an int? No, it's not.

Use strncpy() rather than strcpy() if at all possible

Unless you are absolutely sure there's no risk of buffer overflow. Definitely if the source string comes from a user.

The same applies to using snprintf() rather than sprintf().

Ideally, avoid the libc str* functions entirely and create your own safe library using something like:

typedef struct {
    int len;
    char *str;
} SAFE_STRING;

This also has the benefit of potentially being more efficient of CPU cycles as it avoids the constant and repeated use of strlen(). Think about it.

Return an error code

If at all possible, always return an int to indicate success or failure eg. 0 for success, anything else for an error. Or return a pointer on success and NULL on failure.

Document all parameters and return values

Just do it. No, the source code is not enough documentation. And keep it up to date as you code. You might as well use doxygen format in case the employer cultivates a taste for it.

exit values etc

Unless you've got a really good excuse, the program should exit(0) on success and non-zero on failure (note that exit code can only be up to 8-bits long). As with DecentBash, the program should send error messages (prefixed with the program name) to stderr, output to stdout. It should recognise -h (no matter what) and possibly –help and print help to stdout (and return 0!). -v and –verbose are also pretty standard.

In fact, why not use argp(3) to make option processing easy?

Small functions

Functions must fit on a single editor page. OK I have a hi-res screen and get 70 lines on it, but that's OK. I also stick to K&R code layout (as above) so more context fits on a single screen.

valgrind

You're not done until valgrind says you're done and your program leaks no memory!!

Keep it simple

As Kernighan is reputed to have said, debugging (and tuning) is twice as hard as coding so if you code to the limit of your ability, then by definition, you will not be able to debug or tune it! This sin is most often commited by new programmers eager to show how clever they are. I'm not usually very impressed.

Have it reviewed

I've learned so much from having others (constructively) critique my code.

c.txt · Last modified: 2016/01/03 23:44 by admin