C: how do dynamically allocate & free memory for a char array? -
how dynamically allocate , free memory char array?
my malloc below not work. error thrown 'expected ';' @ end of declaration list.'
char *string_buffer = malloc(128 * sizeof(char)); below attempt @ freeing string_buffer[128] char array:
void rmv(struct tree_node *cwd) { struct list_node *child = cwd -> first_child; free(string = child -> tree -> string_buffer); printf("string memory free successful\n"); //free(cwd -> first_child -> tree); //free(cwd -> first_child); } rest of code:
shell commands:
cd = current directory
ls = list
mkdir = make directory
rmdir = remove directory (unimplemented - need free memory)
#include <stdio.h> /* fgets(), fprintf() , printf() */ #include <stdlib.h> /* exit_failure */ #include <ctype.h> /* isspace() */ #include <string.h> #include <stddef.h> // commands char exitprogram[] = "exit"; char listdirectory[] = "ls"; char commanddirectory[] = "cd"; char makedirectory[] = "mkdir"; char removedirectory[] = "rmdir"; // holds string buffer of length 128 // pointer parent // pointer first node of list of children typedef struct tree_node { char string_buffer[128]; struct tree_node *parent; struct list_node *first_child; // first node of list of children? } tree_node; // pointer tree node // pointer next item in list typedef struct list_node { struct tree_node *tree; struct list_node *next; } list_node; // allocates right amount of space tree node // sets pointers null // initialises string buffer given name // returns pointer tree node struct tree_node *make_tree_node(char *name) { tree_node *instance = malloc(sizeof(*instance)); instance -> parent = null; instance -> first_child = null; strcpy(instance -> string_buffer, name); return instance; } // allocates new list node // assigns fields tree node references , next list node // returns pointer new list node struct list_node *make_list_node(struct tree_node *v, struct list_node *next) { list_node *instance = malloc(sizeof(*instance)); instance -> tree = v; instance -> next = next; return instance; } // checks whether directory named arg exists in current working directory // creates directory if not exist // if arg null, function should print error message void do_mkdir(struct tree_node *cwd, char *arg) { //printf("making directory name %s\n", arg); if (null == arg || !strcmp(arg, "")) { printf("please specify directory name not null: mkdir directoryname\n"); } else { // check if directory exists struct list_node *subdir = cwd->first_child; while (subdir != null) { if (strcmp(subdir -> tree -> string_buffer, arg) == 0) { printf("directory name exists. please provide name.\n"); return; } subdir = subdir -> next; } // adding new directory struct tree_node *newsubdir = make_tree_node(arg); newsubdir -> parent = cwd; struct list_node *newlistnode = make_list_node(newsubdir, null); // putting new node in alphabetical order struct list_node *prev = null; if (subdir == null) { newlistnode -> next = cwd -> first_child; cwd -> first_child = newlistnode; } else { while (subdir != null) { // todo: use case insensitive compare if (strcmp(arg, subdir -> tree -> string_buffer) > 0) { // inserting in front newlistnode -> next = subdir -> next; prev->next = newlistnode; break; } prev = subdir; subdir = subdir -> next; } if (subdir == null) { subdir->next = newlistnode; } } } } // prints children of directory cwd (not recursively) void do_ls(struct tree_node *cwd) { printf("listing directories...\n"); struct list_node *subdir = cwd -> first_child; while (subdir != null) { printf("%s\n", subdir ->tree -> string_buffer); subdir = subdir->next; } } // *checks whether cwd has subdirectory named arg // *if yes, function returns corresponding tree node (and become new working directory) // *if no, prints error message // *handle cd , cd .. struct tree_node *do_cd(struct tree_node *cwd, struct tree_node *root, char *arg) { // initialising subdir cwd's first child struct list_node *subdir = cwd -> first_child; // initialising pardir cwd's parent struct tree_node *pardir = cwd -> parent; if (pardir != null) { // root directory not have parent directory if (strcmp(arg, "..") == 0) { cwd = pardir; printf("returning parent directory.\n"); return cwd; } } // checks if user input argument "." if (strcmp(arg, ".") == 0) { return cwd; } // checks if user input argument null or "" if (null == arg || !strcmp(arg, "")) { printf("returning root directory.\n"); return root; } // checks if cwd has subdirectory named arg while (subdir != null) { if (strcmp(subdir -> tree -> string_buffer, arg) == 0) { printf("subdirectory exists!\n"); cwd = subdir-> tree; printf("making & entering subdirectory: %s\n", arg); return cwd; } subdir = subdir-> next; } printf("directory not exist!\n"); return cwd; } // *prints prompt (e.g. /bar/baz/ >) in every iteration of main loop // *show directories on path in correct order void show_prompt(struct tree_node *cwd) { struct tree_node *originaldir = cwd; if (cwd -> parent != null) { show_prompt(cwd -> parent); printf("/%s", cwd -> string_buffer); } cwd = originaldir; } // *removes node child parent node dir // *take care correctly support corner cases child being 1 // *all memory occupied child should freed void remove_child(struct tree_node *cwd, struct tree_node *child) { child = cwd -> first_child -> tree; cwd = child -> parent; if (cwd -> first_child -> tree == null) { printf("subdirectory not empty!\n"); } else if (cwd -> first_child != null) { free(child); } } void rmv(struct tree_node *cwd) { struct list_node *child = cwd -> first_child; printf("%s\n", child -> tree -> string_buffer); strcpy(child -> tree -> string_buffer, ""); printf("string memory free successful\n"); free(child -> tree -> string_buffer); //free(cwd -> first_child -> tree); //free(cwd -> first_child); } void rmv2(struct tree_node *cwd) { struct tree_node *child = cwd -> first_child -> tree; free(child); } void rmv3(struct tree_node *cwd) { struct tree_node *child = cwd -> first_child -> tree; cwd = child -> parent; if (strcmp(cwd -> first_child -> tree -> string_buffer, "\0") == 0) { printf("subdirectory not empty!\n"); } else if (cwd -> first_child != null) { free(cwd -> first_child -> tree -> string_buffer); free(cwd -> first_child -> tree); free(cwd -> first_child); } } // implement rmdir (follow same pattern previous exercise) // *if specified directory not empty, rmdir should fail error message void do_rmdir(struct tree_node *cwd, char *arg) { struct list_node *subdir = cwd->first_child; // checks if user input argument null or "" if (null == arg || !strcmp(arg, "")) { printf("please specify directory not null: rmdir directoryname.\n"); } while(subdir != null) { if (strcmp(subdir -> tree -> string_buffer, arg) != 0) { printf("directory not exist!\n"); } subdir = subdir -> next; } if (strcmp(subdir -> tree -> string_buffer, arg) == 0) { if (subdir -> tree == null) { free(subdir); } while (subdir != null) { if (strcmp(subdir -> tree -> string_buffer, arg) == 0) { printf("directory name exists. please provide name.\n"); return; } subdir = subdir -> next; } } else { printf("directory not exist!\n"); } } int main(void) { char directoryname[129] = ""; printf("\nprogram start... \n\n"); // getting user input string // , capturing argument user input char inputstring[129]; char delimiter[] = " "; char *line; char *arg; struct tree_node *root = make_tree_node("root\0"); struct tree_node *cwd = root; printf("/ >"); while (fgets(inputstring, 128, stdin)) { inputstring[strlen(inputstring) - 1] = '\0'; // gets argument , stores arg line = malloc(strlen(inputstring) + 1); strcpy(line, inputstring); arg = strsep(&line, delimiter); arg = strsep(&line, delimiter); //printf("your argument: %s\n", arg); if(strncmp(inputstring, "exit", 4) == 0) { printf("breaking...\n"); break; } //////////////////////////////// handling ls //////////////////////////// else if(strncmp(inputstring, "ls ", 3) == 0) { printf("the ls command should used without arguments.\n"); } else if(strncmp(inputstring, "ls", 2) == 0) { do_ls(cwd); show_prompt(cwd); printf("/ >"); } /////////////////////////// handing cd (extra safe) ////////////////////////////// else if(strncmp(inputstring, "cd ..", 5) == 0) { cwd = do_cd(cwd, root, arg); show_prompt(cwd); printf("/ >"); } else if(strncmp(inputstring, "cd .", 4) == 0) { cwd = do_cd(cwd, root, arg); show_prompt(cwd); printf("/ >"); } else if(strncmp(inputstring, "cd ", 3) == 0) { cwd = do_cd(cwd, root, arg); show_prompt(cwd); printf("/ >"); } else if(strncmp(inputstring, "cd", 2) == 0) { cwd = root; printf("returning root directory.\n"); show_prompt(cwd); printf("/ >"); } ////////////////////////////// handling mkdir ///////////////////////////////////// else if(strncmp(inputstring, "mkdir ", 7) == 0) { printf("please specify directory name not null: mkdir directoryname\n"); show_prompt(cwd); printf("/ >"); } else if(strncmp(inputstring, "mkdir ", 6) == 0) { do_mkdir(cwd, arg); show_prompt(cwd); printf("/ >"); } else if(strncmp(inputstring, "mkdir", 5) == 0) { printf("please specify directory name: mkdir directoryname\n"); show_prompt(cwd); printf("/ >"); } ////////////////////////////// handling rmdir ///////////////////////////////////// else if(strncmp(inputstring, "rmdir ", 7) == 0) { printf("please specify directory name not null: rmdir directoryname\n"); show_prompt(cwd); printf("/ >"); } else if(strncmp(inputstring, "rmdir ", 6) == 0) { printf("removing directory name...\n"); do_rmdir(cwd, arg); show_prompt(cwd); printf("/ >"); } else if(strncmp(inputstring, "rmv", 3) == 0) { printf("removing first child test\n"); struct tree_node *child = cwd -> first_child -> tree; rmv(cwd); show_prompt(cwd); printf("/ >"); } else if(strncmp(inputstring, "rmdir", 5) == 0) { printf("please specify directory name: rmdir directoryname\n"); show_prompt(cwd); printf("/ >"); } else { printf("unrecognised input: please try again.\n"); show_prompt(cwd); printf("/ >"); } } printf("\nprogram quit... \n\n"); return 0; }
for compiler issue such diagnostic, must trying define global variable non-constant initializer:
char *string_buffer = malloc(128 * sizeof(char)); this cannot done @ global level.
you must define string_buffer constant initializer such null, or no initializer @ all, have same effect, , allocate memory @ startup, in main() function:
char *string_buffer; int main(void) { string_buffer = malloc(128); ... } note if not need reallocate string_buffer during course of program, not need make pointer dynamic memory, defining global array should suffice:
char string_buffer[128];
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