int base64(char *s,char *d)
{
char CharSet[64]={
'A','B','C','D','E','F','G','H',
'I','J','K','L','M','N','O','P',
'Q','R','S','T','U','V','W','X',
'Y','Z','a','b','c','d','e','f',
'g','h','i','j','k','l','m','n',
'o','p','q','r','s','t','u','v',
'w','x','y','z','0','1','2','3',
'4','5','6','7','8','9','+','/'};
unsigned char In[3];
unsigned char Out[4];
int cnt=0;

if(!s||!d) return 0;

for(;*s != 0;)
{
if(cnt+4>76)
{
*d++='\n';
cnt=0;
}
if(strlen(s)>=3)
{
In[0]=*s;
In[1]=*(s+1);
In[2]=*(s+2);
Out[0]=In[0]>>2;
Out[1]=(In[0]&0x03)<<4|(In[1]&0xf0)>>4;
Out[2]=(In[1]&0x0f)<<2|(In[2]&0xc0)>>6;
Out[3]=In[2]&0x3f;
*d=CharSet[Out[0]];
*(d+1)=CharSet[Out[1]];
*(d+2)=CharSet[Out[2]];
*(d+3)=CharSet[Out[3]];
s+=3;
d+=4;
}
else if(strlen(s)==1)
{
In[0]=*s;
Out[0]=In[0]>>2;
Out[1]=(In[0]&0x03)<<4|0;
*d=CharSet[Out[0]];
*(d+1)=CharSet[Out[1]];
*(d+2)='=';
*(d+3)='=';
s+=1;
d+=4;
}
else if(strlen(s)==2)
{
In[0]=*s;
In[1]=*(s+1);
Out[0]=In[0]>>2;
Out[1]=(In[0]&0x03)<<4|(In[1]&0xf0)>>4;
Out[2]=(In[1]&0x0f)<<2|0;
*d=CharSet[Out[0]];
*(d+1)=CharSet[Out[1]];
*(d+2)=CharSet[Out[2]];
*(d+3)='=';
s+=2;
d+=4;
}
cnt+=4;
}
*d='\0';
return 1;
}

这是我找到的一个base64编码的源码,那位高手解释一下为什么:
if(cnt+4>76)
{
*d++='\n';
cnt=0;
}
不直接写成:
if(cnt>72)
..............