在malloc中为C语言中的“char”数组铸造错误
使用结构变量和结构数组读取bmp图像的代码。 请建议我以正确的方式对malloc进行类型转换（错误代码如下）：
#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,"%dn",readbuffer[(i-1)*pix_x + j].r);
fprintf(greenpix,"%dn",readbuffer[(i-1)*pix_x + j].g);
fprintf(bluepix,"%dn",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;
}
参考文献：如何在C语言中正确使用malloc获取结构数组
malloc结构指针数组与结构数组
错误列表：
bmpread_check.c：在函数'main'中：bmpread_check.c：24：19：warning：从不兼容的指针类型初始化> [默认情况下启用] band readbuffer =（char）malloc（n_pix * sizeof（* readbuffer））; ^ bmpread_check.c：29：33：error'expected'='，'，';'，'asm'或' attribute '> before'。' token unsigned char readbuffer [n] .b =（char）malloc（sizeof（readbuffer [n] .b））; ^ bmpread_check.c：29：33：错误：''之前的预期表达式'。' token'bmpread_check.c：30：33：error'expected'='，'，'，';'，'asm'或' attribute '> before'。' token unsigned char readbuffer [n] .g =（char）malloc（sizeof（readbuffer [n] .g））; ^ bmpread_check.c：30：33：错误：''之前的预期表达式'。' token'bmpread_check.c：31：33：error'expected'='，'，'，';'，'asm'或' attribute '> before'。' 代币
unsigned char readbuffer [n] .r =（char）malloc（sizeof（readbuffer [n] .r））; ^ bmpread_check.c：31：33：错误：''之前的预期表达式'。' 令牌bmpread_check.c：69：5：warning：传递'free'的参数1使指针从>整数而不需要强制转换[默认启用] free（readbuffer [n] .b）; ^从bmpread_check.c包含的文件中：3：0：c： mingw include stdlib.h：357：38：note：expected'void'但参数的类型为'unsigned char'_CRTIMP void __cdecl __MINGW_NOTHROW free（无效）; ^ bmpread_check.c：70：5：warning：传递'free'的参数1使指针从>整数而不需要强制转换[默认启用] free（readbuffer [n] .g）; ^从bmpread_check.c包含的文件中：3：0：c： mingw include stdlib.h：357：38：note：expected'void'但参数的类型为'unsigned char'_CRTIMP void __cdecl __MINGW_NOTHROW free（无效）; ^ bmpread_check.c：71：5：warning：传递'free'的参数1使指针从>整数而不需要强制转换[默认启用] free（readbuffer [n] .r）; ^从bmpread_check.c包含的文件中：3：0：c： mingw include stdlib.h：357：38：note：expected'void'但参数的类型为''unsigned char'_CRTIMP void __cdecl __MINGW_NOTHROW free无效）; ^
这个：
bands *readbuffer=(bands*)malloc(n_pix*sizeof(bands));
（注意：不是*readbuffer ，它是bands ）
已经为所有n_pix频段分配了内存。
没有必要为b, g, r分配内存b, g, r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。
变量b ， g和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一个具有像素总数大小的结构数组，即宽度乘以图像高度。
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));
该表述将分配结构n_pix时间，以便您可以初始化并访问像素值b ， g ＆ r在每个单独的像素位置，如下所示。
readbuffer[i]-> b = 20;
readbuffer[i]-> g = 80;
readbuffer[i]-> r = 40;
i可以从0到n_pix-1任何地方
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