How to print a line to STDERR and STDOUT in C++?
Posted on In QAIn C++, how to print a string as a line to STDOUT? That is, the string and the newline character, nicely?
And similarly, how to print the line to STDERR?
In C++, you may print the string and then 'n' or std::endl to STDOUT by operating on the std::cout stream:
std::cout << your_string << std::endl;
or
std::cout << your_string << 'n';
Example:
$ cat a.cpp
#include <iostream>
int main()
{
std::cout << "hello world!" << std::endl;
std::cout << "happy Dtivl!" << 'n';
return 0;
}
$ g++ a.cpp -o a && ./a
hello world!
happy Dtivl!
In C++, std::cerr is a stream to the STDERR.
You can use the common I/O operators like << or std::cerr to print content to the STDERR.
One example is in stderr.cc:
#include <iostream>
int main()
{
std::cerr << "hello world!n";
}
Built and run it:
$ g++ stderr.cc -o s && ./s
hello world!
Title should be “How to print a line to stderr OR stdout in C++?” as I was looking for how to print to both at the same time.
I guess the “and” in the title should be the “and” in “buy an apple and a banana” rather than the “&&” in “a && b” ;-)
If you want to print to STDOUT && STDERR **atomically**, it will be an interesting problem. The operations of printing a line to STDOUT or STDERR themselves are not atomic operations.
One way that is close to that may be to use a producer/consumer like printing mechanism:
– one buffer using an array like data structure for holding each item/line to be printed out and 2 status marks for each item
– 2 threads waiting for the buffer content and print them to STDOUT and STDERR each; after prints it, the thread sets that corresponding buffer status
– 1 cleaner thread to clean the items if both statuses are set
The printing function writes the item/line to be printed out to the buffer.
The outputs to STDERR and STDOUT may run in parallel though not strictly synchronized or atomically.
In Linux, you can use output redirect, for example,
https://unix.stackexchange.com/questions/20469/difference-between-21-output-log-and-21-tee-output-log