The X509_verify_cert() function attempts to discover and validate a certificate chain based on parameters in ctx. A complete description of the process is contained in the verify(1) manual page.
If a complete chain can be built and validated this function returns 1, otherwise it return zero, in exceptional circumstances it can also return a negative code.
If the function fails additional error information can be obtained by examining ctx using, for example X509_STORE_CTX_get_error().
Applications rarely call this function directly but it is used by OpenSSL internally for certificate validation, in both the S/MIME and SSL/TLS code.
A negative return value from X509_verify_cert() can occur if it is invoked incorrectly, such as with no certificate set in ctx, or when it is called twice in succession without reinitialising ctx for the second call. A negative return value can also happen due to internal resource problems or if a retry operation is requested during internal lookups (which never happens with standard lookup methods). Applications must check for <= 0 return value on error.
This function uses the header x509.h as opposed to most chain verification functiosn which use x509_vfy.h.