Abrufen und Entfernen von verknüpften Listenergebnissen Segmentierungsfehler

Question

Das Abrufen einer Nachricht aus einer verknüpften Liste und das Entfernen dieser Nachricht aus der Liste führt zu einem Segmentierungsfehler.

erste Nachricht und Server Message-Struktur:

// used to store messages. 
struct server_message { 
    char message[80]; 
    char id[80]; 
    struct server_message *next_msg; 
}; 

//head of the list 
static struct server_message *first_message = NULL; 

Get Nachrichtenfunktion

char *get_message(char *id) { 
    char *message; 
    char *not_found =(char *) malloc(sizeof(char) * 20); 
    not_found = "No messages available"; 
    struct server_message *curr_msg = first_message; 
    struct server_message *prev_msg = first_message; 

    if (curr_msg != NULL && (strcmp(curr_msg->id, id) != 0)) { 
     strcpy (message, curr_msg->message); 
     //Remove message 
     first_message = NULL; 
     return message; 
    } 


    while (curr_msg->next_msg != NULL) { 
     curr_msg = curr_msg->next_msg; 

     if (strcmp(curr_msg->id, id) != 0) { 
      strcpy (message, curr_msg->message); 
      //Remove message 
      prev_msg->next_msg = curr_msg->next_msg; 
      return message; 
     } else { 
      prev_msg = curr_msg; 
     } 
    } 
    return not_found; 
} 

Aktualisiert-Code Noch seg Verwerfungen

char *get_message(char *id) { 
    char *message = (char *) malloc(sizeof(char) * 80); 
    char *not_found=(char *) malloc(sizeof(char) * 20); 
    strcpy(not_found, "No messages available"); 
    struct server_message *curr_msg = first_message; 
    struct server_message *prev_msg = NULL; 

    while (curr_msg->next_msg != NULL) { 

     if (strcmp(curr_msg->id, id) != 0) { 
      strcpy (message, curr_msg->message); 
      //Remove message 
      if (prev_msg == NULL) { 
       first_message = curr_msg->next_msg; 
      } else { 
       prev_msg->next_msg = curr_msg->next_msg; 
      } 
      return message; 
     } else { 
      prev_msg = curr_msg; 
      curr_msg = curr_msg->next_msg; 
     } 
    } 
    return not_found; 
} 

Answer 1

Überarbeitete komplettes Beispiel:

#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 

// used to store messages. 
struct server_message { 
    char message[80]; 
    char id[80]; 
    struct server_message *next_msg; 
}; 

//head of the list 
static struct server_message *first_message = NULL; 

char *get_message(char *id) { 
    struct server_message *curr_msg = first_message; 
    struct server_message *prev_msg = NULL; 

    while (curr_msg != NULL) {    /* FIX1: iterate until we do not have an item */ 
     if (strcmp(curr_msg->id, id) == 0) { /* FIX2: 0 is equal */ 
      //Remove message 
      if (prev_msg == NULL) { 
       first_message = curr_msg->next_msg; 
      } else { 
       prev_msg->next_msg = curr_msg->next_msg; 
      } 
      return strdup(curr_msg->message); 
     } else { 
      prev_msg = curr_msg; 
      curr_msg = curr_msg->next_msg; 
     } 
    } 
    return strdup ("No messages available"); 
} 

struct server_message d = {"foo-4", "4", 0}; 
struct server_message c = {"foo-3", "3", &d}; 
struct server_message b = {"foo-2", "2", &c}; 
struct server_message a = {"foo-1", "1", &b}; 

int main(int argc, char *argv[]) 
{ 
     char *t; 

     first_message = &a; 

     t = get_message("1"); 
     printf ("1: %s\n", t); 
     free (t); 

     t = get_message("3"); 
     printf ("3: %s\n", t); 
     free (t); 

     t = get_message("10"); 
     printf ("10: %s\n", t); 
     free (t); 

     return 0; 
}