C pointer issues... Segmentation fault!

[Cconfused]

Howdy!

I'm mega confused with all this pointer malarky in C and it's resulting in an ever annoying segmentation fault which just won't go away!

Essentially the program I am building has a Get() function which reads 1 record of data from a file, another function adds this record (or struct) to a data structure, then the Get function reads and returns the next record.

A record consists of a surname, address (with spaces), and postcode (with spaces), however the file format is:
line 1. surname, firstname (although I only need surname)
line 2. address
line 3. postcode and so on.

My code succesfully reads the first 2 records (as I make it print it to screen to check), and then I am presented with a "error while dumping state <probably corrupted stack>"

The code is as follows:

// read surname up until a comma //
        memset(namebuf, 0, MAXLINESIZE);
    printf("Name : ");
    i = 0;
    ch = fgetc(fd);
    while(ch != ',' && ch != EOF) {
    namebuf[i] = tolower(ch);
    ch = fgetc(fd);
        i++; }
 
    test.surname = malloc(MAXLINESIZE);
    strcpy(test.surname, namebuf);
    printf("%s", test.surname); 
    putchar('\n');
 
// get the first name, and don't do anything with it //
        memset(namebuf, 0, MAXLINESIZE);
    firstname = malloc(MAXLINESIZE);
    strcpy(firstname, fgets((namebuf +1), MAXLINESIZE, fd));
 
    // get address //
    printf("Address : ");
    test.full_address = malloc(MAXLINESIZE);
    strcpy (test.full_address, fgets((addressbuf +1), MAXLINESIZE, fd));
    printf("%s", test.full_address);    
 
    // get postcode //
    //memset(buf, '\0', MAXLINESIZE);
    printf("Postcode : ");
    test.postcode = malloc(MAXLINESIZE);
    strcpy (test.postcode, fgets((postcodebuf +1), MAXLINESIZE, fd));
    printf("%s", test.postcode);
    putchar('\n'); 

Any ideas? I've been stumped for days, and I'm very new to the whole malloc concept...!

Any help would be vastly appreciated!

[Cucumber]

    printf("Postcode : ");
    test.postcode = malloc(MAXLINESIZE);
    strcpy (test.postcode, fgets((postcodebuf +1), MAXLINESIZE, fd));
    printf("%s", test.postcode);
    putchar('\n'); 
 

Im not sure, perhaps postcodebuf has not been allocated before using it

[Cconfused]

I used:

char postcodebuf[MAXLINESIZE];

[mattmao]

Hi.

Just one thing: where is your struct test ? I couldn't find it inside your example code.

From my personal experience in studying linked list, use a pointer to struct testClass would be a better choice, such like:

struct testClass
{
  int number;
  char address[SIZE];
  struct testClass *next;
}
typedef struct testClass test;

In your code, you can use this to allocate memory space:

test *a= (test *) malloc(sizeof(test));

Then access the content of this struct like this:

a->number = 100;
strcpy(a->address, from);
a->next = b;
 

Most of the references books I read recommanded this way.

[Cconfused]

Ok - here's the full code:

typedef struct entry {
char *surname;
char *postcode;
char *full_address;
} Entry;

/* get returns next file entry, or NULL if end of file*/
Entry *get(FILE *fd)
{
int i;
char addressbuf[MAXLINESIZE];
char postcodebuf[MAXLINESIZE];
char namebuf[MAXLINESIZE];
char ch;
char *firstname;
Entry test;

// get surname //
memset(namebuf, '\0', MAXLINESIZE -1);
printf("Name : ");
i = 0;
ch = fgetc(fd);
while(ch != ',' && ch != EOF) {
namebuf[i] = tolower(ch);
ch = fgetc(fd);
i++; }

test.surname = malloc(MAXLINESIZE);
strcpy(test.surname, namebuf);
printf("%s", test.surname);
putchar('\n');

memset(namebuf, '\0', MAXLINESIZE);
firstname = malloc(MAXLINESIZE);
strcpy(firstname, fgets((namebuf +1), MAXLINESIZE, fd));

printf("Address : ");
test.full_address = (char *)malloc(strlen(addressbuf)+1);
test.full_address = (fgets(addressbuf, MAXLINESIZE, fd));
strcpy(test.full_address, addressbuf);
printf("%s", test.full_address);

printf("Postcode : ");
test.postcode = (char *)malloc(strlen(postcodebuf)+1);
test.postcode =(fgets(postcodebuf, MAXLINESIZE, fd));
strcpy(test.postcode, postcodebuf);

printf("%s", test.postcode);
}

[mattmao]

Hi.

Can you quote your code into CODE tags? Just for the ease to read...

And I don't quite get your idea of separately allocate memory space for each element of a struct variable. You use something like:

test.surname = malloc(MAXLINESIZE);

If you declared Entry test, then test.surname would be of type char *,
malloc() would return you a pointer of type void, so at least you've missed a cast:

test.surname = (char *) malloc(MAXLINESIZE);

And, I guess you've got this:

#define MAXLINESIZE 100

which means this:

test.surname = (char *) malloc(100);

You are asking for the malloc method to allocate you the memory space for 100.
This is not legal, I guess.
It should be:

char array[MAXLINESIZE];
test.surname = (char *) malloc(sizeof(array[MAXLINESIZE]));

Still, why wouldn't you allocate a memory space set for one struct element in one go, as I mentioned before?

Sorry I must go for lesson now, can't check for the rest of your code.

[RRick]

Your problem lies in the sections of code that deal with address and postcode.

You are malloc'ing on the strlen of addressbuf and postcodebuf, but they don't contain anything. Strlen will not tell you big your buffer is, it will only tell you how large the string in the buffer is. When strlen finds the first '\000', it stops. In the case of postcodebuf (which is where your program blows up), strlen probably returns a length of 0. After that, you copy a bigger string into a smaller string.

A simple solution is to switch the malloc and fgets lines. This should fix the problem, but be sure to do for address and postcode

 
    printf("Postcode : ");
    test.postcode =(fgets(postcodebuf, MAXLINESIZE, fd));
    test.postcode = (char *)malloc(strlen(postcodebuf)+1);
    strcpy(test.postcode, postcodebuf);
 

Also, the get routine needs to be cleaned up. You are using fgetc and fgets to get the values. Stick with one. Also, you use two different ways to malloc the buffers.

[Cconfused]

Howdy!

Thanks for your help there, I understand what you meant, and actually thought it would fix the problem... but it hasn't! :(

The reason I use two methods to read the file, is that I *only* need the surname of the first line, not the first name (it's discarded), thus I must read only up to the comma.

Now it still reads in only 2 records, then hits a segmentation fault. Code is now:

typedef struct entry { 
    char *surname;
    char *postcode;
    char *full_address;
} Entry;
 
Entry *me_get(FILE *fd)
{
    int i;
    char addressbuf[MAXLINESIZE];
    char postcodebuf[MAXLINESIZE];
    char namebuf[MAXLINESIZE];
    char fnamebuf[MAXLINESIZE];
        char ch;
    char *firstname;
    Entry test;
 
    // get surname //
    memset(namebuf, '\0', MAXLINESIZE -1);
    printf("Name : ");
    i = 0;
    ch = fgetc(fd);
    while(ch != ',' && ch != EOF) {
    namebuf[i] = tolower(ch);
    ch = fgetc(fd);
    i++; }
 
    test.surname = (char *)malloc(strlen(namebuf)+ 1);
    strcpy(test.surname, namebuf);
    printf("%s", test.surname); 
    putchar('\n');
 
    // get first name //
    memset(fnamebuf, '\0', MAXLINESIZE);
    fgets(fnamebuf, MAXLINESIZE, fd);
    firstname = (char *)malloc(strlen(fnamebuf)+ 1);
    strcpy(firstname, fnamebuf);
 
    // get address //
    printf("Address : ");
    fgets(addressbuf, MAXLINESIZE, fd);
    test.full_address = (char *)malloc(strlen(addressbuf)+ 1);
    strcpy(test.full_address, addressbuf);
    printf("%s", test.full_address); 
 
    // get postcode //
    printf("Postcode : ");
    fgets(postcodebuf, MAXLINESIZE, fd);
    test.postcode = (char *)malloc(strlen(postcodebuf)+ 1);
    strcpy(test.postcode, postcodebuf);
    printf("%s", test.postcode);  

[Cucumber]

I think its time to let the code tell us where the error is. I.e.

 
if( fgets( postcodebuf, MAXLINESIZE, fd ) == NULL )
{
   printf("Error at fgets\n");
   exit(-1);
}
test.postcode = (char *)malloc(strlen(postcodebuf)+ 1);
if( test.postcode == NULL )
{
   printf("Error at malloc\n");
   exit(-1);
}
strncpy(test.postcode, postcodebuf,strlen(postcodebuf)+ 1);
printf("%s", test.postcode); 
 
 

[weaknessforcats]

Entry *me_get(FILE *fd)
{
int i;
char addressbuf[MAXLINESIZE];
char postcodebuf[MAXLINESIZE];
char namebuf[MAXLINESIZE];
char fnamebuf[MAXLINESIZE];
char ch;
char *firstname;
Entry test;

I can't see your entire function but I notice it returns an Entry*. I certainly hope it's not returning the address of test because that is a local variable which is destroyed when this function returns.

I suspect that's what you are doing as you are loading up the members of test.

If , in fact, that is what you are doing, then you will need to allocate test on the heap.

[Cconfused]

You are exactly correct - and I have now done this, and all is compiling well! I also fiddled a few other bits... Thank you all for your help!

Now I've gotta figure out how to stick this in a hash table! Eek!! I may be back!