管道命令 ./a.out | cat, printf("line\n") in a.out 未执行且无输出
pipeline command ./a.out | cat, printf("line\n") in a.out is not excuted and no output
一个很奇怪的事情，a.out只是printf()一行然后进入死循环，当a.out单执行时，我可以在terminal中看到该行，但是如果pipeline a.out with cat，然后我们什么也看不到。
#include <stdio.h>
#include <unistd.h>
int main(int argc, char **argv)
{
printf("----------\n");
while (1) {
sleep(1000);
}
return 0;
}
运行 结果
$ cc test.c
$ ./a.out
----------
^C
$ ./a.out | cat
^C
如果我跟踪 a.out | cat, write(1) 系统调用未被调用
$ strace ./a.out | cat
execve("./a.out", ["./a.out"], 0x7ffdaa23b200 /* 65 vars */) = 0
brk(NULL) = 0x5567446dd000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=94391, ...}) = 0
mmap(NULL, 94391, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f977ba22000
close(3) = 0
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "7ELF[=15=][=15=][=15=][=15=][=15=][=15=][=15=][=15=][=15=]>[=15=][=15=][=15=][=15=]0l[=15=][=15=][=15=][=15=][=15=]"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=2000480, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f977ba20000
mmap(NULL, 2008696, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f977b835000
mmap(0x7f977b85a000, 1519616, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x25000) = 0x7f977b85a000
mmap(0x7f977b9cd000, 299008, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x198000) = 0x7f977b9cd000
mmap(0x7f977ba16000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1e0000) = 0x7f977ba16000
mmap(0x7f977ba1c000, 13944, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f977ba1c000
close(3) = 0
arch_prctl(ARCH_SET_FS, 0x7f977ba21500) = 0
mprotect(0x7f977ba16000, 12288, PROT_READ) = 0
mprotect(0x556743cd8000, 4096, PROT_READ) = 0
mprotect(0x7f977ba64000, 4096, PROT_READ) = 0
munmap(0x7f977ba22000, 94391) = 0
fstat(1, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
brk(NULL) = 0x5567446dd000
brk(0x5567446fe000) = 0x5567446fe000
nanosleep({tv_sec=1000, tv_nsec=0}, ^C{tv_sec=994, tv_nsec=769383373}) = ? ERESTART_RESTARTBLOCK (Interrupted by signal)
strace: Process 5050 detached
如果我 strace a.out 单身，那么有 write(1)
$ strace ./a.out
execve("./a.out", ["./a.out"], 0x7ffe09a7c360 /* 65 vars */) = 0
brk(NULL) = 0x564b085a8000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=94391, ...}) = 0
mmap(NULL, 94391, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fac37df5000
close(3) = 0
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "7ELF[=16=][=16=][=16=][=16=][=16=][=16=][=16=][=16=][=16=]>[=16=][=16=][=16=][=16=]0l[=16=][=16=][=16=][=16=][=16=]"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=2000480, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fac37df3000
mmap(NULL, 2008696, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fac37c08000
mmap(0x7fac37c2d000, 1519616, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x25000) = 0x7fac37c2d000
mmap(0x7fac37da0000, 299008, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x198000) = 0x7fac37da0000
mmap(0x7fac37de9000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1e0000) = 0x7fac37de9000
mmap(0x7fac37def000, 13944, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fac37def000
close(3) = 0
arch_prctl(ARCH_SET_FS, 0x7fac37df4500) = 0
mprotect(0x7fac37de9000, 12288, PROT_READ) = 0
mprotect(0x564b076e3000, 4096, PROT_READ) = 0
mprotect(0x7fac37e37000, 4096, PROT_READ) = 0
munmap(0x7fac37df5000, 94391) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
brk(NULL) = 0x564b085a8000
brk(0x564b085c9000) = 0x564b085c9000
write(1, "----------\n", 11----------
) = 11
nanosleep({tv_sec=100
为什么？
输出流可以处于三种不同模式之一，无缓冲、行缓冲或完全缓冲。在 unbuffered 模式下，输出会立即写入。在 line buffered 模式下，输出首先写入内部流缓冲区，直到缓冲区已满或写入换行符，然后缓冲区被刷新到输出。在 fully buffered 模式下，输出首先写入内部流缓冲区，直到缓冲区已满，然后将缓冲区刷新到输出。 （某些实现也可能在其他时间刷新输出，例如从交互式设备读取输入时。）
输入流也可以处于相同的三种模式，这决定了读取输入何时可供读取流的函数的调用者使用。
该实现在 [=15= 之前初始化标准输入 (stdin)、标准输出 (stdout) 和标准错误输出 (stderr) 流的模式] 函数被调用（或至少在访问流之前）。在某些情况下，允许实现将标准输入或标准输出初始化为 fully buffered 模式。当且仅当实现可以确定它们未链接到交互式设备（例如终端）时，标准输入和输出流才被初始化为完全缓冲模式。 （标准错误输出流从未初始化为完全缓冲模式。）
通常，POSIX 系统的 C 运行time 库将调用 isatty 标准输入和输出流的底层文件描述符，并将流设置为完全缓冲mode if isatty returns 0。这发生在调用 main 函数之前。
当您 运行 "./a.out" 将输出发送到终端时，C 运行time 库确定输出将发送到交互式设备并执行 不 将stdout 设置为完全缓冲模式。它将被设置为其他模式之一，通常是行缓冲模式。然而，当你 运行 "/a.out" 输出到管道时，C 运行time 库确定输出是 not交互式设备并且确实将stdout设置为完全缓冲模式。这就是输出没有立即写入管道的原因。
有两种方法可以解决您的问题。第一种是在第一次调用 printf:[=22 之前将标准输出流更改为 line buffered 模式或 unbuffered 模式=]
setvbuf(stdout, NULL, _IOLBF, 0); // set standard output to line buffered mode
setvbuf(stdout, NULL, _IONBF, 0); // set standard output to unbuffered mode
另一种方法是按需刷新标准输出：
fflush(stdout); // write buffered standard output contents
一个很奇怪的事情，a.out只是printf()一行然后进入死循环，当a.out单执行时，我可以在terminal中看到该行，但是如果pipeline a.out with cat，然后我们什么也看不到。
#include <stdio.h>
#include <unistd.h>
int main(int argc, char **argv)
{
printf("----------\n");
while (1) {
sleep(1000);
}
return 0;
}
运行 结果
$ cc test.c
$ ./a.out
----------
^C
$ ./a.out | cat
^C
如果我跟踪 a.out | cat, write(1) 系统调用未被调用
$ strace ./a.out | cat
execve("./a.out", ["./a.out"], 0x7ffdaa23b200 /* 65 vars */) = 0
brk(NULL) = 0x5567446dd000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=94391, ...}) = 0
mmap(NULL, 94391, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f977ba22000
close(3) = 0
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "7ELF[=15=][=15=][=15=][=15=][=15=][=15=][=15=][=15=][=15=]>[=15=][=15=][=15=][=15=]0l[=15=][=15=][=15=][=15=][=15=]"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=2000480, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f977ba20000
mmap(NULL, 2008696, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f977b835000
mmap(0x7f977b85a000, 1519616, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x25000) = 0x7f977b85a000
mmap(0x7f977b9cd000, 299008, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x198000) = 0x7f977b9cd000
mmap(0x7f977ba16000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1e0000) = 0x7f977ba16000
mmap(0x7f977ba1c000, 13944, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f977ba1c000
close(3) = 0
arch_prctl(ARCH_SET_FS, 0x7f977ba21500) = 0
mprotect(0x7f977ba16000, 12288, PROT_READ) = 0
mprotect(0x556743cd8000, 4096, PROT_READ) = 0
mprotect(0x7f977ba64000, 4096, PROT_READ) = 0
munmap(0x7f977ba22000, 94391) = 0
fstat(1, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
brk(NULL) = 0x5567446dd000
brk(0x5567446fe000) = 0x5567446fe000
nanosleep({tv_sec=1000, tv_nsec=0}, ^C{tv_sec=994, tv_nsec=769383373}) = ? ERESTART_RESTARTBLOCK (Interrupted by signal)
strace: Process 5050 detached
如果我 strace a.out 单身，那么有 write(1)
$ strace ./a.out
execve("./a.out", ["./a.out"], 0x7ffe09a7c360 /* 65 vars */) = 0
brk(NULL) = 0x564b085a8000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=94391, ...}) = 0
mmap(NULL, 94391, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fac37df5000
close(3) = 0
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "7ELF[=16=][=16=][=16=][=16=][=16=][=16=][=16=][=16=][=16=]>[=16=][=16=][=16=][=16=]0l[=16=][=16=][=16=][=16=][=16=]"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=2000480, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fac37df3000
mmap(NULL, 2008696, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fac37c08000
mmap(0x7fac37c2d000, 1519616, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x25000) = 0x7fac37c2d000
mmap(0x7fac37da0000, 299008, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x198000) = 0x7fac37da0000
mmap(0x7fac37de9000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1e0000) = 0x7fac37de9000
mmap(0x7fac37def000, 13944, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fac37def000
close(3) = 0
arch_prctl(ARCH_SET_FS, 0x7fac37df4500) = 0
mprotect(0x7fac37de9000, 12288, PROT_READ) = 0
mprotect(0x564b076e3000, 4096, PROT_READ) = 0
mprotect(0x7fac37e37000, 4096, PROT_READ) = 0
munmap(0x7fac37df5000, 94391) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
brk(NULL) = 0x564b085a8000
brk(0x564b085c9000) = 0x564b085c9000
write(1, "----------\n", 11----------
) = 11
nanosleep({tv_sec=100
为什么？
输出流可以处于三种不同模式之一，无缓冲、行缓冲或完全缓冲。在 unbuffered 模式下，输出会立即写入。在 line buffered 模式下，输出首先写入内部流缓冲区，直到缓冲区已满或写入换行符，然后缓冲区被刷新到输出。在 fully buffered 模式下，输出首先写入内部流缓冲区，直到缓冲区已满，然后将缓冲区刷新到输出。 （某些实现也可能在其他时间刷新输出，例如从交互式设备读取输入时。）
输入流也可以处于相同的三种模式，这决定了读取输入何时可供读取流的函数的调用者使用。
该实现在 [=15= 之前初始化标准输入 (stdin)、标准输出 (stdout) 和标准错误输出 (stderr) 流的模式] 函数被调用（或至少在访问流之前）。在某些情况下，允许实现将标准输入或标准输出初始化为 fully buffered 模式。当且仅当实现可以确定它们未链接到交互式设备（例如终端）时，标准输入和输出流才被初始化为完全缓冲模式。 （标准错误输出流从未初始化为完全缓冲模式。）
通常，POSIX 系统的 C 运行time 库将调用 isatty 标准输入和输出流的底层文件描述符，并将流设置为完全缓冲mode if isatty returns 0。这发生在调用 main 函数之前。
当您 运行 "./a.out" 将输出发送到终端时，C 运行time 库确定输出将发送到交互式设备并执行 不 将stdout 设置为完全缓冲模式。它将被设置为其他模式之一，通常是行缓冲模式。然而，当你 运行 "/a.out" 输出到管道时，C 运行time 库确定输出是 not交互式设备并且确实将stdout设置为完全缓冲模式。这就是输出没有立即写入管道的原因。
有两种方法可以解决您的问题。第一种是在第一次调用 printf:[=22 之前将标准输出流更改为 line buffered 模式或 unbuffered 模式=]
setvbuf(stdout, NULL, _IOLBF, 0); // set standard output to line buffered mode
setvbuf(stdout, NULL, _IONBF, 0); // set standard output to unbuffered mode
另一种方法是按需刷新标准输出：
fflush(stdout); // write buffered standard output contents