以二进制格式组合两个文件

Question

我写了这段代码来测试合并两个文件:

 long getFileSize(char *filename)
{
     FILE* fp=fopen(filename,"rb");
     fseek(fp,0,SEEK_END);
     long size=ftell(fp); 
     fclose(fp); 
     return size;   
}



 long lengthA = getFileSize(argv[1]);
   long lengthB = getFileSize(argv[2]);
   printf("sizeof %s is:%d\n",argv[1],lengthA);
   printf("sizeof %s is %d\n",argv[2],lengthB);

   void *pa;
   void *pb;
   FILE* fp=fopen(argv[1],"rb");
   fread(pa,1,lengthA,fp);
   fclose(fp);
   FILE* fpn=fopen(argv[2],"rb");
   fread(pb,1,lengthB,fpn);
   fclose(fpn);
   printf("pointerA is:%p;pointerB is:%p\n",pa,pb);

   FILE *ff=fopen("test.pack","wb");
   fwrite(pa,1,lengthA,ff);
   fwrite(pb,1,lengthB,ff);
   fclose(ff);

   long lengthFinal = getFileSize("test.pack");

   printf("Final size:%i\n",lengthFinal);

但是我不知道数据是否等于 getFileSize 的返回值，控制台打印清楚地表明它有问题，但我无法弄清楚:

sizeof a.zip is:465235
sizeof b.zip is 107814
pointerA is:0x80484ec;pointerB is:0x804aff4
Final size:255270

因为我知道每个文件的长度，所以我可以使用 fseek 来恢复它们，对吗？这就是我的想法。

Answer 1

*pa 和 *pb 需要指向一些内存，文件内容将被读取到。

因此，使用lengthA*sizeof(char) 和lengthB*sizeof(char) 为这两个缓冲区执行malloc 并传递它们为 fread 分配缓冲区:

pa = malloc(lengthA*sizeof(char));
pb = malloc(lengthB*sizeof(char));
...
fread(pa,sizeof(char),lengthA,fp);
...
fread(pb,sizeof(char),lengthB,fpn);

此外，fread 返回实际读取的项目数。还要检查这个!

摘自 man fread :

fread() and fwrite() return the number of items successfully read or written (i.e., not the number of characters). If an error occurs, or the end-of-file is reached, the return value is a short item count (or zero).