C語言中「char」數組在malloc中的Casting錯誤

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使用結構變量和結構數組讀取bmp圖像的代碼。請給我建議正確的方式做類型轉換對malloc（下面的代碼錯誤列表）：C語言中「char」數組在malloc中的Casting錯誤

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

typedef struct bands{ 
/* .bmp image store pixel colors in "bgr" sequence */ 
unsigned char b,g,r; //in 24bit bmp, we need to use 8bit datatype for each color 
}bands; 

int main() 
{ 
FILE *bmpimage; //ptr to read image file 
FILE *redpix,*greenpix,*bluepix; //ptr to create band/color wise file 
unsigned short pix_x=223,pix_y=197; /*pix_x: no. of pixels in a row, pix_y: no. of pixels in a column of input image*/ 
unsigned short n_pix=pix_x*pix_y; /*variable to count total no. of pixels*/ 

bmpimage=fopen("blocks223x197.bmp","r"); //24 bit bmpimage 
redpix=fopen("redpixels.txt","w"); 
greenpix=fopen("greenpixels.txt","w"); 
bluepix=fopen("bluepixels.txt","w"); 

/* Define a pointer to a memory block,'*readbuffer', 
that has 'n_pix' no. of memory blocks each of size same as struct bands */ 
bands *readbuffer=(char*)malloc(n_pix*sizeof(*readbuffer)); 

int n; 
//Create memory for each of 'n_pix' no. of pixel array of each color 
for(n=0;n<n_pix;n++){ 
    unsigned char *readbuffer[n].b = (char*) malloc(sizeof(readbuffer[n].b)); 
    unsigned char *readbuffer[n].g = (char*) malloc(sizeof(readbuffer[n].g)); 
    unsigned char *readbuffer[n].r = (char*) malloc(sizeof(readbuffer[n].r)); 
} 

if(!bmpimage){printf("Error reading bmpimage!");return 1;} 
if(readbuffer==NULL){printf("NULL buffer"); exit(1);} 

/* Go to 54th byte to access pixelvalue data (since, 24bit bmp format) */ 
fseek(bmpimage,54,SEEK_SET); 

/* Read 'n_pix' no. of 'bgr' blocks each of which are of the size same as "struct bands" */ 
fread(readbuffer,sizeof(bands),n_pix,bmpimage); /*read 'n_pix' no. of 'bgr' blocks each of which are of the size same as "struct bands" to the memory address, 'readbuffer' or '&readbuffer[0]' */  

int n_blocks=(sizeof(readbuffer)/sizeof(bands)); 
printf("no. of blocks read= %d, n_pix=%d",n_blocks,n_pix); 


int i,j; int count; count=0; 
/* logic to print pixel values in correct order*/ 

for(i=pix_y;i>0;i--){ /*for accessing row data. Choose to print from bottom to top*/ 
for(j=1;j<=pix_x;j++){ /*for accessing column data. Print from left to right*/ 

    if(j!=pix_x){ 
    fprintf(redpix,"%d,",readbuffer[(i-1)*pix_x + j].r); 
    fprintf(greenpix,"%d,",readbuffer[(i-1)*pix_x + j].g); 
    fprintf(bluepix,"%d,",readbuffer[(i-1)*pix_x + j].b); 
    } 
    else{ 
     count++; 
    fprintf(redpix,"%d\n",readbuffer[(i-1)*pix_x + j].r); 
    fprintf(greenpix,"%d\n",readbuffer[(i-1)*pix_x + j].g); 
    fprintf(bluepix,"%d\n",readbuffer[(i-1)*pix_x + j].b); 
    } 
    } 
} 

// free allocated memory 
for(n=0;n<n_pix;n++){ 
    free(readbuffer[n].b) ; 
    free(readbuffer[n].g) ; 
    free(readbuffer[n].r) ; 
} 


fclose(bmpimage);fclose(redpix);fclose(bluepix);fclose(greenpix); 

return 0; 

}

參考文獻： How to properly malloc for array of struct in C

malloc an array of struct pointers vs array of structs

錯誤列表：

bmpread_check.c: In function 'main': bmpread_check.c:24:19: warning: initialization from incompatible pointer type >[enabled by default] bands readbuffer=(char)malloc(n_pix*sizeof(*readbuffer)); ^ bmpread_check.c:29:33: error: expected '=', ',', ';', 'asm' or 'attribute' >before '.' token unsigned char readbuffer[n].b = (char)malloc(sizeof(readbuffer[n].b)); ^ bmpread_check.c:29:33: error: expected expression before '.' token bmpread_check.c:30:33: error: expected '=', ',', ';', 'asm' or 'attribute' >before '.' token unsigned char readbuffer[n].g = (char)malloc(sizeof(readbuffer[n].g)); ^ bmpread_check.c:30:33: error: expected expression before '.' token bmpread_check.c:31:33: error: expected '=', ',', ';', 'asm' or 'attribute' >before '.' token
unsigned char readbuffer[n].r = (char)malloc(sizeof(readbuffer[n].r)); ^ bmpread_check.c:31:33: error: expected expression before '.' token bmpread_check.c:69:5: warning: passing argument 1 of 'free' makes pointer from >integer without a cast [enabled by default] free(readbuffer[n].b) ; ^ In file included from bmpread_check.c:3:0: c:\mingw\include\stdlib.h:357:38: note: expected 'void ' but argument is of >type 'unsigned char' _CRTIMP void __cdecl __MINGW_NOTHROW free (void); ^ bmpread_check.c:70:5: warning: passing argument 1 of 'free' makes pointer from >integer without a cast [enabled by default] free(readbuffer[n].g) ; ^ In file included from bmpread_check.c:3:0: c:\mingw\include\stdlib.h:357:38: note: expected 'void ' but argument is of >type 'unsigned char' _CRTIMP void __cdecl __MINGW_NOTHROW free (void); ^ bmpread_check.c:71:5: warning: passing argument 1 of 'free' makes pointer from >integer without a cast [enabled by default] free(readbuffer[n].r) ; ^ In file included from bmpread_check.c:3:0: c:\mingw\include\stdlib.h:357:38: note: expected 'void ' but argument is of type >'unsigned char' _CRTIMP void __cdecl __MINGW_NOTHROW free (void); ^

來源

2016-02-08 Sai Krishna

你的問題是什麼？ –

請看圖片鏈接「錯誤列表」 –

'band * readbuffer =（bands *）malloc（n_pix * sizeof（* readbuffer））;''或者更好的'band * readbuffer = malloc（n_pix * sizeof（* readbuffer））;'。成員b，g，r不是指針，你不需要爲它們分配。 –

這：

bands *readbuffer=(bands*)malloc(n_pix*sizeof(bands));

（注意：不是*readbuffer。這是bands）

已經爲所有n_pix頻段分配了內存。

沒有必要爲b, g, r分配內存，因爲它們不是指針。

所以，

//Create memory for each of 'n_pix' no. of pixel array of each color 
// And allocating using for loop

是沒有必要的。

來源

2016-02-08 06:27:14 GNKeshava

而類型轉換必須更改爲'（bands *）'或完全刪除。 –

@GNKeshava根據我的驗證，我使用sizeof（band）還是sizeof（* readbuffer）並不重要，因爲它們具有相同的大小。 –

你的建議很有用Martin Zabel。 –

變量b，g & r不是指針，而是無符號的8位變量。因此，爲這種情況分配內存的正確方法是分配一個具有像素總數大小的結構數組，即寬度乘以圖像高度。

這可以通過如下動態分配結構指針bands*來實現。

bands *readbuffer = malloc(n_pix * sizeof(bands));

這statment將分配結構n_pix時間，這樣你就可以初始化和存取像素在每個單獨的像素位置如下值b，g & r。

readbuffer[i]-> b = 20; 
readbuffer[i]-> g = 80; 
readbuffer[i]-> r = 40;

哪裏i可以從0到n_pix-1

來源

2016-02-08 07:10:09

C語言中「char」數組在malloc中的Casting錯誤

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