getchar/putchar/gets/puts小讨

example1.c

#include
void main()
{
char *ptr;
int c;
while((c = getchar()) != EOF)
putchar(c);
}

运行：
# strace ./example1

...
read(0, 123456
"123456\n"..., 1024) = 7
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb80a9000
write(1, "123456\n"..., 7123456
) = 7
...

由于stdout默认是行缓冲的，所以putchar仅仅是把字符保存在缓冲区中，当碰到换行符时才一次性调用write写出
example2.c

#include
void main()
{
char *ptr;
int c;
setbuf(stdout, NULL);
while((c = getchar()) != EOF)
putchar(c);
}

运行：
#strace ./example2

...
read(0, 123456
"123456\n"..., 1024) = 7
write(1, "1"..., 11) = 1
write(1, "2"..., 12) = 1
write(1, "3"..., 13) = 1
write(1, "4"..., 14) = 1
write(1, "5"..., 15) = 1
write(1, "6"..., 16) = 1
write(1, "\n"..., 1
) = 1
...

在本例中，将stdout修改为不带缓冲，所以，每次putchar都会激发系统调用write
example3:

#include
void main()
{
char buf[1024];
char *ptr;
while((ptr = gets(buf)) != EOF)
puts(buf);
}

运行：
#strace ./example3

...
read(0, 123456
"123456\n"..., 1024) = 7
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb8027000
write(1, "123456\n"..., 7123456
) = 7
...

puts将一个以null符终止的字符串写到标准输出，终止符不写出，但是puts在字符串后面添加换行符（若字符串本身已有换行符，则不会重复添加）
example4:

#include <stdio.h>
void main()
{
    char buf[1024];
    char *ptr;
    setbuf(stdout, NULL);
    while((ptr = gets(buf)) != EOF)
        puts(buf);
}

运行:strace ./example4

...
read(0, 123456
"123456\n"..., 1024) = 7
write(1, "123456"..., 6123456) = 6
write(1, "\n"..., 1
) = 1
...

这种输出说明了什么呢？待解！