释放c循环中的子字符串 [英] freeing substring in c - loop
问题描述
我正在尝试获取结构'structs
'的每个成员的子字符串,然后将该子字符串分配给temp_struct
的新成员.
我遇到的问题是如何在每次迭代中释放子字符串,出于某种原因代码会运行,但是valgrind
抛出了Invalid read of size 1
,我以为我正在读取内存块.>
如何释放子字符串?
谢谢
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct st_ex {
char product[16];
float price;
};
struct st_temp {
char *prod;
};
char *temp = NULL;
// from stackoverflow
char* substr( const char* source, size_t start, size_t end )
{
char* dest = malloc( end - start + 1) ;
memcpy( dest, &source[start], end - start ) ;
dest[end - start] = 0 ;
return dest ;
}
int main()
{
struct st_ex structs[] = {{"mp3 player", 2.0f}, {"plasma tv", 20.0f},
{"notebook", 10.0f}, {"smartphone", 49.9f},
{"dvd player", 10.0f}, {"matches", 0.2f }};
struct st_temp **temp_struct;
size_t j, i;
temp_struct = malloc(sizeof *temp_struct * 6);
for (j = 0; j < 6; j++)
temp_struct[j] = malloc(sizeof *temp_struct[j]);
size_t structs_len = sizeof(structs) / sizeof(struct st_ex);
for(i=0; i<structs_len; i++){
temp = substr(structs[i].product, 0, 4);
temp_struct[i]->prod = temp;
free(temp);
temp = NULL;
}
for(i=0; i<6; i++ )
printf("%s\n",temp_struct[i]->prod);
for(i=0; i<6; i++ )
free(temp_struct[i]);
free(temp_struct);
return 0;
}
1)您正在释放子字符串
temp = substr(structs[i].product, 0, 4);
temp_struct[i]->prod = temp;
free(temp);
上面第三行释放了您在substr
中分配的内存.
2)因为您要释放此处的内存,所以引入了一个错误.
释放它之后访问malloc的内存是无效的,因此尝试打印temp_struct[i]->prod
是无效的.
解决方案?
不要free(temp)
,而是在释放temp_struct[i]
的循环中,您首先需要释放temp_struct[i]->prod
,像这样
for(i=0; i<6; i++ )
{
free(temp_struct[i]->prod);
free(temp_struct[i]);
}
I'm trying to get a sub-string for each member of the struct 'structs
' and then assign that sub-string to a new member of the temp_struct
.
The problem I'm having is how to free the sub-string on each iteration, for some reason the code runs, however valgrind
throws an Invalid read of size 1
, which I assume I'm reading off the block of memory.
How could I free the sub-string?
Thanks
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct st_ex {
char product[16];
float price;
};
struct st_temp {
char *prod;
};
char *temp = NULL;
// from stackoverflow
char* substr( const char* source, size_t start, size_t end )
{
char* dest = malloc( end - start + 1) ;
memcpy( dest, &source[start], end - start ) ;
dest[end - start] = 0 ;
return dest ;
}
int main()
{
struct st_ex structs[] = {{"mp3 player", 2.0f}, {"plasma tv", 20.0f},
{"notebook", 10.0f}, {"smartphone", 49.9f},
{"dvd player", 10.0f}, {"matches", 0.2f }};
struct st_temp **temp_struct;
size_t j, i;
temp_struct = malloc(sizeof *temp_struct * 6);
for (j = 0; j < 6; j++)
temp_struct[j] = malloc(sizeof *temp_struct[j]);
size_t structs_len = sizeof(structs) / sizeof(struct st_ex);
for(i=0; i<structs_len; i++){
temp = substr(structs[i].product, 0, 4);
temp_struct[i]->prod = temp;
free(temp);
temp = NULL;
}
for(i=0; i<6; i++ )
printf("%s\n",temp_struct[i]->prod);
for(i=0; i<6; i++ )
free(temp_struct[i]);
free(temp_struct);
return 0;
}
1) You are freeing the substring
temp = substr(structs[i].product, 0, 4);
temp_struct[i]->prod = temp;
free(temp);
The third line above frees the memory you malloc'd in substr
.
2) because you're freeing the memory here, you've introduced a bug.
It's invalid to access the malloc'd memory after you free it, therefore it's invalid to try to print temp_struct[i]->prod
.
The solution?
Don't free(temp)
, instead in your loop to free temp_struct[i]
, you first need to free temp_struct[i]->prod
, like this
for(i=0; i<6; i++ )
{
free(temp_struct[i]->prod);
free(temp_struct[i]);
}
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