Overwrite file using append mode for fopen

Question

I am using fopen to write to a binary file and using the cstdio (stdio.h) library due to legacy code and it must be cross-platform compatible with Windows and Linux.

For the prototype, FILE * fopen ( const char * filename, const char * mode );, I am using const char * mode = "ab", which will append to a binary file. Writing operations append data at the end of the file. The file is created if it does not exist.

I have N number of input files where I process data from and write to one output file for each type, where I have M types. I process one input file and write the data to each corresponding output file. I then will close that ith input file and open (i + 1)th, and repeat the process by appending the data from the input file to the output files.

If an output file exists at the beginning on the executable, I want it deleted. If it exists and I don't delete it, then when I use the "wb" mode, then it will just append data to the output file, which will result in duplication I don't want. I am open to a boost solution and I like to maintain standards best as possible (i.e avoid POSIX if possible)

fopen C++ reference

Answer 1

If you want to overwrite rather than append, why not just use the mode "wb"? "w" overwrites the file when writing.

Answer 2

Here is one way

char* infile[N] = //input names
char* outfile[M] = //output names

int i, j;
for (i = 0; i < N; i++){
  //process input
  char* mode = "ab";
  if (i == 0) mode = "wb";
  for (j = 0; j < M; j++){
    FILE* f = fopen(outfile[j], mode);
    //write to file
    fclose(f);
  }
}

The "w" mode should overwrite the file. It is "a" mode that will avoid deleting a file that already exists.

EDIT: You can also remove (const char * filename) if you want to delete the files at the beginning of execution. If that's the case then you never have to use the "wb" mode.

Answer 3

One possibility would be to use open (_open for Windows) to create the appropriate file handle and then use fdopen (_fdopen for Windows) to create a stdio handle out of it. You will need some preprocessor magic to handle the fact that the names are not exactly the same in Linux and Windows:

// Allow us to use Posix compatible names on Windows
#ifdef WINDOWS
#define open      _open
#define O_CREAT   _O_CREAT
#define O_TRUNC   _O_TRUNC
#define O_APEND   _O_APPEND
#define S_IREAD   _S_IREAD
#define S_IWRITE  _S_IWRITE
#define fdopen    _fdopen
#endif

int fd = open(filename, O_CREAT | O_TRUNC | O_APPEND, S_IREAD | S_IWRITE);
FILE *fp = fdopen(fd, "a");