[英]Segfault in a char ** pointer (learning malloc and pointers usage)
我目前正在學習指針和malloc
function 的工作原理。 我正在嘗試在 C 中編寫一個 function ,它將一個字符串作為參數。 基本上,它將這個字符串的每個單詞存儲在一個char **ptr
中,並且在找到一個空格/制表/'\n' char 時會區分單詞。 例如,字符串“hello world”將存儲為ptr[0] = "hello
, ptr[1] = world
。
到目前為止,這是我寫的:
#include "libft.h"
#include <stdlib.h>
char **ft_split_whitespaces(char *str)
{
int i;
int j;
int k;
char **tab;
i = 0;
j = 0;
k = 0;
tab = (char **)malloc(sizeof(char) * 8);
*tab = (char *)malloc(sizeof(char) * 8);
while(str[i])
{
k = 0;
while(str[i] == 9 || str[i] == 32 || str[i] == 10 || str[i] == '\0') // if a whitespace or a tabulation or a '\n' char is found, we go further in the string and do nothing
{
i++;
}
while(str[i] != 9 && str[i] != 32 && str[i] != 10 && str[i] != '\0') // if this isn't the case, we put the current char str[i] in the new array
{
tab[j][k] = str[i];
k++;
i++;
}
if(str[i] == 9 || str[i] == 32 || str[i] == 10 || str[i] == '\0') // now if a new whitespace is found we're going to store the next word in tab[j+1]
{
j++;
}
i++;
}
return (tab);
}
int main(void)
{
char str[] = " hi im a new bie learning malloc\nusage";
ft_split_whitespaces(str);
}
請注意,我已經嘗試 malloc 幾個值,但它似乎沒有改變任何東西:當嘗試將單詞“im”(第二個單詞)的字符“i”復制到我的數組中時,它會出現段錯誤。
你們能指導我並告訴我我錯過了什么嗎?
非常感謝提前!
這一行:
tab = (char **)malloc(sizeof(char) * 8);
只分配 8 個字節來存儲所有指針。 假設 8 是指針的最大數量,它應該是:
tab = (char **)malloc(sizeof(char *) * 8);
這一行:
*tab = (char *)malloc(sizeof(char) * 8);
為第一個字分配空間,最多可包含 7 個字符,外加一個 null 終止符。
但是,這只會為tab[0]
指向的第一個單詞分配 memory 。 代碼沒有為剩余的字分配任何 memory。
ft_split_whitespaces
應該如何表示它從輸入中拆分出來的單詞數? 一種方法是在指向列表中最后一個字的指針之后添加 null 指針。
這是一個工作演示。 它使用strspn
和strcspn
函數來處理輸入字符串的空白和非空白部分。 它掃描輸入字符串兩次——一次確定字數,一次分配和復制字。 它為空終止的指針列表分配一個 memory 塊,並為每個字分配一個 memory 塊。
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <stdio.h>
/* Free NULL-terminated list of strings. */
void ft_free_strlist(char **strings)
{
if (strings)
{
char **ss = strings;
while (*ss)
{
free(*ss++);
}
free(strings);
}
}
char **ft_split_whitespaces(const char *input)
{
static const char white[] = "\t\n\v\f\r ";
size_t nwords;
size_t i;
size_t wordlen;
char **words;
const char *s;
/* Determine number of words in input string. */
nwords = 0;
s = input;
s += strspn(s, white); /* Skip initial whitespace. */
while (*s)
{
/* Found non-whitespace at beginning of word. */
nwords++;
s += strcspn(s, white); /* Skip non-whitespace. */
s += strspn(s, white); /* Skip whitespace. */
}
/* Allocate memory for NULL-terminated list of words. */
words = malloc((nwords + 1) * sizeof(words[0]));
if (words != NULL)
{
/* Rescan input, allocate and copy words. */
s = input;
for (i = 0; i < nwords; i++)
{
s += strspn(s, white); /* Skip whitespace. */
wordlen = strcspn(s, white); /* Determine word length. */
/* Allocate memory for null-terminated word. */
words[i] = malloc((wordlen + 1) * sizeof(*s));
if (words[i] == NULL)
{
/* Error will be dealt with later. */
break;
}
/* Copy word (source is not null-terminated). */
memcpy(words[i], s, wordlen * sizeof(*s));
words[i][wordlen] = 0; /* Append null terminator to word. */
s += wordlen; /* Skip past the copied word. */
}
/* NULL-terminate the list of words. */
words[i] = NULL;
if (i < nwords)
{
/* Could not allocate enough memory. Free what we got. */
ft_free_strlist(words);
words = NULL; /* Will return NULL later. */
}
}
if (words == NULL)
{
/* There was a memory allocation error. */
errno = ENOMEM;
}
/* Return the NULL-terminated list of words, or NULL on error. */
return words;
}
/* Print NULL-terminated list of strings. */
void ft_print_strlist_const(const char *const *strings)
{
if (strings)
{
while (*strings)
{
puts(*strings);
strings++;
}
}
}
void ft_print_strlist(char *const *strings)
{
ft_print_strlist_const((const char *const *)strings);
}
int main(void)
{
static const char str[] = " hi im a new bie learning malloc\nusage";
char **words;
words = ft_split_whitespaces(str);
if (words == NULL)
{
perror("Split whitespace");
return EXIT_FAILURE;
}
ft_print_strlist(words);
ft_free_strlist(words);
return EXIT_SUCCESS;
}
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