potential dangers of merging two files of unknown size?

I have a binary file that I need to insert a header at the beginning of. I was thinking of opening a new file, writing the header data, and then copying the data from the binary fi开发者_StackOverflow社区le to this new file. Since the binary file is about 1 megabyte, are there any dangers to making this file using fwrite? One specific concern would be something like unintentionally overwriting data, similar to what happens if using gets and the input is longer than the buffer.

There's no risk. Allocate a buffer of a given size, read that many bytes into it from the source file, write the buffer back out to the destination file. The operations (file read / file write) all take a maximum number of bytes so as long as your buffer is the size you claim it is, it won't be overrun.

Also, the approach you describe is pretty much the only way to do it. I've never heard of a filesystem that has an "insert these bytes at the beginning of this file" operation.