I read the previous questions on dynamic arrays in C however I was not able to relate the answers to my question.
I am taking commands from stdin
using fgets, removing the newline character and then want to store each command delimited by a space in a dynamically allocated array of strings. I however am having a lot of trouble with the correct way to allocated and reallocate memory. I am compiling with clang
and keep getting segmentation fault 11. I then used -fsanitize=address
and keep getting:
==2286==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60200000eeb8 at pc 0x000108fb6f85 bp 0x7fff56c49560 sp 0x7fff56c49558 WRITE of size 8 at 0x60200000eeb8 thread T0
Here is my code:
// Sets a delimiter to split the input
const char *seperator = " ";
char *token = strtok(line, seperator);
char **cmds = (char **) malloc(sizeof(char) * sizeof(*cmds));
// Adds first token to array of delimited commands
cmds[0] = token;
int count = 1;
while (token != NULL) {
token = strtok(NULL, sep);
if (token != NULL) {
cmds = (char **) realloc(cmds, sizeof(char) * (count + 1));
// Adds next token array of delimited commands
cmds[count] = token;
count++;
}
}
You're not allocating enough memory. cmds
is an array of pointers, so each element is sizeof(char *)
bytes, not sizeof(char)
bytes.
On the initial allocation you want 1 char *
, then on subsequent allocations you want count + 1
.
Also, don't cast the return value of malloc
, as that can hide other problems, and don't forget to check for failures.
char **cmds = malloc(sizeof(char *) * 1);
if (cmds == NULL) {
perror("malloc failed");
exit(1);
}
...
cmds = realloc(cmds, sizeof(char *) * (count + 1));
if (cmds == NULL) {
perror("reallocfailed");
exit(1);
}
First, sizeof(char)
is always 1 by definition. And coding that does not make your code more readable.
But a pointer to char
needs sizeof(char*)
bytes (depending on the machine & ABI , that is often 8 or 4 bytes). I would at least suggest to compile your code with gcc -Wall -Wextra -g
if using GCC .
At last, I find your code a bit inefficient. You are calling realloc
at every loop. I would maintain a variable containing the allocated size
int allocsize = 4; // allocated size in number of elements
char **cmds = malloc(allocsize*sizeof(char*));
if (!cmds) { perror("malloc"); exit(EXIT_FAILURE); };
(BTW, always check the result of malloc
; it can fail).
And to avoid realloc
-ing every time, I would grow the allocated size in a geometric fashion, so inside the loop:
if (count>=allocsize) {
int newallocsize = (4*allocsize)/3+10;
cmds = realloc (cmds, newallocsize*sizeof(char*));
if (!cmds) { perror("realloc"); exit(EXIT_FAILURE); };
allocsize = newallocsize;
}
Alternatively, instead of keeping three variables: cmds
, count
, allocsize
you could use a single struct
ending with a flexible array member (and keeping its allocated and used sizes).
the right way would be..
// strtok modifies the string. So use writable string
char line[80] = "Hello my name is anand";
char *token = strtok(line, sep);
int count = 0;
// Alloc array of char* for total number of tokens we have right now
char **cmds = (char **) malloc(sizeof(char*) * (count + 1));
while (token != NULL)
{
/**
* Alloc memory for the actual token to be stored..
* token returned by strtok is just reference to the existing string
* in 'line'
*/
cmds[count] = malloc(sizeof(char) * ((strlen(token) + 1)));
// Adds tokens to array of delimited commands
strcpy(cmds[count], token);
count++;
token = strtok(NULL, sep);
if (token != NULL)
{
// resize array of tokens to store an extra token
char ** newCmds = (char **) realloc(cmds, sizeof(char*) * (count + 1));
// only if realloc was successful then use it.
if (newCmds != NULL)
{
cmds = newCmds;
}
}
}
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