CODEWITHSUNDEEP
c++readfilefreadseek
Frank Escobar
Frank Escobar

Reputation: 376

Weird behaviour with fread

I'm reading a binary uint32_t data from a file that indicates the size of the next binary block, after that I read that block but the reading pointer is "moving" wrong.

FILE* file = fopen("file.zip", "r");
long pointerA = ftell(file);
uint32_t streamSize = 0;
fread(reinterpret_cast<char*>(&streamSize), sizeof streamSize,1,file);
long pointerB = ftell(file);
char* zipData = new char[streamSize];
fread(zipData, sizeof(char),streamSize,file);

long pointerC = ftell(file);
fseek( file, pointerA + 4 + streamSize, SEEK_SET );
long pointerD = ftell(file);
qDebug()<<"streamSize"<<streamSize<<"Positions"<<pointerA<<pointerB<<pointerC<<pointerD;

PointerA is the original position, PointerB the position after reading that uint32_t, PointerC is the pointer after reading all that binary data and PointerD is just a check about what I suppose that should be the right behaviour.

Now let's see the debug:

streamSize 2653 Positions 151 156 4627 2808

Why the stream read position has moved too 4627 instead 2808?

Thank you in advance for any tip!

Upvotes: 3

Views: 153

Answers (2)

Alan Birtles
Alan Birtles

Reputation: 36389

You need to open your file in binary mode. When a file is opened in text mode some characters are changed as you read them. For example on Windows when reading '\n' "\r\n" is returned. To open in binary mode add 'b' to your open mode, e.g:

FILE * file = fopen("file.txt", "rb");

Note you need to do the same when writing binary files otherwise the same transformations take place.

std::fstream also needs std::ios_base::binary to be passed to the constructor/open to avoid the same issue.

Upvotes: 2

Frank Escobar
Frank Escobar

Reputation: 376

Both users @alan-birtles and @remy-lebeau were right, I opened it as text instead binary that was the issue.

Unfortunatly I cannot mark this as solved.

PS. For begginers this means open file with "rb" instead "r".

Upvotes: 3

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