Porting Guide for Dummies

The problems addressed here were encountered while working on fixing PATH_MAX and MAXPATHLEN.

Test on Hurd

Installing the required files

As apt-get source will download and extract many files, you may want to create a dedicated folder for the package and work from there.

sudo apt-get build-dep PACKAGE
apt-get source PACKAGE

Trying to build the package

dpkg-buildpackage -us -uc -rfakeroot -tc

Test a quick fix

In all the files that use PATH_MAX, include those lines at the beginning.

#ifndef PATH_MAX
#define PATH_MAX 4196

Try to rebuild the package and see if it's solved the problem.
If yes, you can start working on the package.

Basic things

Maintaining a original version

mkdir old

Coding style

Follow the conventions used in the source code!

if (condition) {

is not the same as:

if (condition) 

and is not the same as:

if (condition)

Pay attention to spaces surrounding, or not, arithmetic signs and symbols:

a = do_smthg( b + c );
a = do_smthg(b+c);


By default use 8 spaces as the size for 1 tab. Then figure out if the code uses tab + 1/2 tab:

....if (condition) {

or tab only:

------->if (condition) {

Creating a patch


Known problems

Dynamically allocated buffer returned by a function

Use a static buffer

Buffer used to format an expression containing an INTEGER

The length of an INTEGER in a string can be up to sizeof (int) * 3 + 1.

The usual trick for "%d" is to use the constant 'sizeof (int) * 3 + 1'. I included + 1 for the sign, but it's not really necessary if we exepect sizeof(int) >= 2, which we probably should. Jérémie Koenig

 = log(2 ^ 32)
 = 32 * log(2)
 = 4 * 8 * log(2)
 = sizeof(int) * 2.40823997
 < sizeof(int) * 3

Proper use of realloc()

use a new_buff to check if everything went fine Free buf if realloc failed (and prog doesn't exit)

Reading lines from file

Function to read line (no size limit, ending with "\n") from a file.

static char *get_line(FILE *f)
    char *buff  = NULL;
    char *new_buff  = NULL;
    size_t buff_size = 0;
    size_t last = 0;

    while (!feof(f)) {
        buff_size = buff_size ? buff_size * 2 : BUFSIZ;
        new_buff = realloc(buff, buff_size);
        if (new_buff == NULL) {
            return NULL;
        buff = new_buff;
        if (fgets(buff + last, buff_size - last, f) == NULL) {
            return NULL;
        last = strlen(buff);
        if (buff[last - 1] == '\n')
            return buff;
    return buff;

Proper use of readlink()

One has to rely on lstat() to get the size of the link that readlink() returns.

Declare what you need:

char *linkname = NULL;
struct stat sb;
ssize_t len = -1;

Call lstat() and check return value:

if (lstat(filename, &sb) == -1) {

Create a buffer of the appropriate size and check the return value:

linkname = malloc(sb.st_size + 1);
if (linkname == NULL) {

Call readlink(), check return value and set the null char in the linkname:

len = readlink(filename, linkname, sb.st_size + 1);
if (len < 0 || len > sb.st_size) {
linkname[sb.st_size] = '\0';

Alternative use of readlink(): readlink_malloc()

In some cases the above approch doesn't work.for instance when reading from /proc/*/exe on Linux. In this case you can try the following function.
The code comes from https://buildsecurityin.us-cert.gov/bsi/articles/knowledge/coding/806-BSI.html

static char *readlink_malloc(const char *filename)
    int size = 100;

    while (1) {
        char *buff = malloc(size);
        if (buff == NULL)
            return NULL;
        int nchars = readlink(filename, buff, size);
        if (nchars < 0) {
            return NULL;
        if (nchars < size) {
            buff[nchars] = '\0';
            return buff;
        free (buff);
        size *= 2;