Dynamic pointer array to strings

[Richard Gilmore]

Ok I need to create a dynamic array of pointers to strings, the number of strings is determined by the typed value at the keyboard, and then the length of each string is determined as it's typed in

char **zerg;
char ch;
 int num;
 int i;
 char buffer[BUFSIZ];
printf("Enter the number of names you wish to store>");
 scanf("%d",&num);
zerg = (char **)malloc(num);
for(i = 0; i < num; i++)
 {
  fgets(buffer,BUFSIZ,stdin); 
 if (buffer[strlen(buffer)-1] == '\n') buffer[strlen(buffer)-1] = '\0';
 
 zerg[i] = malloc(sizeof(char)*(strlen(buffer)+1));
 strcpy(zerg[i],buffer);
 

I think I've referenced correctly using the **char but when I run the program I do not get the desired results from the output or for the memory address anyone able to help?

[newb16]

You should
malloc(num*sizeof(char*));
because malloc accepts number of bytes to allocate.

[Banfa]

In C you should cast the output of malloc

zerg = (char **)malloc(num);

should be

zerg = malloc(num);

In the C and C++ languages sizeof(char) is by definition 1 always.

[newb16]

@Banfa
Actually argument for malloc should be
malloc(num*sizeof(char*)),
because size of pointer is not 1.

[Banfa]

@newb16
Actually my whole post was wrong because I wrote "should" instead of "shouldn't" :D

So stating again and recapping, when using malloc in C you should

1. Make sure you allocate the right amount of data taking into account the size of the objects you are allocating and the number of objects you are allocating.
2. Not cast the output of malloc which by passes C's (admittedly not brilliant) pointer conversions.

[Richard Gilmore]

Ok so I've got the program running as I wanted, except it automatically allocates 16 bytes for each string even if the length is less than that. so for example turkey still gets allocated 16 instead of just 7 bytes. It does however expand correctly for longer strings.
Richard starts at address 00331A18
Gilmore starts at address 00331A28
Paul starts at address 00331A38
How can I get gilmore to start at say 00331A21 straight after the terminating character of richard.

[Banfa]

You should understand that when you call malloc all you provide is a minimum size. That is the returned pointer to a pointer to a block of data of at least that many bytes.

However compiler/platform actually implements the memory allocation is implementation defined as long as it meets that requirement. Many implementations will allocate additional data for organisational or book keeping purposes. Many implementations have a minimum block size, any request for the blocks below the minimum size returns a block of the minimum size.

Finally malloc has to return a block that meets the alignment requirements of the most stringent structure that could be put into memory block. That is for instance it can't return a byte aligned block since words require word alignment.

If you have some additional requirement of how your data items are aligned with respect to each other malloc will not do it for you. You have to do it for yourself so you would need to allocate a block of memory large enough to hold all the strings and pack them into that data block yourself (obviously holding pointers to the start of each string).

If on the other hand you are just worrying about the the wasted space of the unused bytes don't. Just accept it as a fact of the implementation you are using.