[英]segmentation fault in a linked list while loop?
我正在嘗試在C中設置一個圖形。我使用用戶輸入嘗試了該圖形,並且它運行完美。 但是,我試圖實現從文件讀取。 最后的else語句是錯誤的出處,因為當我注釋掉它時,它可以毫無問題地進行編譯。 我已經對我認為存在問題的區塊發表了評論。 請讓我知道這個問題是否還有其他需要。
#include <stdio.h>
#include <stdlib.h>
struct node{
int data;
struct node* next;
};
//int counter and mainVertex would be used to determine if graph is connected.
// void graphConnection(){
//
//
//
//
//
//
// }
char* deblank(char* input)
{
int i,j;
char *output=input;
for (i = 0, j = 0; i<strlen(input); i++,j++)
{
if (input[i]!=' ')
output[j]=input[i];
else
j--;
}
output[j]=0;
return output;
}
struct node *G[1000];
int counter = 0;
char *mainVertex;
void readingEachLine(){
FILE * fp;
char * line = NULL;
size_t len = 0;
ssize_t read;
//Read file and exit if fail
fp = fopen("test.txt", "r");
if (fp == NULL)
exit(EXIT_FAILURE);
while ((read = getline(&line, &len, fp)) != -1) {
line = deblank(line);
int i = 0;
struct node* cursor = malloc(sizeof(struct node));
struct node* secondcursor = malloc(sizeof(struct node));
struct node* tempitem;
while(line[i] != '\n'){
//If its the first of the line look into the array and set struct cursor to the corresponding
//array position
if (i == 0){
mainVertex[counter] = line[0];
int convertor = line[i] - '0';
cursor = G[convertor];
counter++;
}
//if its not the first, then set a struct with that number as data
else{
tempitem = malloc(sizeof(struct node));
int convertor = line[i] - '0';
tempitem->data = convertor;
tempitem->next = NULL;
}
//if there is no element connected to the struct in array, connect the tempitem
if (cursor->next == NULL){
cursor->next = tempitem;
}
//If there are already connected elements, loop until the end of the linked list
//and append the tempitem
//ERROR: I GET SEGMENTATION FAULT FROM HERE. TRIED AFTER COMMENTING IT OUT
else{
secondcursor = cursor;
while(secondcursor->next != NULL){
secondcursor = secondcursor->next;
}
secondcursor->next = tempitem;
}
i++;
}
printf("\n");
}
}
int main(void){
for (int i = 1; i < 1000; i++)
{
G[i]= malloc(sizeof(struct node));
G[i]->data = i;
G[i]->next = NULL;
}
readingEachLine();
}
編輯:這是文本文件的外觀:
1 3 4
2 4
3 1 4
4 2 1 3
您的代碼有幾個misconceoptions:
G
開始時不要為所有1,000個節點分配內存。 最初,所有列表都是空的,而空鏈接列表是沒有節點且頭為NULL
。 在您的示例中, cursor
用於迭代已經存在的指針,因此請不要為其分配內存。 如果您有這樣的代碼:
struct node *p = malloc(...); // next use of p: p = other_node;
你不應該分配。 您將覆蓋p
並失去分配內存的句柄。 並非所有指針都必須使用malloc
初始化; 僅在創建節點時分配。
strtol
。 mainVertex
應該是什么。 分配給它時,只能使用一次。 您將其視為一個數組,但是它是一個初始化為NULL
的全局指針。 取消引用它時,您會得到不確定的行為,這可能是您的細分錯誤的來源。 這是一個執行您想要做的程序。 (為簡單起見,它總是在頭插入節點,並且應該進行更多的分配檢查。)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
enum {
maxNodes = 1000
};
struct node{
int data;
struct node* next;
};
struct node *G[maxNodes];
size_t nnode = 0;
int read_graph(const char *fn)
{
FILE * fp;
char * line = NULL;
size_t len = 0;
fp = fopen(fn, "r");
if (fp == NULL) return -1;
while (getline(&line, &len, fp) != -1) {
char *p;
char *end;
int id;
int n;
id = strtol(line, &end, 10);
if (end == line) continue;
if (id < 1 || id > maxNodes) break;
if (id > nnode) nnode = id;
id--;
p = end;
n = strtol(p, &end, 10);
while (p != end) {
struct node *nnew = malloc(sizeof(*nnew));
nnew->data = n - 1;
nnew->next = G[id];
G[id] = nnew;
p = end;
n = strtol(p, &end, 10);
}
}
fclose(fp);
free(line);
return 0;
}
int main(void)
{
if (read_graph("test.txt") < 0) {
fprintf(stderr, "Couldn't gread raph.\n");
exit(1);
}
for (int i = 0; i < nnode; i++) {
struct node *p = G[i];
if (p) {
printf("%d:", i + 1);
for (; p; p = p->next) {
printf(" %d", p->data + 1);
}
puts("");
}
}
for (int i = 0; i < nnode; i++) {
struct node *p = G[i];
while (p) {
struct node *old = p;
p = p->next;
free(old);
}
}
return 0;
}
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