在C中动态分配字符串数组的内存

Question

I'm trying to store a list of every mount point on a Linux system in a string array with C. I'm focused on this piece of code.

int i = 0;
char **mountslist = malloc(1024 * sizeof(char *));

/*
 * Make sure that the number entries in the array are less than the allocated
 * 1024 in the absurd case that the system has that many mount points.
 */
while (i < 1024 && (ent = getmntent(mounts))) {
    /*
     * The output of getmntent(mounts) goes down to
     * the next mount point every time it is called.
     */
    mountslist[i] = strdup(ent->mnt_dir);
    i++;
}

I was wondering how I could dynamically allocate the number of entries in the mountslist array (currently statically set at 1024) to avoid that limit and wasting memory. If I had the final value of i when declaring mountslist I could use char *mountslist[i]; or char **mountslist = malloc(i * sizeof(char *));

Answer 1

You can use realloc to change the size of an allocated memory block:

int i = 0;
int len = 10;
char **mountslist = malloc(len * sizeof(char *));

while ((ent = getmntent(mounts)) != NULL) {
    if (i >= len) {
        len *= 2;
        char **tmp = realloc(mountslist, len * sizeof(char *));
        if (tmp) {
            mountslist = tmp;
        }
    }
    mountslist[i] = strdup(ent->mnt_dir);
    i++;
}

As shown above, a good rule is to double the amount of space you allocate when you run out of space. That prevents excessive calls to realloc which may move the allocated memory each time.