PostgreSQL psql Error: Library Not Loaded /usr/local/opt/readline/lib/libreadline.6.2.dylib

• Library not loaded: This indicates that a required library, which is a piece of software code, is missing or cannot be found by the program.
• /usr/local/opt/readline/lib/libreadline.6.2.dylib: This specifies the path and filename of the missing library.
  • /usr/local/opt/readline/lib/: This directory likely contains libraries associated with a software package named readline.
  • libreadline.6.2: This is the name of the library itself, likely version 6.2.
  • .dylib: This extension is common for dynamic libraries on macOS. These libraries are loaded at runtime by the program.

What is readline and Why is it Needed?

• Readline is a software library that provides a set of functions for editing command lines in a more user-friendly way.
• It allows features like history, command completion, and arrow key navigation, making the command-line interface (CLI) more interactive.
• Psql, the PostgreSQL interactive terminal program, likely uses readline for these editing functionalities.

Potential Causes and Solutions:

• Missing readline library: The library might not be installed on your system. You can install it using your system's package manager.
• Version Mismatch: The error message specifies that psql is looking for version 6.2 of the readline library, but you might have a different version installed (e.g., 7.0).
  • In this case, reinstalling the PostgreSQL client library with readline support (if available) might resolve the issue. This would ensure compatibility between psql and the installed readline library.
• Incorrect Linking: There might be an issue with how psql is linked to the readline library. Refer to the PostgreSQL documentation for troubleshooting steps specific to your system and configuration.

#include <stdio.h>
#include <readline/readline.h>
#include <readline/history.h>

int main() {
  char *line;

  while ((line = readline("psql > ")) != NULL) {
    if (strlen(line) > 0) {
      add_history(line); // Add line to history
    }
    printf("You entered: %s\n", line);
    free(line); // Free memory allocated by readline
  }

  return 0;
}

This code demonstrates using readline to get user input with editing features. It includes:

• Including the necessary readline headers.
• Using readline("psql > ") to prompt the user and read the line.
• Checking if the line is empty and adding it to history if not.
• Printing the entered line for demonstration.
• Freeing the memory allocated by readline.

Disabling Filename Completion (C):

#include <stdio.h>
#include <readline/readline.h>

int main(int argc, char *argv[]) {
  // Disable filename completion if -d argument is provided
  if (argc > 1 && strcmp(argv[1], "-d") == 0) {
    rl_bind_key('\t', rl_insert); // Insert TAB character instead of completion
  }

  char *line;
  while ((line = readline("psql > ")) != NULL) {
    // ... (rest of your code)
  }

  return 0;
}

This code snippet shows how to potentially address a version mismatch issue. By checking for a -d flag and disabling filename completion (a feature that might rely on a specific readline version), you might avoid conflicts with your installed library.

If the readline library is missing entirely, reinstalling it might be the simplest solution. The specific method depends on your system's package manager.

• On macOS with Homebrew:

brew reinstall readline

• On Debian/Ubuntu:

sudo apt install libreadline-dev

• On other systems:

Consult your system's documentation for installing the readline development libraries.

Reinstall PostgreSQL client with readline support (if available):

If the issue is due to a version mismatch between psql and the installed readline library, reinstalling the PostgreSQL client libraries with readline support (if available) might help.

brew reinstall postgresql --with- readline

This flag (--with-readline) tells Homebrew to configure PostgreSQL to use the available readline library during installation.

The specific flag or option for enabling readline support during PostgreSQL client installation might vary. Refer to your system's package manager documentation or the PostgreSQL installation guide.

Update PostgreSQL (if applicable):

Sometimes, newer versions of PostgreSQL might be compiled against a more recent version of readline. Upgrading PostgreSQL could potentially resolve the issue if the version mismatch is causing the problem.

brew upgrade postgresql

Consult your system's package manager documentation for upgrading PostgreSQL.

Manually Specify Library Path (Advanced):

This approach is for experienced users and should be attempted with caution as modifying environment variables can have unintended consequences. If the library exists in a non-standard location, you can try setting the LD_LIBRARY_PATH environment variable to include the directory containing libreadline.6.2.dylib.

Here's a warning: Setting LD_LIBRARY_PATH can interfere with other programs that rely on finding libraries in standard locations. It's generally recommended to fix the root cause (missing library or incompatibility) instead of relying on this method.

Check for Conflicting Readline Versions:

If you have multiple versions of readline installed on your system, it might lead to conflicts. Try identifying any conflicting versions and removing them if possible. This could involve using your system's package manager to remove unnecessary readline installations.