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How can we know the name of the stream to which output is being redirected through a C++ program?
For example, I have a program 1.cpp whose output is being redirected to a file a.txt
i.e., a.out > a.txt
Now, I want to know the name of the file to which output is being redirected in the program itself.
How can I do that?

Thanks in advance.

Edited by rtyui
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As asked: a program cannot be guaranteed to know a redirected filename because the shell/OS arranges descriptors for the child. As pointed out, pipelines and utilities (tee, sockets, deleted files, etc.) mean there may be no single, recoverable pathname — only best‑effort, platform‑specific queries are possible.

On Linux the pragmatic sequence is: check whether stdout is a terminal (isatty), use fstat on file descriptor 1 to see if it’s a regular file, and — if so — read the /proc/self/fd/1 symlink. This is a best‑effort trick (it will return things like "pipe:[12345]" for pipes, and may show "(deleted)" if the file was unlinked). Example (minimal):

#include <unistd.h>
#include <limits.h>
#include <sys/stat.h>
#include <stdio.h>

struct stat st;
if (!isatty(STDOUT_FILENO) && fstat(STDOUT_FILENO, &st) == 0 && S_ISREG(st.st_mode)) {
    char buf[PATH_MAX];
    ssize_t n = readlink("/proc/self/fd/1", buf, sizeof(buf) - 1);
    if (n > 0) { buf[n] = '\0'; printf("stdout -> %s\n", buf); }
}

On macOS/BSD use the platform facility instead of /proc: fcntl(fd, F_GETPATH, buf) (or inspect /dev/fd/1) to obtain the path when available. On Windows, obtain the HANDLE from the CRT (_get_osfhandle) or GetStdHandle, check the handle type (GetFileType) and, for disk files, call GetFinalPathNameByHandle to get a filename (it may use the device namespace like \\?\).

Important caveats: these are heuristics, not guaranteed. Pipes, redirection through other processes, network sockets, or files unlinked after open will break the result. The most robust design is to have the caller supply the output path explicitly (command argument or environment variable) or have the program open the output file itself so the name is known reliably.

Member Avatar for L7Sqr

No. Redirection, piping and general screen printing are facilities of the OS, not your program. Additionally, there are other tools, such as tee which would complicate the matter if you could. Suppose you had ./a.out | tee a.txt would you want that tee was receiving your output or that it was ultimately sent to a.txt?

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