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Help with C, character pointers and such
TwistedJester
Registered User regular
Registered User regular
For an assignment I'm supposed to implement a crude file system in linux, and to get a better idea of how it's going to work, I'm messing around in C rather than going directly into the syscalls. I'm planning to make a linked list, where each node has the file's name, a pointer to the data (a string, basically), and a pointer to the next node (obviously). So my struct is basically
I decided to make the struct contain a char pointer because I don't want to just make it a huge character array as that would waste space if it was just a single word. However, I'm having a hell of a time getting the char pointer to actually work. Every time I go to print out the linked list I get a segmentation violation. Here's how I make the "files", as it is, it just takes a string (filename here) and sets both the filename and data to that string, for simplicity:
It's possible I'm just using pointers totally wrong, I haven't really used them in a long time. So... anyone have any idea what I'm doing wrong here?
char *data; char file[30]; struct node *next;
I decided to make the struct contain a char pointer because I don't want to just make it a huge character array as that would waste space if it was just a single word. However, I'm having a hell of a time getting the char pointer to actually work. Every time I go to print out the linked list I get a segmentation violation. Here's how I make the "files", as it is, it just takes a string (filename here) and sets both the filename and data to that string, for simplicity:
struct node *new_node;
new_node = malloc(sizeof( struct node));
char *ptr = malloc(strlen(filename));
strcpy(ptr, filename);
(*new_node).data = ptr;
strcpy((*new_node).name, filename);
printf("data = %s\n", (*new_node).data);
(*last).next = new_node;
last = new_node;
The data is set after assigning it here, but when traversing the nodes I get a segmentation violation. At first I thought it was a memory lifetime issue, but if it were, getting the filename itself would cause a seg violation, and it didn't do that until I added the code to print out the data. For further reference, here is my code to print the nodes
printf("Node #%d has name: %s and data contents: %s\n", i, (*curr_node).name,
(*curr_node).data);
It's possible I'm just using pointers totally wrong, I haven't really used them in a long time. So... anyone have any idea what I'm doing wrong here?
TwistedJester on
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Also, it looks like your second code listing is part of a function. It would be helpful if you posted the entire function so we can see what the parameters are to the function, etc.
An unrelated aside: would usually be written . There's no difference, but the second form is easier to read.
#include <stdio.h> #include <stdlib.h> struct node { char *data; char name[30]; struct node *next; }; int main() { struct node *first, *last; first = malloc(sizeof(struct node)); last = first; strcpy(first->name, "First node"); first->next = NULL; char vartype[30]; while(1) { printf("Enter the name. If you wish to quit, enter 0\n"); scanf("%s", vartype); if (!strncmp(vartype, "0", 1)) { printf("exiting program and printing linked list\n"); break; } else { struct node *new_node; new_node = malloc(sizeof( struct node)); char *ptr = malloc(strlen(vartype)); strcpy(ptr, vartype); new_node->data = ptr; strcpy(new_node->name, vartype); printf("data = %s\n", new_node->data); last->next = new_node; new_node->next = NULL; last = new_node; } } struct node *curr_node; curr_node = first; int i = 1; printf("Before printing nodes\n"); while(curr_node != NULL) { printf("Node #%d has name: %s and data contents: %s\n", i, curr_node->name, curr_node->data); i++; curr_node = curr_node->next; } return 0; }Out of curiosity though, do you have any suggestions regarding how to cut down on that?
If you need to see just how much memory you're leaking, there's a tool called valgrind that's standard on most Linux systems. Just do "valgrind --tool=memcheck --show_unreachable=yes your_program" (I believe that's right, at any rate) to see how bad it is. Additionally, you also declare variables all willy-nilly, which is generally discouraged in C. To catch that, we always had our students (I used to be a C TA) compile their programs with the following GCC command: "gcc -Wall -Werror -ansi -pedantic". This will turn on all warnings, make all warnings errors, compile in C89 mode, and be really pedantic about C89 syntax (respectively).
I don't see any printk's in your code above, so obviously it's changed a little bit. Try posting it again (in a spoiler if you want to save space) so we can see where you're at now.