Segmentation fault and incompatible pointer type warning with scandir()

Question

I wrote a post earlier about scandir(), but figured I'd make a new post since I believe the requirement is to only have one question per post.

I'm writing a program that uses scandir() and getting a incompatible pointer type warning when I run the program.

According to the man-page for scandir(), the function takes in a "triple pointer" to a dirent struct:

int scandir(const char *dirp, struct dirent ***namelist,
              int (*filter)(const struct dirent *),
              int (*compar)(const struct dirent **, const struct dirent **));

However, the page also provides the example below:

#define _DEFAULT_SOURCE
       #include <dirent.h>
       #include <stdio.h>
       #include <stdlib.h>

       int
       main(void)
       {
           struct dirent **namelist;
           int n;

           n = scandir(".", &namelist, NULL, alphasort);
           if (n == -1) {
               perror("scandir");
               exit(EXIT_FAILURE);
           }

           while (n--) {
               printf("%s\n", namelist[n]->d_name);
               free(namelist[n]);
           }
           free(namelist);

           exit(EXIT_SUCCESS);
       }

In my program, I decided to follow the example and have the code in my program below (distilled to only the statements relevant to scandir()):

int numOfDirectories = 0;
struct dirent **dirEntries;
numOfDirectories = scandir(".", &dirEntries, NULL, alphasort);

When I run the program, I then get the warning below:

Why would it be giving me this error when I'm essentially following the example it has in the man-page?

Further, according to information received from my other post, passing in &dirEntries when dirEntries is a double-pointer should be a triple-pointer.

I did try changing the program by declaring dirEntries as a triple pointer:

int numOfDirectories = 0;
struct dirent ***dirEntries;
numOfDirectories = scandir(".", dirEntries, NULL, alphasort);

I don't get any compiler errors, but when I run the program I then get a Segmentation fault (core dumped) error.

I did have a filter function as the 3rd parameter, but decided to just set to NULL in-case it was my filter function causes the error. Still get the same segmentation fault error using NULL.

I also tried running the debugger, which it does show this:

Then essentially after I declare my dirEntries I have an if-statement that is entered and immediately runs numOfDirectories = scandir(".", &dirEntries, NULL, alphasort);

I'm at a loss of what is going on. Any ideas what it could be? Thank you. Appreciate your help and feedback.

Do not paste images of text. Copy and paste the exact text. You can use `
` and `
` tags to mark it as preformatted text. — Eric Postpischil, Dec 13 '20 at 00:49

score 1 · Accepted Answer · answered Dec 13 '20 at 00:56

Why would it be giving me this error when I'm essentially following the example it has in the man-page?

You are not following the example. The example declares a thing with type struct dirent ** and passes its address, using &. You declare a thing with type struct dirent ** and pass it, not its address, not using &.

Change your code to pass &dirEntries, not dirEntries.

I did try changing the program by declaring dirEntries as a triple pointer: … I don't get any compiler errors, but when I run the program I then get a Segmentation fault (core dumped) error.

The thing you pass had the right type, but it did not have a correct value. scandir does not just want you to pass something of type struct dirent **, it wants you to pass the address of a struct dirent ** that you provide. This is because it is going to fill in that struct dirent ** with some value. (That value will be a struct dirent **; it will point to a struct dirent *.)

When you make dirEntries with struct dirent ***dirEntries;, you do not give it any value, so passing it as an argument is wrong. In contrast, struct dirent **dirEntries makes a pointer, and then passing &dirEntries gives the address of that pointer to scandir. It uses that address to put a value in dirEntries.

Thanks for the feedback. Regarding your first point, I actually did use the address-of operator to pass the address of `struct dirent **`. On your second point, in a sense I see what you're saying, but having difficulty wrapping my head around it. The documentation says it want's a triple pointer, so I'm passing a triple pointer. — GainzNerd, Dec 13 '20 at 01:08
One thing I realized is with my "incompatible pointer type" warning, it was due to another statement later on that was just using `dirEntries` and not `&dirEntries`. So I commented that out and now everything compiles and runs correctly. Still not sure about the issue regarding double vs. triple pointer. — GainzNerd, Dec 13 '20 at 01:17
Re “The documentation says it want's a triple pointer, so I'm passing a triple pointer”: So? The documentation for `sqrt` says it takes a `double`, but if you do `double x; printf("%g\n", sqrt(x));`, what do you get? `x` was never initialized, so it has no determined value. When you pass `scandir` an uninitialized pointer, it gets a pointer with no determined value. When it tries to use that pointer, something goes wrong. You have to pass a particular value, not just something matching in type. — Eric Postpischil, Dec 13 '20 at 01:20

Segmentation fault and incompatible pointer type warning with scandir()

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