[英]C Malloc of array within a struct
我正在嘗試使用main值分配一個結構。 我一直在尋找實現的方法,但是找不到答案。 我有3種硬幣我想投入RET其價格的。 我如何申報RET從結構?
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
typedef struct
{
double *ret;
}coin;
void ini(int a)
{
ret = (double*)malloc(a*sizeof(double));
}
int main(void){
long int a=250;
int n_coins=3;
coin *m = (coin*)malloc(n_coins*sizeof(coin));
ini(a);
m[0].ret[0] = 2000;
printf("%lf", m[0].ret[0]);
return 0;
}
首先, return
是C中的保留keyword
,您不能將保留關鍵字用作變量名。
其次,如果要在其他函數中為任何數據類型的數組分配內存,則在該函數中聲明一個變量,調用malloc
,通過malloc分配所需的空間並返回已分配空間的第一個元素的地址。返回被調用函數(這里的main()
)不知道分配的空間的地址,並且它無法訪問分配的內存空間。您可以這樣執行:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
typedef struct
{
double *var;
}moeda;
double *ini(int n)
{
double *arr;
arr = malloc(n*sizeof(*arr));
return arr;
}
int main(void){
long int a=250;
moeda m;
m.var=ini(a);
m.var[0] = 2000;
printf("%lf", m.var[0]);
return 0;
}
如果我有您的代碼並必須對其進行改進,我會爭取
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
// The kernel style guide https://www.kernel.org/doc/html/v4.10/process/coding-style.html discourages typedefs for structs
typedef struct moeda {
double *return_value;
} moeda;
// return a struct here:
moeda initialize_return(int a)
{
moeda ret;
ret.return_value = malloc(a*sizeof(double));
return ret;
}
int main(void) {
long int a=250;
moeda m = initialize_return(a);
m.return_value[0] = 2000;
printf("%lf", m.return_value[0]);
return 0;
}
(最好將所有標識符都用英語表示)。
這將是第一步。 然后我可能會意識到並不需要該結構,並替換它:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
double * initialize_double_array(int a)
{
return malloc(a*sizeof(double));
}
int main(void) {
long int a=250;
double * arr = initialize_double_array(a);
arr[0] = 2000;
printf("%lf", arr[0]);
return 0;
}
OTOH,如果在所述結構中還有其他字段,我可能會決定是否應該將它們與該數組一起初始化。
一些變體:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
// The kernel style guide https://www.kernel.org/doc/html/v4.10/process/coding-style.html discourages typedefs for structs
struct moeda {
int num_values;
double *values;
};
// only fill a struct here:
// i. e. take a pre-initialized struct and work with it:
void moeda_alloc_values(struct moeda * data)
{
data->return_value = malloc(data->num_values * sizeof(double));
}
// return a struct here:
struct moeda initialize_moeda(int num)
{
struct moeda ret;
ret.num_values = num;
ret.return_value = malloc(num * sizeof(double));
// or just moeda_alloc_values(&ret);
return ret;
}
int main(void) {
long int a=250;
struct moeda m = initialize_return(a);
m.return_value[0] = 2000;
printf("%lf", m.return_value[0]);
struct moeda m2;
m2.num_values = 20;
moeda_alloc_values(&m2);
m2.return_value[0] = 2000;
printf("%lf", m2.return_value[0]);
return 0;
}
struct返回函數的優點是返回后具有“立即填充”的結構。
另一個通過指針修改結構的函數的優點是,它可以在任何可能預先填充的,可能已分配的結構上工作,並且可以在單個字段上工作,而不必考慮所有字段。
return是c中的關鍵字。 您不能將其用作變量名。 我也不清楚這個問題。 什么是“ moeda m”? 萌達在這里? 如果這不是英語C,我很抱歉。
我假設您只是意味着您正在嘗試從main中調用的函數向結構分配內存。 為了清楚起見,我更改了您的變量名。 因此,首先,正如其他人所說,您不能將return用作變量名。 我還建議您使用結構的大小,而不是僅使用兩倍,因為將來您在結構中可能會有多個變量。
如果要使用main函數,則必須將指針傳遞給該函數,然后使用malloc為該函數分配內存,然后返回它。 或者,您可以將struct指針設置為全局選項。
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#include "stdafx.h"
#include <malloc.h>
typedef struct
{
double number;
}example;
example *allocateMemory(int a, example *s)
{
s = (example*)malloc(a * sizeof(example));
return s;
}
int main() {
long int a = 250;
example *structure = NULL;
structure = allocateMemory(a, structure);
structure[0].number = 2000;
printf("%lf\n", structure[0].number);
//cleaning up memory
free(structure);
structure = NULL;
return 0;
}
只是其他一些說明,我讓我的示例結構等於null,因為編譯器抱怨未初始化的局部變量。
在您的代碼中,您有這個。
m.retorno[0] = 2000;
但我假設您想訪問struct數組中的第一個數字,因此應該是:
structure[0].number = 2000;
對於此特定示例,我將執行以下操作:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
typedef struct
{
size_t nLen; // number of elements allocated for var
double *var; // pointer to a list of double variables
} moeda;
// struct is small so just initialize the whole thing and return it
// from the initialization routine.
moeda iniMoeda (size_t n)
{
moeda x = {0};
x.var = malloc(n * sizeof(double)); // try to allocate the requested number of doubles
if (x.var) x.nLen = n; // if allocated then remember number of doubles
return x;
}
// free a moeda variable. we require a pointer so that we can reset
// the moeda variable to a known state of NULL pointer and zero allocation
// length so that we can easily catch using the variable after the memory
// has been freed. Hope for Address exception on a NULL pointer if this
// variable is used after freeing.
void freeMoeda (moeda *x)
{
// free the allocated doubles and clear everything.
// if x->var is NULL then free() does nothing.
free (x->var); x->var = NULL; x->nLen = 0;
}
int main(void)
{
size_t a = 250;
moeda m = iniMoeda (a);
if (m.var) {
// allocation worked so lets test our space
m.var[0] = 2000;
printf("%lf", m.var[0]);
} else {
printf ("m.var is NULL.\n");
}
freeMoeda (&m);
return 0;
}
以下建議的代碼:
注意:代碼必須一致,可讀並執行所需的功能
現在,建議的代碼:
// for ease of readability and understanding:
// 1) insert a space:
// after commas,
// after semicolons,
// inside brackets,
// inside parens,
// around C operators
// 2) separate code blocks
// ( 'for' 'if' 'else' 'while' 'do...while' 'switch' 'case' 'default' )
// via a single blank line
// 3) variable (and parameter) names should indicate
// 'content' or 'usage' (or better, both)
#include <stdio.h> // printf(), perror()
#include <stdlib.h> // malloc(), free(), exit(), EXIT_FAILURE
// do not include header files those contents are not used
//#include <math.h>
//#include <string.h>
// added 'sCOIN' tag name to make it easier to use debugger
// since most debuggers use the tag name to reference fields inside a struct
typedef struct sCOIN
{
double *ret;
} coin;
int main( void )
{
// 'malloc()' expects its parameters to be of type 'size_t'
size_t n_coins=3;
coin mycoin;
// do not cast the returned value from 'malloc()', 'calloc()', 'realloc()'
// as the returned type is 'void*' which can be assigned to any pointer
//coin *m = (coin*)malloc(n_coins*sizeof(coin));
mycoin.ret = malloc( n_coins * sizeof( double ) );
// always check to assure the operation was successful
if( !mycoin.ret )
{
// 'perror()' outputs the enclosed text
// and the text of why the system thinks the error occurred
// to 'stderr'
perror( "malloc failed" );
exit( EXIT_FAILURE );
}
// implied else, malloc successful
// the field in 'coin' is declared a DOUBLE so assign a double
// not a integer. I.E include a decimal point '.'
mycoin.ret[ 0 ] = 2000.0;
printf( "%lf", mycoin.ret[ 0 ] );
// code should always clean up after itself
// I.E. don't leave a mess nor depend on the OS to cleanup.
free( mycoin.ret );
return 0;
}
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