C语言中"char"数组的malloc转换错误

Question

使用结构变量和结构数组读取 bmp 图像的代码。请建议我对 malloc 进行类型转换的正确方法（代码下方列出的错误）：

#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;   

}

参考资料： How to properly malloc for array of struct in C

malloc an array of struct pointers vs array of structs

错误列表：

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); ^

Answer 1

这个：

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

（注意：不是*readbuffer。是bands）

已经为所有 n_pix 个频段分配了内存。

不需要为 b, g, r 分配内存，因为它们不是指针。

所以，

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

不需要。

Answer 2

变量 b , g & r 不是指针而是无符号的 8 位变量。因此，为这种情况分配内存的正确方法是分配该结构的数组，其大小为像素总数，即图像的宽度乘以高度。

这可以通过如下动态分配结构指针bands*来实现。

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

该语句将分配结构 n_pix 次，因此您可以在每个像素位置初始化和访问像素值 b、g 和 r，如下所示如下。

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

其中 i 可以是从 0 到 n_pix-1

的任何值

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