$ init --reference_
FixerCode Error Code Reference
Find your error code.
An independent reference for HTTP status codes, web errors, operating-system errors, Git errors, and software error codes — concise explanations of what each code means, where it appears, and related context.
[01]OS
Independent reference for operating-system error codes, process exit codes, kernel signals, and errno constants from Linux and Windows with clear explanations.
[02]Software
Independent reference for software error codes and messages from developer tools like Git and SQL Server with clear explanations, common causes, and context.
[03]Web
Independent reference for HTTP status codes, Nginx web server errors, Cloudflare CDN edge errors, Chromium network errors, and CORS cross-origin policy errors with clear explanations, common causes, and context.
[01]Most Common Error Codes
High severity first — the most frequently searched error codes
Process Killed — the process was terminated by a SIGKILL signal
Signal 11 (SIGSEGV) — kernel signal for invalid memory reference segmentation fault
Exit Code 139 — process terminated by segmentation fault (Signal 11 / SIGSEGV)
Input/output error EIO — a read or write operation could not complete
Out of Memory (ENOMEM) — the kernel cannot allocate the requested amount of memory
Illegal Instruction — the CPU reached an opcode it cannot decode or execute
[02]Common Error Code Comparisons
Similar codes explained side by side — the differences that matter most
404 vs 410
Not Found vs GoneThe server cannot find the resource at this URL. It may or may not have existed before.
The resource was intentionally removed. The server signals deliberate and lasting retirement.
401 vs 403
Unauthorized vs ForbiddenValid authentication credentials are missing. The server does not know who you are.
The server knows who you are but denies access. Authentication will not help.
502 vs 503 vs 504
Gateway ErrorsThe proxy received an invalid response from the upstream server.
The server is temporarily overloaded or under maintenance.
The upstream server did not respond within the timeout window.
[03]Understanding Error Codes
Error codes are short identifiers that systems, protocols, and applications produce when something goes wrong. They range from three-digit HTTP status codes like 404 and 500 that govern web communication, to process exit codes like 137 that indicate a Linux process was killed by a signal, to structured HRESULT values like 0x80070005 that Windows uses for access-denied errors.
The challenge with error codes is not finding them — logs, browsers, and terminal output surface them constantly. The challenge is understanding what each code means in context, what commonly causes it, and how it differs from related codes that look similar but carry different semantics. A 404 and a 410 both indicate a missing resource, but only one signals deliberate removal. A 502 and a 504 both involve gateway failures, but one means the upstream returned garbage and the other means it never responded at all.
FixerCode organizes 279+ error codes across 3 categories — web protocols, operating systems, and software tools — with each page providing a concise explanation, common causes, typical scenarios, and links to related codes. The reference is independent and not affiliated with any vendor, manufacturer, or standards body mentioned on the site.
[04]Frequently Asked Questions
An error code is a short identifier — numeric, alphanumeric, or text — that a system, protocol, or application produces when something goes wrong. FixerCode documents 279+ error codes across web protocols, operating systems, and developer tools, with each page explaining what the code means, common causes, and related context.
4xx codes indicate client errors — the request was invalid, unauthorized, or malformed, and the client needs to fix the request before retrying. 5xx codes indicate server errors — the server failed to fulfill a valid request, and the client may retry without changes.
A 404 means the server cannot find the resource at the requested URL — the resource may or may not have existed before. A 410 means the resource was intentionally removed and is not expected to return. Search engines treat 410 as a stronger retirement signal than 404.
502 Bad Gateway means a proxy or gateway received an invalid response from the upstream server. 503 Service Unavailable means the server is temporarily overloaded or under maintenance. 504 Gateway Timeout means the upstream server did not respond within the timeout window.