while compiling app am getting the error like this - iphone

Question

This is the error log, which I got
Ld /Users/apple/Library/Developer/Xcode/DerivedData/SMTPExample-dkhosyetbsajvtcdyyalnckswjgd/Build/Products/Debug-iphonesimulator/SMTPExample.app/SMTPExample normal i386
cd /Users/apple/Desktop/SMTPExample
setenv MACOSX_DEPLOYMENT_TARGET 10.6
setenv PATH "/xcode4/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/xcode4/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
/xcode4/Platforms/iPhoneSimulator.platform/Developer/usr/bin/llvm-gcc-4.2 -arch i386 -isysroot /xcode4/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.3.sdk -L/Users/apple/Library/Developer/Xcode/DerivedData/SMTPExample-dkhosyetbsajvtcdyyalnckswjgd/Build/Products/Debug-iphonesimulator -F/Users/apple/Library/Developer/Xcode/DerivedData/SMTPExample-dkhosyetbsajvtcdyyalnckswjgd/Build/Products/Debug-iphonesimulator -F/Users/apple/Desktop/SMTPExample -filelist /Users/apple/Library/Developer/Xcode/DerivedData/SMTPExample-dkhosyetbsajvtcdyyalnckswjgd/Build/Intermediates/SMTPExample.build/Debug-iphonesimulator/SMTPExample.build/Objects-normal/i386/SMTPExample.LinkFileList -mmacosx-version-min=10.6 -Xlinker -objc_abi_version -Xlinker 2 -framework Security -framework CFNetwork -framework UIKit -framework Foundation -framework CoreGraphics -framework DropboxSDK -o /Users/apple/Library/Developer/Xcode/DerivedData/SMTPExample-dkhosyetbsajvtcdyyalnckswjgd/Build/Products/Debug-iphonesimulator/SMTPExample.app/SMTPExample
ld: duplicate symbol _EstimateBas64EncodedDataSize in /Users/apple/Desktop/SMTPExample/DropboxSDK.framework/DropboxSDK and /Users/apple/Library/Developer/Xcode/DerivedData/SMTPExample-dkhosyetbsajvtcdyyalnckswjgd/Build/Intermediates/SMTPExample.build/Debug-iphonesimulator/SMTPExample.build/Objects-normal/i386/Base64Transcoder.o for architecture i386
collect2: ld returned 1 exit status
Command /xcode4/Platforms/iPhoneSimulator.platform/Developer/usr/bin/llvm-gcc-4.2 failed with exit code 1

Answer 1

This is a linker error (the "ld" in the output indicates this, because the output error has been generated by the GNU linker program).
What has happened is that two identical declarations have been compiled separately from two separate source code files. One of these source files might be named Base64Transcoder.m or Base64Transcoder.cpp (as the compiled file that's caused the problem is Base64Transcoder.o). The declaration in question is _EstimateBas64EncodingDataSize. This symbol is being declared multiple times within different source code files, and then the linker is throwing an error because the two compiled source code files are different. You need to have a look through the entire source code that's being compiled and resolve the conflict.
Hope this helps and makes sense, I've always found linker issues difficult to understand and deal with. I'd really strongly suggest reading about the linker stages of program compilation for C/C++/Objective-C code (this is a very good guide). Then you should be able to better understand what's going on under the hood! :)