Document ERR library.

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diff --git a/doc/crypto/err.pod b/doc/crypto/err.pod
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+=pod
+
+=head1 NAME
+
+err - Error codes
+
+=head1 SYNOPSIS
+
+ #include <openssl/err.h>
+
+ unsigned long ERR_get_error(void);
+ unsigned long ERR_peek_error(void);
+ unsigned long ERR_get_error_line(const char **file, int *line);
+ unsigned long ERR_peek_error_line(const char **file, int *line);
+ unsigned long ERR_get_error_line_data(const char **file, int *line,
+         const char **data, int *flags);
+ unsigned long ERR_peek_error_line_data(const char **file, int *line,
+         const char **data, int *flags);
+
+ int ERR_GET_LIB(unsigned long e);
+ int ERR_GET_FUNC(unsigned long e);
+ int ERR_GET_REASON(unsigned long e);
+
+ void ERR_clear_error(void);
+
+ char *ERR_error_string(unsigned long e, char *buf);
+ const char *ERR_lib_error_string(unsigned long e);
+ const char *ERR_func_error_string(unsigned long e);
+ const char *ERR_reason_error_string(unsigned long e);
+
+ void ERR_print_errors(BIO *bp);
+ void ERR_print_errors_fp(FILE *fp);
+
+ void ERR_load_crypto_strings(void);
+ void ERR_free_strings(void);
+
+ void ERR_remove_state(unsigned long pid);
+
+ void ERR_put_error(int lib, int func, int reason, const char *file,
+         int line);
+ void ERR_add_error_data(int num, ...);
+
+ void ERR_load_strings(int lib,ERR_STRING_DATA str[]);
+ unsigned long ERR_PACK(int lib, int func, int reason);
+ int ERR_get_next_error_library(void);
+
+=head1 DESCRIPTION
+
+When a call to the OpenSSL library fails, this is usually signalled
+by the return value, and an error code is stored in an error queue
+associated with the current thread. The B<err> library provides
+functions to obtain these error codes and textual error messages.
+
+The L<ERR_get_error(3)|ERR_get_error(3)> manpage describes how to
+access error codes.
+
+Error codes contain information about where the error occurred, and
+what went wrong. L<ERR_GET_LIB(3)|ERR_GET_LIB(3)> describes how to
+extract this information. A method to obtain human-readable error
+messages is described in L<ERR_error_string(3)|ERR_error_string(3)>.
+
+L<ERR_clear_error(3)|ERR_clear_error(3)> can be used to clear the
+error queue.
+
+Note that L<ERR_remove_state(3)|ERR_remove_state(3)> should be used to
+avoid memory leaks when threads are terminated.
+
+=head1 ADDING NEW ERROR CODES TO OPENSSL
+
+See L<ERR_put_error(3)> if you want to record error codes in the
+OpenSSL error system from within your application.
+
+The remainder of this section is of interest only if you want to add
+new functionality to OpenSSL.
+
+=head2 Reporting errors
+
+Each sub-library has a specific macro XXXerr() that is used to report
+errors. Its first argument is a function code B<XXX_F_...>, the second
+argument is a reason code B<XXX_R_...>. Function codes are derived
+from the function names; reason codes consist of textual error
+descriptions. For example, the function ssl23_read() reports a
+"handshake failure" as follows:
+
+ SSLerr(SSL_F_SSL23_READ, SSL_R_SSL_HANDSHAKE_FAILURE);
+
+When you are using new function or reason codes, run B<make errors>.
+The necessary B<#define>s will then automatically be added to the
+sub-library's header file.
+
+=head2 Adding new libraries
+
+When adding a new sub-library to OpenSSL, assign it a library number
+B<ERR_LIB_XXX>, define a macro XXXerr() (both in B<err.h>), add its
+name to B<ERR_str_libraries[]> (in B<crypto/err/err.c>), and add
+C<ERR_load_XXX_strings()> to the ERR_load_crypto_strings() function
+(in B<crypto/err/err_all.c>). Finally, add an entry
+
+ L	XXX	xxx.h	xxx_err.c
+
+to B<crypto/err/openssl.ec>, and add B<xxx_err.c> to the Makefile.
+Running B<make errors> will then generate a file B<xxx_err.c>, and
+add all error codes used in the library to B<xxx.h>.
+
+=head1 INTERNALS
+
+The error queues are stored in a hash table with one B<ERR_STATE>
+entry for each pid. ERR_get_state() returns the current thread's
+B<ERR_STATE>. An B<ERR_STATE> can hold up to B<ERR_NUM_ERRORS> error
+codes. When more error codes are added, the old ones are overwritten,
+on the assumption that the most recent errors are most important.
+
+Error strings are also stored in hash table. The hash tables can
+be obtained by calling ERR_get_err_state_table(void) and
+ERR_get_string_table(void) respectively.
+
+=head1 SEE ALSO
+
+L<ERR_get_error(3)|ERR_get_error(3)>,
+L<ERR_GET_LIB(3)|ERR_GET_LIB(3)>,
+L<ERR_clear_error(3)|ERR_clear_error(3)>,
+L<ERR_error_string(3)|ERR_error_string(3)>,
+L<ERR_print_errors(3)|ERR_print_errors(3)>,
+L<ERR_load_crypto_strings(3)|ERR_load_crypto_strings(3)>,
+L<ERR_remove_state(3)|ERR_remove_state(3)>,
+L<ERR_put_error(3)|ERR_put_error(3)>,
+L<ERR_load_strings(3)|ERR_load_strings(3)>,
+L<SSL_get_error(3)|SSL_get_error(3)>
+
+=cut