Adding values to char* [][] - c

Question

I have been working around with building a simple shell in C. I am wanting to add a history built in function, but I need to know how to do the following:
I have a global variable commanHistory which, I believe is a pointer to an array of characters? (correct me if I am wrong please).
char *commandHistory[MAX_COMMANDS][MAX_LINE_LENGTH + 1];
Within my read line function, I want to store the ith line in the ith row of commandHistory. Here is what I am doing:
char *lsh_read_line(void)
{
int bufsize = MAX_LINE_LENGTH;
int position = 0;
char *buffer = malloc(sizeof(char) * bufsize);
int c;
int i = 0;
if (!buffer) {
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
while (1) {
// Read a character
c = getchar();
// If we hit EOF, replace it with a null character and return.
if (c == EOF || c == '\n') {
buffer[position] = '\0';
return buffer;
} else {
buffer[position] = c;
}
position++;
// If we have exceeded the buffer, reallocate.
if (position >= bufsize) {
bufsize += MAX_LINE_LENGTH;
buffer = realloc(buffer, bufsize);
if (!buffer) {
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
}
}
commandHistory[i++][0] = buffer; // wanting to store command i in commandHistory (also only want to keep track of 10 at a time)
}
When I call my built in function I just get 10 (null) printing out from this:
int lsh_history(char **args)
{
int i;
for (i = 0; i < MAX_COMMANDS; i++) {
printf(" %s\n", commandHistory[i][0]);
}
}
EDIT: I am required to use a 2d array. This is the last part of building a shell that I am having trouble with. While I believe this can be done with a 1 dimensional array, I am following this part of instructions:
Internally, your shell should save the command history in a 2-dimensional array:
char commandHistory[MAX_COMMANDS][MAX_LINE_LENGTH + 1];
Each row of this table will store one command. Viewing the rows arranged in a circle, a data
structure similar to a queue can be constructed. Unlike a traditional queue, your command
history will never overflow — as we continue to add commands, older commands will simply
be overwritten.

Answer 1

Your commandHistory is actually a two-dimensinal array of strings/char pointers, while you want a one dimensional one. You should declare it like this:
char *commandHistory[MAX_COMMANDS];
and you don't need to worry about the string length at this point, since each command is allocated dynamically. And then to access the i'th string from this array you just need commandHistory[i], which is of type char *.
Update:
If you (or the instructor) insist on having commandHistory declared as static two dimensional array:
char commandHistory[MAX_COMMANDS][MAX_LINE_LENGTH + 1];
You should not allocate the buffer dynamically as you do, but copy the commands into corresponding place in your statically pre-allocated array commandHistory (i.e. instead of buffer[position]=.. do commandHistory[i][position] = ...).

Answer 2

You either need to make your command history an array of char*'s , i.e.:
char *commandHistory[MAX_COMMANDS];
and copy the buffer pointer into the history entry (be careful; the history now "owns" the malloced space)
An alternative would be to make it a 2D array of chars, but then you couldn't accept your overly long commands (the ones for which you realloced buffer).