Working with linked list + struct C

Question

Here is the linked list and the struct:

#define MAX_PATH_SIZE (256)
#define MAX_NAME_SIZE (50)
struct Frame
{
    char *name;
    unsigned int duration;
    char *path;  // may change to FILE*
};

typedef struct Frame frame_t;
struct Link
{
    frame_t *frame;
    struct Link *next;
};

typedef struct Link link_t;

And here's my function:

link_t* createFrame(char name[], int duration, char path[]){
frame_t * temp = (frame_t*)malloc(sizeof(frame_t));
temp->duration = duration;

strncpy(temp->name, name,MAX_NAME_SIZE);
strncpy(temp->path, path,MAX_PATH_SIZE);
link_t* newFrame = (link_t*)malloc(sizeof(link_t));

newFrame->frame = temp;
return newFrame;
}

The problem is that the function stop working in the line "strncpy(temp->name)..", the weird thing is that the temp->duration is working but it doesn't work with strings. Error: "Unhandled exception at 0x0F744645 (msvcr120d.dll)"

Possible duplicate of 'strcpy' with 'malloc'?

jpw
– jpw

2017-06-08 16:30:10 +00:00
Commented Jun 8, 2017 at 16:30 — jpw
– jpw, Commented Jun 8, 2017 at 16:30

unalignedmemoryaccess · Accepted Answer · 2017-06-08 18:09:01Z

2

You didn't allocate memory for your name, now they point to unknown location and is undefined behaviour.

temp->name = malloc((MAX_NAME_SIZE + 1) * sizeof(*temp->name));
temp->path = malloc((MAX_PATH_SIZE + 1) * sizeof(*temp->path));
temp->name[MAX_NAME_SIZE] = 0; //Manually add null termination
temp->name[MAX_PATH_SIZE] = 0; //Manually add null termination
strncpy(temp->name, name,MAX_NAME_SIZE);

Now, memory is allocated for name and path and you are able to copy data for name and path.

Or, if you want, you can define your structure like this:

struct Frame {
    char name[MAX_NAME_SIZE + 1];
    unsigned int duration;
    char path[MAX_PATH_SIZE + 1];
};

Then you won't need to call malloc for name and path separatelly as memory will be allocated on first malloc for structure already.

edited Jun 8, 2017 at 18:09

answered Jun 8, 2017 at 16:27

unalignedmemoryaccess

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4 Comments

chux Over a year ago

Note With strncpy(temp->name, name,MAX_NAME_SIZE);, temp->name, name may lack a null character.

chux Over a year ago

The allocation is OK, yet name[MAX_NAME_SIZE]; remains uninitialized with strncpy(temp->name, name,MAX_NAME_SIZE);.

chux Over a year ago

Let us continue this discussion in chat.

Saga Over a year ago

I tried the first way and in the first run(in loop) it works, but in the second it doesn't work, do I need to free something in the loop?

Sokre · Accepted Answer · 2017-06-08 16:30:07Z

0

You need to allocate memory to store strings with strncpy(), in your malloc you allocated only enough to store sizeof(struct Frame) bytes.

You may want to try this instead of strncpy...

temp->name = strndup(name, MAX_NAME_SIZE);
temp->path = strndup(path,MAX_PATH_SIZE);

...if you are insisting on restricting the max size of strings.

answered Jun 8, 2017 at 16:30

Sokre

1146 bronze badges

3 Comments

chux Over a year ago

Note that strndup() is not a standard C library function. The implementations I have seen the 2nd argument is 1 less than the max size. It is more like a max string length.

Sokre Over a year ago

What does it mean "not standard"? Which standard? This one - man7.org/linux/man-pages/man3/strndup.3p.html ? And that there are implementation deviating from the standard, is natural. Talking of standard, "the man" seems to be using Microsoft C. He might not have strndup() at all.

chux Over a year ago

"Which standard?" --> "standard C library" is referring to the library of functions specified in the C11 spec. ref. Deviating from the C standard makes the implementation non-compliant. As strndup() is not part of the standard C library function, its signature and functionality may vary. Using strndup() to solve OP's issues is an OK idea yet it lacks portability.

Adam · Accepted Answer · 2017-06-08 17:30:27Z

0

Try this :

#define MAX_PATH_SIZE (256)
#define MAX_NAME_SIZE (50)

struct Frame
{
    char name[MAX_NAME_SIZE];
    unsigned int duration;
    char path[MAX_PATH_SIZE];  // may change to FILE*
};

typedef struct Frame frame_t;
struct Link
{
    frame_t *frame;
    struct Link *next;
};

typedef struct Link link_t;

link_t* createFrame(char name[MAX_NAME_SIZE], int duration, char path[MAX_PATH_SIZE]){
    frame_t * temp = (frame_t*)malloc(sizeof(frame_t));
    temp->duration = duration;

    strncpy(temp->name, name, MAX_NAME_SIZE);
    strncpy(temp->path, path, MAX_PATH_SIZE);
    link_t* newFrame = (link_t*)malloc(sizeof(link_t));

    newFrame->frame = temp;
    return newFrame;
}

answered Jun 8, 2017 at 17:30

Adam

335 bronze badges

2 Comments

unalignedmemoryaccess Over a year ago

What if string is long MAX_NAME_SIZE characters and you copy it to temp->name. You have undefined behaviour because of null termination.

chux Over a year ago

strncpy(temp->name, name, MAX_NAME_SIZE); does not certainly null character terminate temp->name. Could append temp->name[MAX_NAME_SIZE - 1] = 0; to insure proper string termination.

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