Why does write() print before printf() in output redirection?

Question

So I know printf() is higher level than write() and ends up using write(). Printf() is buffered and write() makes system calls.

Example 1, if I were to run a program with printf() before write() then it would output the value of printf() before the value of write().

Example 2, if I were to run the same program and have it go through output redirection into a file, the value of write() outputs before printf().

#include <stdio.h>
#include <unistd.h>

int main()
{
    printf("This is a printf test\n");
    write(STDOUT_FILENO, "This is a write test\n", 21);
    return 0;
}

I don't understand what is happening here. In example 1, is the program waiting for printf()s output before running write()? In example 2, is the program redirecting the first output that is ready? And because write() is lower level, and does not need to buffer like printf() then it is printed first?

Answer 1

You answered your own question.

printf is buffered and write is not.

For output to a terminal, the C stdio system has a feature that it flushes the buffers whenever it sees a newline '\n'. For more about stdio buffering look at the documentation for setvbuf.

But when output is to a file to increase speed the stdio system does not flush the buffer. That is why write output appears first.

Here is some of the strace output from running on my Linux system:

fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f7880b41000
write(1, "This is a printf test\n", 22) = 22
write(1, "This is a write test\n\0", 22) = 22

The fstat is where the stdio system detects the type of output file descriptor 1 is connected to. I believe it looks at st_mode and sees that it is a character device. A disk file would be a block device. The mmap is the memory allocation for the stdio buffer, which is 4K. Then the output is written.