X-Git-Url: https://git.openssl.org/gitweb/?a=blobdiff_plain;f=FAQ;h=a381d1d862d7c522cb84f6f46a2fdb033e6fd935;hb=51012a0969eb46b02078228d13eb7dc4783fb6ae;hp=4497b1c7f438126fc0aea76b162a69bc8c91b2b8;hpb=84b65340e1cf5b0c427d87e89f42382bff2a5b63;p=openssl.git

diff --git a/FAQ b/FAQ
index 4497b1c7f4..a381d1d862 100644
--- a/FAQ
+++ b/FAQ
@@ -10,6 +10,7 @@ OpenSSL  -  Frequently Asked Questions
 * Why does the linker complain about undefined symbols?
 * Where can I get a compiled version of OpenSSL?
 * I've compiled a program under Windows and it crashes: why?
+* How do I read or write a DER encoded buffer using the ASN1 functions?
 * I've tried using <M_some_evil_pkcs12_macro> and I get errors why?
 * I've called <some function> and it fails, why?
 * I just get a load of numbers for the error output, what do they mean?
@@ -182,6 +183,43 @@ otherwise the conflict will cause a program to crash: typically on the
 first BIO related read or write operation.
 
 
+* How do I read or write a DER encoded buffer using the ASN1 functions?
+
+You have two options. You can either use a memory BIO in conjunction
+with the i2d_XXX_bio() or d2i_XXX_bio() functions or you can use the
+i2d_XXX(), d2i_XXX() functions directly. Since these are often the
+cause of grief here are some code fragments using PKCS7 as an example:
+
+unsigned char *buf, *p;
+int len;
+
+len = i2d_PKCS7(p7, NULL);
+buf = OPENSSL_Malloc(len); /* or Malloc, error checking omitted */
+p = buf;
+i2d_PKCS7(p7, &p);
+
+At this point buf contains the len bytes of the DER encoding of
+p7.
+
+The opposite assumes we already have len bytes in buf:
+
+unsigned char *p;
+p = buf;
+p7 = d2i_PKCS7(NULL, &p, len);
+
+At this point p7 contains a valid PKCS7 structure of NULL if an error
+occurred. If an error occurred ERR_print_errors(bio) should give more
+information.
+
+The reason for the temporary variable 'p' is that the ASN1 functions
+increment the passed pointer so it is ready to read or write the next
+structure. This is often a cause of problems: without the temporary
+variable the buffer pointer is changed to point just after the data
+that has been read or written. This may well be uninitialized data
+and attempts to free the buffer will have unpredictable results
+because it no longer points to the same address.
+
+
 * I've tried using <M_some_evil_pkcs12_macro> and I get errors why?
 
 This usually happens when you try compiling something using the PKCS#12