Dynamically grown string array

Question

I am trying to build a dynamically grown array of strings. At compile time, neither the number of lines nor the length of each line is known. Here is the code that I have come up with so far (it's just me playing with the syntax):

char **result = NULL; char *temp = NULL; result = (char **)realloc (result,sizeof(char *) * 1); temp= (char *)realloc(temp,5 * sizeof(char)); strcat(temp,"hello"); temp= (char *)realloc(temp,10 * sizeof(char)); strcat(temp," world"); printf ("%s \n", temp); result[0]=temp; free(temp); printf ("%s \n", result[0]); result = (char **)realloc (result, sizeof(char *) * 2); temp= (char *)realloc(temp,10 * sizeof(char)); strcat(temp,"0123456789"); temp= (char *)realloc(temp,15 * sizeof(char)); strcat(temp,"asdfg"); printf ("%s \n", temp); result[1]=temp; free(temp); printf ("%s \n", result[0]); printf ("%s \n", result[1]);) 

Now, when I print the result [0] or the result [1], its just an empty line, why does not it result [1] = temp; work?

Here is what I tried before, but it didn’t work, I continued to get "wrong size" errors when using realloc () in this last line:

 char **result = NULL; result = (char **)realloc (result,sizeof(char *) * 1); result[0]= (char *)realloc(result[0],5 * sizeof(char)); strcat(result[0],"hello"); printf ("%s \n", result[0]); result[0]= (char *)realloc(result[0],10 * sizeof(char)); strcat(result[0]," world"); printf ("%s \n", result[0]); result = (char **)realloc (result, sizeof(char *) * 2); result[1]= (char *)realloc(result[1],10 * sizeof(char)); strcat(result[0],"0123456789"); result[0]= (char *)realloc(result[1],15 * sizeof(char)); strcat(result[0],"asdfg"); 

If anyone can help me get any version, I would really appreciate it.

UPDATE: Well, I have both versions of the code. Now, when I try to use this format in my real program, I get errors like

 *** glibc detected *** ./uvicfmt3: realloc(): invalid next size: 0x08ed3170 *** 

Now in my program the "result" is declared as a global variable (using the second version of my code), and the realloc functions are called in different subprograms. Is that what causes the problem? How could I get around this?

Answer 1

In the following expression, both the result[0] and temp characters point to the same memory address:

 result[0]=temp; 

Performing the above assignment, you then free(temp) and try to access result[0] :

 free(temp); printf ("%s \n", result[0]); 

This behavior is undefined, since you are accessing newly freed memory.

The same goes for the identical code for result[1] .

Answer 2

• In the first example, you use the string after freeing it
• In the second example, you strcat for unallocated memory (plus uneducated realloc(result[0] does not make sense)

You can try the following:

 char **result = NULL; result = realloc(result, sizeof(char *) * 1); result[0] = strdup("hello"); /* ... */ result = realloc(result, sizeof(char *) * 2); result[1] = strdup(" world"); 

Now strdup not standard, but it is not difficult to steal / fake it. Here is one try:

 char *strdup2(const char *str) { size_t len; char *rval = NULL; len = strlen(str); /* We should probably check this. */ rval = malloc(len + 1); /* And this. */ memcpy(rval, str, len); rval[len] = 0; return rval; } 

EDIT

I was under the (possibly wrong) impression that you want to change in the future. If not, just save them (without strdup ):

 result[0] = "hello";