鏈表值被改變

Question

相關的代碼：

常量缺省值：

#define MAX_NAME_LEN 15 /* Maximum length of any attribute or relation is 15. */ 

STRUCT缺省值：

typedef struct schnode { 
char name[MAX_NAME_LEN + 1]; /* The name of the attribute. */ 
char type; /* The type of the attribute ('S' or 'I'). */ 
int len; /* The length of the attribute. */ 
struct schnode *next; /* A pointer to the next node. */ 
} SCHNODE; 

變量缺省值：

FILE *schemafile; /* A text file. */ 
SCHNODE *head = NULL; /* Head of the linked list. */ 
SCHNODE *ptrnode = NULL; /* A variable node in the linked list. */ 
SCHNODE *p = NULL; /* A variable node in the linked list. */ 
SCHNODE *tail = NULL; /* Tail of the linked list. */ 
char *attr_name; /* Holds the name of the current attribute. */ 
char *attr_type; /* Holds the current attribute type (S or I). */ 
int attr_len; /* Holds the current attribute length. */ 
int attr_loc = 0; /* Attribute location in respect to the tuple. */ 
int i; /* Index for loops. */ 
char attr_match = 0; /* 1 if the attribute is valid, 0 otherwise. */ 

常量缺省值：

#define BUFFER_START_SIZE 20 
#define BUFFER_INC_SIZE 10 

NextLine功能：

char* NextLine (FILE *f) { 
int size = BUFFER_START_SIZE; 
char *buf; //buffer storing current line as a char array 
int i = 0; //index for array 
int c; //a character 

if (f == NULL){ //check if the file pointer is NULL 
    printf("Unable to find the file.\n"); fflush(stdout); 
    return NULL; //nothing to read, so return NULL 
} 

if((c = getc(f)) == EOF) //check if the first 'char' in the line is EOF 
    return NULL; //if so, line is empty, so return NULL 

if((buf = (char *) malloc(size)) == NULL){ //make room for the buffer 
    printf("Failed to create buffer.\n"); fflush(stdout); } 

buf[0] = (char) c; //put the first char into the buffer string 
i = 1; //and increment the index 

while(1){ 
    while(i < size){ //loop through the buffer char array 
     if((c = getc(f)) == '\n'){ //if end of line 
      buf[i] = '\0'; //instead put '\0' to terminate string 
      return buf; //and return the completed string 
     } 
     buf[i] = (char) c; //otherwise, just keep copying the characters 
     i++; //increment the index 
    } 

    //IF REACHED THIS POINT, LINE IS TOO LONG TO FIT IN BUFFER -> REALLOC 
    size += BUFFER_INC_SIZE; 
    if((buf = (char*) realloc(buf,size)) == NULL){ //make buffer bigger 
     printf("Unable to realloc memory.\n"); fflush(stdout); } 
    //loop and continue adding chars from where you left off 
} 
} 

執行的代碼：

for (i = 0; i < num_attr; i++) { 
    if ((i > 0) && !attr_match) attr_loc += attr_len; 
    attr_name = NextLine(schemafile); 
    attr_name = strtok(attr_name, " \t\n"); 
    attr_type = strtok(NULL, " \t\n"); 
    attr_len = atoi(strtok(NULL, " \t\n")); 

    /* Construct linked list of schema file data. */ 
    /* If empty: */ 
    if (head == NULL) { 
     if ((head = (SCHNODE*) malloc(sizeof(SCHNODE))) == NULL) { 
      fprintf(stderr, "Unable to Malloc space.\n"); 
      return; 
     } 
     strcpy(head->name, attr_name); 
     head->type = attr_type[0]; 
     head->len = attr_len; 
     head->next = NULL; 
    } 

    /* If the list already has a head defined: */ 
    else { 
     /* Start from the head if tail is NULL (only 1 element) otherwise start from tail. */ 
     if(tail == NULL) ptrnode = head; 
     else ptrnode = tail; 
     /* Malloc space for the tail node. */ 
     if ((tail = (SCHNODE*) malloc(sizeof(SCHNODE))) == NULL) { 
      fprintf(stderr, "Unable to Malloc space.\n"); 
      return; 
     } 
     strcpy(tail->name, attr_name); 
     tail->type = attr_type[0]; 
     tail->len = attr_len; 
     tail->next = NULL; 
     /* Insert tail node at the end. */ 
     ptrnode->next = tail; 
     if (strcmp(tail->name, "Instr") == 0) 
      p = tail; 
     if (p != NULL) printf("%d: %s\n", i, p->name); 
    } 

輸入：

CName S 25 
CId S 8 
Instr S 10 
Credits I 4 

輸出：

2: Instr 
3: Insur 

沒有其他值改變（據我所知）。有人能解釋爲什麼這個特殊的價值總是被改變嗎？ （Instr - > Insur）。我只想讓我讀的條目（Instr）在整個閱讀過程中保持不變。

Answer 1

在您的程序中，您的數據結構的name字段被定義爲pointer。因此，當您strcpy時，將會有內存損壞，因爲沒有空間分配給pointer。因此，對於每個節點，或者malloc爲name的空間或者將name定義爲n元素的數組。