EC
EBADF
Linux / POSIXERRORNotableFile SystemHIGH confidence

Bad File Descriptor

What this means

An operation was attempted on a file descriptor that is not open or is invalid. This commonly occurs after a double-close bug, using a descriptor after the file was closed, or using a read-only descriptor for writing.

Why it happens
  1. 1The file descriptor was already closed (double-close bug).
  2. 2The descriptor number is negative or beyond the process limit.
  3. 3Attempting to write to a file descriptor opened with O_RDONLY.
  4. 4Using a file descriptor after fork() in the child process when it was not meant to be inherited.
How to reproduce

Reading from a file descriptor after it has been closed.

trigger — this will error
trigger — this will error
int fd = open("/tmp/test.txt", O_RDONLY);
close(fd);
// fd is now invalid
char buf[64];
read(fd, buf, sizeof(buf)); // returns -1, errno = EBADF

expected output

read() returned -1, errno = EBADF (Bad file descriptor)

Fix

Set descriptor to -1 after closing

WHEN In C/C++ code that might close a descriptor and later reference it

Set descriptor to -1 after closing
close(fd);
fd = -1; // prevent accidental reuse

// Later:
if (fd >= 0) { read(fd, buf, n); }

Why this works

Setting fd to -1 after close makes subsequent validity checks fail safely instead of operating on a recycled descriptor.

Sources
Official documentation ↗

Linux Programmer Manual errno(3)

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