CRC校验和网络通信中writen、readn函数

1.对网络传输的数据进行CRC32校验。

 #include <stdint.h>
 #include <stdio.h>
 #include <string.h>

 static const uint32_t crc32tab[] = {
  0x00000000L, 0x77073096L, 0xee0e612cL, 0x990951baL,
  0x076dc419L, 0x706af48fL, 0xe963a535L, 0x9e6495a3L,
  0x0edb8832L, 0x79dcb8a4L, 0xe0d5e91eL, 0x97d2d988L,
  0x09b64c2bL, 0x7eb17cbdL, 0xe7b82d07L, 0x90bf1d91L,
  0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL,
  0x1adad47dL, 0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L,
  0x136c9856L, 0x646ba8c0L, 0xfd62f97aL, 0x8a65c9ecL,
  0x14015c4fL, 0x63066cd9L, 0xfa0f3d63L, 0x8d080df5L,
  0x3b6e20c8L, 0x4c69105eL, 0xd56041e4L, 0xa2677172L,
  0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL, 0xa50ab56bL,
  0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L,
  0x32d86ce3L, 0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L,
  0x26d930acL, 0x51de003aL, 0xc8d75180L, 0xbfd06116L,
  0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L, 0xb8bda50fL,
  0x2802b89eL, 0x5f058808L, 0xc60cd9b2L, 0xb10be924L,
  0x2f6f7c87L, 0x58684c11L, 0xc1611dabL, 0xb6662d3dL,
  0x76dc4190L, 0x01db7106L, 0x98d220bcL, 0xefd5102aL,
  0x71b18589L, 0x06b6b51fL, 0x9fbfe4a5L, 0xe8b8d433L,
  0x7807c9a2L, 0x0f00f934L, 0x9609a88eL, 0xe10e9818L,
  0x7f6a0dbbL, 0x086d3d2dL, 0x91646c97L, 0xe6635c01L,
  0x6b6b51f4L, 0x1c6c6162L, 0x856530d8L, 0xf262004eL,
  0x6c0695edL, 0x1b01a57bL, 0x8208f4c1L, 0xf50fc457L,
  0x65b0d9c6L, 0x12b7e950L, 0x8bbeb8eaL, 0xfcb9887cL,
  0x62dd1ddfL, 0x15da2d49L, 0x8cd37cf3L, 0xfbd44c65L,
  0x4db26158L, 0x3ab551ceL, 0xa3bc0074L, 0xd4bb30e2L,
  0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL,
  0x4369e96aL, 0x346ed9fcL, 0xad678846L, 0xda60b8d0L,
  0x44042d73L, 0x33031de5L, 0xaa0a4c5fL, 0xdd0d7cc9L,
  0x5005713cL, 0x270241aaL, 0xbe0b1010L, 0xc90c2086L,
  0x5768b525L, 0x206f85b3L, 0xb966d409L, 0xce61e49fL,
  0x5edef90eL, 0x29d9c998L, 0xb0d09822L, 0xc7d7a8b4L,
  0x59b33d17L, 0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL,
  0xedb88320L, 0x9abfb3b6L, 0x03b6e20cL, 0x74b1d29aL,
  0xead54739L, 0x9dd277afL, 0x04db2615L, 0x73dc1683L,
  0xe3630b12L, 0x94643b84L, 0x0d6d6a3eL, 0x7a6a5aa8L,
  0xe40ecf0bL, 0x9309ff9dL, 0x0a00ae27L, 0x7d079eb1L,
  0xf00f9344L, 0x8708a3d2L, 0x1e01f268L, 0x6906c2feL,
  0xf762575dL, 0x806567cbL, 0x196c3671L, 0x6e6b06e7L,
  0xfed41b76L, 0x89d32be0L, 0x10da7a5aL, 0x67dd4accL,
  0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L, 0x60b08ed5L,
  0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L,
  0xd1bb67f1L, 0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL,
  0xd80d2bdaL, 0xaf0a1b4cL, 0x36034af6L, 0x41047a60L,
  0xdf60efc3L, 0xa867df55L, 0x316e8eefL, 0x4669be79L,
  0xcb61b38cL, 0xbc66831aL, 0x256fd2a0L, 0x5268e236L,
  0xcc0c7795L, 0xbb0b4703L, 0x220216b9L, 0x5505262fL,
  0xc5ba3bbeL, 0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L,
  0xc2d7ffa7L, 0xb5d0cf31L, 0x2cd99e8bL, 0x5bdeae1dL,
  0x9b64c2b0L, 0xec63f226L, 0x756aa39cL, 0x026d930aL,
  0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x05005713L,
  0x95bf4a82L, 0xe2b87a14L, 0x7bb12baeL, 0x0cb61b38L,
  0x92d28e9bL, 0xe5d5be0dL, 0x7cdcefb7L, 0x0bdbdf21L,
  0x86d3d2d4L, 0xf1d4e242L, 0x68ddb3f8L, 0x1fda836eL,
  0x81be16cdL, 0xf6b9265bL, 0x6fb077e1L, 0x18b74777L,
  0x88085ae6L, 0xff0f6a70L, 0x66063bcaL, 0x11010b5cL,
  0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L,
  0xa00ae278L, 0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L,
  0xa7672661L, 0xd06016f7L, 0x4969474dL, 0x3e6e77dbL,
  0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L, 0x37d83bf0L,
  0xa9bcae53L, 0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L,
  0xbdbdf21cL, 0xcabac28aL, 0x53b39330L, 0x24b4a3a6L,
  0xbad03605L, 0xcdd70693L, 0x54de5729L, 0x23d967bfL,
  0xb3667a2eL, 0xc4614ab8L, 0x5d681b02L, 0x2a6f2b94L,
  0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL, 0x2d02ef8dL
 };

 uint32_t crc32( const unsigned char *buf, uint32_t size)
 {
      uint32_t i, crc;
      crc = 0xFFFFFFFF;
      for (i = ; i < size; i++)
       crc = crc32tab[(crc ^ buf[i]) & 0xff] ^ (crc >> );
      return crc^0xFFFFFFFF;
 }

 int main(int argc,char **argv){
     char *str="sdfdsfdsfdsfdsfdssdfdsfdsfdsfdddddsssssssssssssfdsfdsfds";
     printf("%u\n",crc32(str,strlen(str)));
     return ;
 }

2.下面两个函数readn和writen的功能是读、写指定的N字节数据，并处理返回值小于要求值的情况。

这两个函数只是按需多次调用read和write直至读、写了N字节数据。
*/

//在要将数据写到上面提到的文件类型上时，就可调用writen，但是只有当事先就知道要接收数据的数量时.
//才调用readn（通常只调用read接收来自这些设备的数据）

//在要将数据写到上面提到的文件类型上时，就可调用writen，但是只有当事先就知道要接
//收数据的数量时，才调用readn（通常只调用read接收来自这些设备的数据）

//注意，若在已经读、写了一些数据后出错，则这两个函数返回已经传输的数据量，而非出错返回。
//与此类似，在读时如达到文件尾，而且在此之前已经成功地读了一些数据，但尚未满足所要求的量，
//则readn返回已复制到调用者缓冲区中的字节

ssize_t writen(int fd, const void *buf, ssize_t lenth)
{
    ssize_t nleft;
    ssize_t nwrite;

    const void *ptr = buf;
    nleft = lenth;

    while(nleft > )
    {
        if((nwrite = write(fd, ptr, nleft )) < )
        {
            if( nleft == lenth )
            {
                printf("func read() error: %s, %d", __FILE__, __LINE__);
                return -;
            }
            else
            {
                printf("剩余 【%d】 个字节，未写进去\n", nleft);
                break;
            }
        }
        else if(nwrite ==  )
        {
            break;
        }

        ptr += nwrite ;            //从剩下的地方继续写
        nleft -= nwrite ;            //剩余的字节数
    }

    return (lenth - nleft);
}

ssize_t readn(int fd, const void *buf, sszie_t lenth)
{
    ssize_t nleft;
    ssize_t nread;

    const void *ptr = buf;
    nleft = lenth;

    while(nleft > )
    {
        if((nread = read(fd, ptr, nleft)) < )
        {
            if( nleft == lenth )
            {
                printf("func read() error: %s, %d", __FILE__, __LINE__);
                return -;
            }
            else
            {
                printf("剩余 【%d】 个字节，未读进去\n", nleft);
                break;
            }
        }
        else if(nread == )    //表示已读完
        {
            break;
        }

        ptr += nread;
        nleft -= nread ;
    }

    return (lenth - nleft);
}