如何分配內存的int數組的結構

Question

我有一個結構

typedef struct 
{ 
    int size; //size of array 
    int array* 
} 

如何使用尺寸變量和malloc的int數組分配內存？

Answer 1

int *ptr; 
ptr = malloc(sizeof *ptr * size); 
//can now reference ptr[0], ptr[1], ... ptr[size-1]. 

上述代碼的作用：當爲一個int數組分配內存，可以分配4個字節的存儲器（的sizeof（int）的），用於每次需要元素。所以，當你使用malloc時，malloc會將元素的大小乘以你需要的元素數量。

Answer 2

您可以創建初始化結構的功能，你可以創建一個這樣的：

typedef struct { 
    int size; //size of array 
    int *array; 
} mystruct; 

mystruct * create_mystruct(int size) { 
    mystruct * st = (mystruct*) malloc(sizeof(mystruct)); 
    st->size = size; 
    st->array = (int *) malloc(sizeof(int) * size); 
    return st; 
} 

Here是工作吡咯烷酮作爲一個例子。

Answer 3

您的問題的答案將取決於您是否聲明struct或指針指向struct。如果你聲明一個結構體（例如mystruct s），那麼你只需要分配內存元素s.array。例如： -

typedef struct mystruct { 
    int size; 
    int *array; 
} mystruct; 
... 

mystruct s; 

s.size = 5; 

/* allocating array based on size */ 
s.array = malloc (s.size * sizeof *s.array); 
if (!s.array) { 
    fprintf (stderr, "error: virtual memory exhausted.\n"); 
    return 1; 
} 

注：你必須驗證你的配置成功和malloc實際上每次調用malloc時間返回一個有效的地址。 （你可以創建一個函數來減少輸入）。完成後您只需要免費s.array。

然而，如果聲明一個指針struct（例如mystruct *msp = NULL;）你現在必須兩個msp和msp->array分配內存。例如：

mystruct *msp = NULL; 

msp = malloc (sizeof *msp); 
if (!msp) { 
    fprintf (stderr, "error: virtual memory exhausted.\n"); 
    return 1; 
} 

msp->size = 5; 

/* allocating array based on size */ 
msp->array = malloc (msp->size * sizeof *msp->array); 
if (!msp->array) { 
    fprintf (stderr, "error: virtual memory exhausted.\n"); 
    return 1; 
} 

必須free你都完成的時候。

以下是示出在兩種情況下爲array分配，使用和存儲器釋放一個簡單的例子：

#include <stdio.h> 
#include <stdlib.h> 

typedef struct mystruct { 
    int size; 
    int *array; 
} mystruct; 

int main (void) { 

    size_t i = 0; 
    size_t nelements = 0; 
    int rd = 0; 

    /* 
    * declaring a struct mystruct 
    */ 
    mystruct s; 

    s.size = 5; 

    /* allocating array based on size */ 
    s.array = malloc (s.size * sizeof *s.array); 
    if (!s.array) { 
     fprintf (stderr, "error: virtual memory exhausted.\n"); 
     return 1; 
    } 

    /* fill array */ 
    for (rd = 0; rd < s.size; rd++) 
     if (scanf ("%d", &s.array[nelements]) == 1) 
      nelements++; 

    for (i = 0; i < nelements; i++) 
     printf (" s.array[%zu] = %d\n", i, s.array[i]); 

    /* free allocated memory */ 
    free (s.array); 

    putchar ('\n'); 

    /* 
    * declaring a pointer to mystruct 
    */ 
    mystruct *msp = NULL; 

    /* allocate memory for msp (mystruct pointer) */ 
    msp = malloc (sizeof *msp); 
    if (!msp) { 
     fprintf (stderr, "error: virtual memory exhausted.\n"); 
     return 1; 
    } 

    msp->size = 5; 

    /* allocating array based on size */ 
    msp->array = malloc (msp->size * sizeof *msp->array); 
    if (!msp->array) { 
     fprintf (stderr, "error: virtual memory exhausted.\n"); 
     return 1; 
    } 

    /* fill array */ 
    rd = 0; 
    nelements = 0; 
    for (rd = 0; rd < msp->size; rd++) 
     if (scanf ("%d", &msp->array[nelements]) == 1) 
      nelements++; 

    for (i = 0; i < nelements; i++) 
     printf (" msp->array[%zu] = %d\n", i, msp->array[i]); 

    /* free allocated memory */ 
    free (msp->array); 
    free (msp); 

    return 0; 
} 

實施例使用/輸出

$ printf "2\n4\n6\n8\n10\n12\n14\n16\n18\n20\n" | ./bin/struct_alloc_int 
s.array[0] = 2 
s.array[1] = 4 
s.array[2] = 6 
s.array[3] = 8 
s.array[4] = 10 

msp->array[0] = 12 
msp->array[1] = 14 
msp->array[2] = 16 
msp->array[3] = 18 
msp->array[4] = 20 

存儲器錯誤檢查

在您編寫動態代碼的任何代碼中y分配內存，所以你必須使用內存錯誤檢查程序。對於Linux valgrind是正常的選擇。濫用一塊可能導致真正問題的內存塊有很多微妙的方法，沒有理由不這樣做。每個平臺都有類似的內存檢查器。它們使用簡單。通過它來運行你的程序。

$ printf "2\n4\n6\n8\n10\n12\n14\n16\n18\n20\n" | valgrind ./bin/struct_alloc_int 
==20515== Memcheck, a memory error detector 
==20515== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al. 
==20515== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info 
==20515== Command: ./bin/struct_alloc_int 
==20515== 
s.array[0] = 2 
<snip> 
msp->array[4] = 20 
==20515== 
==20515== HEAP SUMMARY: 
==20515==  in use at exit: 0 bytes in 0 blocks 
==20515== total heap usage: 3 allocs, 3 frees, 56 bytes allocated 
==20515== 
==20515== All heap blocks were freed -- no leaks are possible 
==20515== 
==20515== For counts of detected and suppressed errors, rerun with: -v 
==20515== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)