Returning a string pointer array from a function in C

I am trying to use a function to split a string in its separate words. The function creates an array, and I can see that the array is working in the function. My problem is when I try to return the array back to the main function as another char array.

Here is my code:

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <unistd.h>
#include <sys/wait.h>
#include <string.h>
#include <assert.h>
#include <fcntl.h>

char *commands(char *command) {
	char *commands[30];
	int i =0;
	char *st = strtok(command, " ");
	while (st != NULL) {
		commands[i] = strsep(&st, " ");
		st = strtok(NULL, " ");
		i++;
	}
	for (int p = 0; p < i; p++) {
		printf("inside the function still: i-> %d, command: %s\n", p, commands
);
	}
	return *commands;
}

int main(int MainArgc, char *MainArgv[]) {
	size_t size = 10;
	char *user_input;
	char *user_commands;
	int flag = 0;
	user_input = (char *)malloc(size);

	while (flag != EOF) {
		printf("witsshell> ");
		flag = getline(&user_input, &size, stdin);
		user_commands = commands(user_input);
		puts(&user_commands[1]);
	}

	return (0);
}

There are multiple problems in your code:

• the function commands modifies its argument string. This may pose problems, in particular, you cannot call this function with a string literal.
• you split the string on spaces only, you should also consider TABs and newlines as word separators.
• you store the word pointers into a local array without checking for overflow, which will occur if the argument string contains more than 30 words.
• you should store a null pointer at the end of the array for the caller to detect the end of the array.
• you return *commands, which is the first word, not the array of strings
• you cannot return a local array at it becomes invalid as soon as the function returns.

Here is an alternative approach using standard string functions that avoid many of the above shortcomings:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char **commands(const char *command) {
char **array;
int count, pos, len;
for (pos = 0, count = 0;; count++) {
// skip whitespace
pos += strspn(command + pos, " \t\r\n");
if (command[pos] == '\0')
break;
// get the word length
len = strcspn(command + pos, " \t\r\n");
// skip the word
pos += len;
}
// allocate memory for the array:
array = malloc(sizeof(*array) * (count + 1));
if (array == NULL)
return NULL;
for (pos = 0, i = 0;; i++) {
// skip whitespace
pos += strspn(command + pos, " \t\r\n");
if (command[pos] == '\0')
break;
// get the word length
len = strcspn(command + pos, " \t\r\n");
// make a copy of the word
array[i] = strndup(command + pos, len);
if (array[i] == NULL) {
while (i-- > 0)
free(array[i]);
free(array);
return NULL;
}
// skip the word
pos += len;
}
array[i] = NULL;
return array;
}
int main(int argc, char *argv[]) {
size_t size = 0;
char *user_input = NULL;
char **user_commands;
for (;;) {
printf("witsshell> ");
if (getline(&user_input, &size, stdin) < 0)
break;
char **user_commands = commands(user_input);
if (user_commands) {
for (int i = 0; user_commands[i]; i++) {
printf("%d: %s\n", i, user_commands[i]);
free(user_commands[i]);
}
free(user_commands);
}
}
free(user_input);
return 0;
}

In the code snippet, you are using a function named commands to split a string based on space characters. However, due to the limited scope of this function, returning the array to the main function is causing issues. Therefore, while processing separate words with the commands function, you might need to create a new array outside of the main function to overcome this limitation.

char** commands(char *command, int *numCommands){
char **commands = (char**)malloc(sizeof(char*) * 30);
int i = 0;
char *st = strtok(command, " ");
while(st != NULL){
commands[i] = strdup(st);
st = strtok(NULL, " ");
i++;
}
*numCommands = i;
return commands;}