[英]Why is this C program segfaulting at this location?
I'm working on making a C program that basically can take a sentence and count how many times each word appears in it. 我正在开发一个C语言程序,该程序基本上可以使用一个句子并计算每个单词出现在其中的次数。 I've made a stripped down version that reproduces the issue exactly.
我制作了一个精简版,可以准确地再现问题。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct testStrut{
char * string;
int uses;
}word;
void grow();
int main(){
int i;
int count = 1;
word ** words;
words = (word **) malloc(count * sizeof(word *));
words[0] = (word *) malloc(sizeof(word));
for(i = 0; i < 10; i++){
printf("Re-looping i: %d \n", i);
printf("words[0]->string = %s \n", words[0]->string);
grow("TEST", words, &count);
}
printf("Done.");
return 0;
}
void grow(char * str, word ** words, int * count){
word ** tmp;
tmp = realloc(words, (*count) * sizeof(word *));
if(tmp){
tmp[(*count)-1] = malloc(sizeof(word));
tmp[(*count)-1]->string = malloc(strlen(str)+1); /*+1 for null terminator as pointed out */
strcpy(tmp[(*count)-1]->string, str);
tmp[(*count)-1]->uses = 1;
words = tmp;
(*count)++;
} else{
printf("Failure to allocate. \n");
exit(0);
}
printf("Count: %d and word[0] %s \n", (*count), str);
}
As well as the output from a run: 以及运行的输出:
Re-looping i: 0
words[0]->string = (null)
Count: 2 and word[0] TEST
Re-looping i: 1
words[0]->string = TEST
Count: 3 and word[0] TEST
Re-looping i: 2
words[0]->string = TEST
Count: 4 and word[0] TEST
Re-looping i: 3
words[0]->string = TEST
Count: 5 and word[0] TEST /*Prints it fine? */
Re-looping i: 4
Segmentation fault (core dumped) /*Suddenly unable to print it? */
I'm not understanding why between ending the grow function and re-going through the loop the value of words[0]->str is suddenly lost. 我不明白为什么在结束增长功能和重新执行循环之间,单词[0]-> str的值突然丢失。 Is there something I'm missing?
有什么我想念的吗?
[I do know that I should be freeing anything I malloc.I also realize my method prototype isn't the correct one but I just wanted to make a quick program that demonstrated my issue] [我确实知道我应该释放我分配的任何东西。我也意识到我的方法原型不是正确的方法,但我只是想制作一个演示我问题的快速程序]
On the first iteration of the for loop the following line is accessing uninitialized memory. 在for循环的第一次迭代中,下一行正在访问未初始化的内存。
printf("words[0]->string = %s \n", words[0]->string);
You have also declared 您还声明了
void grow();
but the actual signature ie 但实际签名即
void grow(char * str, word ** words, int * count)
You first need to call grow
before that line ie. 您首先需要在该行之前调用
grow
。 You are also realloc
and assuming that the pointer words
in main points to the original pointer. 您还要重新
realloc
并假设main中的指针words
指向原始指针。
Try this. 尝试这个。 I've simplified a little bit...
我简化了一点...
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct testStrut{
char * string;
int uses;
} word;
void grow(const char *str, word *words, int count);
int main(){
int i;
word * words;
printf("sizeof word == %zu\n", sizeof(word));
assert(sizeof(word) == 16);
words = malloc(sizeof(word));
for(i = 0; i < 10; i++){
printf("Re-looping i: %d \n", i);
grow("TEST", words, i);
printf("words[0]->string = %s \n", words[0].string);
}
printf("Done.");
return 0;
}
void grow(const char * str, word *words, int count){
word ** tmp;
int idx = count - 1;
printf("size == %zu\n", count * sizeof(word));
tmp = realloc(words, count * sizeof(word));
size_t str_len = strlen(str);
if(tmp != NULL) {
tmp[idx] = malloc(sizeof(word*));
tmp[idx]->string = malloc(str_len + 1);
strcpy(tmp[idx]->string, str);
tmp[idx]->string[4] = '\0';
tmp[idx]->uses = 1;
} else{
printf("Failure to allocate. \n");
exit(0);
}
printf("Count: %d and word[0] %s \n", count, str);
}
The problem is here : 问题在这里:
words = tmp;
This statement has no effect outside of the function. 该语句在函数外部无效。
Since realloc may (or may not) return a new pointer to another memory location, you code may work a few times before crashing. 由于realloc可能会(也可能不会)返回指向另一个内存位置的新指针,因此您的代码在崩溃前可能会工作几次。
You should use a word*** instead of the word** parameter or simply return the new pointer: 您应该使用单词***而不是单词**参数,或者仅返回新指针:
word** grow(char * str, word ** words, int * count){
word ** tmp;
tmp = realloc(words, (*count) * sizeof(word *));
if(tmp){
tmp[(*count)-1] = malloc(sizeof(word));
tmp[(*count)-1]->string = malloc(strlen(str)+1); /*+1 for null terminator as pointed out */
strcpy(tmp[(*count)-1]->string, str);
tmp[(*count)-1]->uses = 1;
(*count)++;
} else{
printf("Failure to allocate. \n");
exit(0);
}
printf("Count: %d and word[0] %s \n", (*count), str);
return tmp;
}
And call it this way: 并这样称呼:
words = grow("TEST", words, &count);
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