I have an char array and I am trying to copy a part of it (tokenization) into 0th index of array of pointer to char using the strncpy function. But during runtime a segmentation fault occurs.
Code example:
char array[30] = "ls -l";
char* args[10];
strncpy(args[0], array + 0, 2);
char *args[10] has the following declaration:
declare args as array 10 of pointer to char
That is to say, we have an array of uninitialized pointers. We'll need to make those pointers point somewhere first, before trying to place characters there. Remembering that we must NUL terminate ('\0') C strings, we can simultaneously allocate and NUL out space for our string by using calloc.
This will make space for just 'l', 's', and our mandatory '\0'.
char original_command[30] = "ls -l";
char *args[10] = { 0 };
args[0] = calloc(3, sizeof (char));
strncpy(args[0], original_command, 2);
Alternatively, we can use malloc, but we must remember the NUL terminating byte.
args[0] = malloc(3);
strncpy(args[0], original_command, 2);
args[0][2] = '\0';
It's generally a good idea to always initialize our variables - see how we initialize our args array to be full of NULL pointers (0). Makes it very clear they don't point anywhere useful yet.
Also note that strncpy does not place a NUL terminating byte if it was not found in the first n bytes of our source string. This is why it's very important to manually terminate our destination string.
Additionally, any call to an *alloc function must be matched later by a call to free, when we are finished using that memory.
/* Do whatever needs to be done */
/* ... */
free(args[0]);
calloc is.You need to allocate space for the copied string content; char* args[10] reserves only space for holding the pointer to the content, not for the content itself. And don't forget to reserve space for the string terminating character '\0' then.
args[0] = malloc(2+1);
strncpy(agrs[0],array+0,2+1);
agrs[0][2] = '\0';
args[0].
strncpy(agrs[0],array+0,2+1) looks fishy anyway.strncpy will copy the first 3 bytes (being 'l', 's', ' '), and not NUL terminate properly.
args. Second time in this thread. ;-)