C语言如何抓取文件中的字符串

一个文件的内容大致是userid=AAAAAA,inname=XXXXXXXX，如何提取inname后的字符串，并写入到一个新文件中？

void f(char * src, char* dest)
{
    char* p = NULL;
    int len = 0;
    char* buf = NULL;
    FILE *fp = fopen(src,"rb");
    if(fp)
    {
        fseek(fp,0,SEEK_END);
        len = ftell(fp);
        fseek(fp,0,SEEK_SET);
        buf = malloc(len+1);
        if(buf)
        {
            memset(buf,0,len+1);
            fread(buf,1,len,fp);
            p = strstr(buf,"inname=");
            if(p)
            {
                char* start = p+8;
                //空格是结束符
                char* p1 = strstr(start," ");
                if(p1)
                {
                    FILE *fpWrite = fopen(dest,"wb");
                    if(fpWrite)
                    {
                        fwrite(start,1,p1-start-1,fp);
                        fclose(fpWrite);
                    }
                }
            }
            free(buf);
        }
        fclose();
    }
}