[英]Malloc'ing pointer-to-pointer member of struct
I'm trying to use a structure to hold a pointer to a data block that I change sometimes when a file is updated, the idea being to free the old data block, malloc a new one of the right size, and assign the pointer to pointer in the structure to the pointer returned by malloc, and this is how I thought I should do it. 我正在尝试使用一种结构来保存指向数据块的指针,该文件有时会在文件更新时更改,该想法是释放旧数据块,malloc正确大小的新块,并将指针分配给结构体中的指针指向malloc返回的指针,这就是我认为应该这样做的方式。 But it seg faults.
但这是错误。 In fact in the larger program that I pared down to make this test program, its not seg faulting but writing to stdout does nothing after the malloc (anywhere in the program after that).
实际上,在我缩减为该测试程序的较大程序中,它不是段错误,而是在malloc之后(向程序中的任何地方)写入stdout不会执行任何操作。 I guess I am writing over the stdout FD, the cause being that I am using the pointers incorrectly when I set the pointer to the
malloc()
ed return value. 我想我正在写stdout FD,原因是当我将指针设置为
malloc()
ed返回值时,我使用的指针不正确。
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>
#include <string.h>
#include <stdint.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <malloc.h>
#include <inttypes.h>
struct mystruct {
int offset;
int ** d;
};
int filecheck (struct mystruct *g, int * count) {
int v, size = 0;
FILE *f = fopen("/home/pi/schedule/default", "rb");
if (f == NULL) {
return 0;
}
fseek(f, 0, SEEK_END);
size = ftell(f);
fseek(f, 0, SEEK_SET);
int schedsize = sizeof(int);
int elementcount = size / schedsize;
// free(*(g->d));
// seg fault next line
if ((*(g->d) = malloc(size))==NULL) return 0;
if (elementcount != fread(*(g->d), schedsize, elementcount, f)) {
free(*(g->d));
return 0;
}
fclose(f);
*count = elementcount;
return 1;
}
void setp (struct mystruct *g) {
// if uncommented, seg fault here
// *(g->d) = NULL;
}
int main (){
struct mystruct g;
setp(&g);
int i, count = 0;
if (filecheck(&g, &count)==0) {
printf("Returned 0\n");
return 0;
}
while (1) {
printf("%d\n", (*(g.d))[i]);
sleep(1);
}
return 0;
}
For convenience I want to set mystruct.d
to NULL
initially, in setp()
, but even if thats comented out the code still fails, so I know its completely wrong. 为了方便起见,我想在
mystruct.d
setp()
中将mystruct.d
初始设置为NULL
,但是即使那没错,代码仍然会失败,所以我知道它是完全错误的。 Maybe I don't need to be using a pointer to pointer but it seems to me I do. 也许我不需要使用指向指针的指针,但是在我看来我确实需要。
EDIT: Modified as per answer, is this OK? 编辑:根据答案修改,可以吗?
struct mystruct {
int offset;
int * d;
};
int filecheck (struct mystruct *g, int * count) {
int v, size = 0;
FILE *f = fopen("/home/pi/schedule/default", "rb");
if (f == NULL) {
return 0;
}
fseek(f, 0, SEEK_END);
size = ftell(f);
fseek(f, 0, SEEK_SET);
int schedsize = sizeof(int);
int elementcount = size / schedsize;
free (g->d);
if ((g->d = malloc(size))==NULL) return 0;
if (elementcount != fread(g->d, schedsize, elementcount, f)) {
free(g->d);
return 0;
}
fclose(f);
*count = elementcount;
return 1;
}
void setp (struct mystruct *g) {
g->d = NULL;
}
int main (){
struct mystruct g;
setp(&g);
int i, count = 0;
if (filecheck(&g, &count)==0) {
printf("Returned 0\n");
return 0;
}
while (1) {
for (i=0;i<count;i++) {
printf("%d %d \n", *((g.d)+i), g.d[i]);
}
sleep(1);
}
return 0;
}
This seems to work but is it correct or am I writing over some part of memory I am not supposed to again with this? 这似乎可行,但这是正确的,还是我应该重写内存的某个部分,我不应该再对此进行处理?
You need to allocate memory to all the pointer elements before using them. 您需要在使用所有指针元素之前为其分配内存。
Here, d
being a pointer to pointer, first you need to allocate memory for d
itself, then you should go on for dereferencing d
(using *d
). 在这里,
d
是指向指针的指针,首先需要为d
本身分配内存,然后应该继续对d
进行解引用(使用*d
)。
For example, either 例如,
void setp (struct mystruct *g) {
g->d = NULL; // no need to derererence d here, but later need to allocate
}
or, ( for better ) 或者,( 为了更好 )
void setp (struct mystruct *g) {
g->d = malloc(32 * sizeof (int *)); // d is allocated
g->d[i] = malloc(16 * sizeof (int)); // or g->d[i] = NULL; or *(g->d) = NULL;
}
should work fine. 应该工作正常。
Also, the recommended siganture of main()
is int main(void)
. 另外,
main()
的推荐符号是int main(void)
。
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