C/C++: Get the size of a file in bytes

 in C / C++  tagged C/C++ / C++ by

The following function accepts the name of a file as a string and returns the size of the file in bytes. If for any reason it cannot get the file information, it will return the value -1.

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In our header file, we used the following pre-processor directives around our declarations

#ifdef __cplusplus
extern "C" {
#endif
#ifdef __cplusplus
}
#endif

to allow c++ code to call our c function.

The usage example code makes three tests:

  1. Getting the size of the currently executing binary, as it will have a non-zero size
  2. Getting the size of a non-existing file, to check that it will properly return -1
  3. Getting the size of an empty file, to be sure it is empty we create it right before the test

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Source file (file_helpers.c)

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#include <sys/stat.h>
#include "file_helpers.h"
 
//It will return the size of the file in bytes OR -1 in case that it cannot get any status information for it
off_t get_file_size(const char *filename) {
  //Specialised struct that can hold status attributes of files.
  struct stat st;
 
  //Gets file attributes for filename and puts them in the stat buffer.
  // Upon successful completion, it returns 0, otherwise and errno will be set to indicate the error.
  if (stat(filename, &st) == 0) {
    //Size of file, in bytes.
    return st.st_size;
  }
 
  return -1;
}

Header file (file_helpers.h)

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#ifndef GM_S_LITTLE_HELPERS_FILE_HELPERS_H
#define GM_S_LITTLE_HELPERS_FILE_HELPERS_H
 
#ifdef __cplusplus
extern "C" {
#endif
 
off_t get_file_size(const char *filename);
 
#ifdef __cplusplus
}
#endif
 
#endif //GM_S_LITTLE_HELPERS_FILE_HELPERS_H

Usage example (main.cpp)

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#include <iostream>
#include "file_helpers.h"
 
void print_file_size(const char *filename) {
  const off_t size_of_file = get_file_size(filename);
  if (size_of_file > 0) {
    printf("The size of '%s' is %zd bytes\n", filename, size_of_file);
  }
  else if (size_of_file == 0) {
    printf("The file '%s' is empty\n", filename);
  }
  else {
    printf("Could not get the status information for file '%s'\n", filename);
  }
}
 
int main(int argc, char *argv[]) {
 
  //Testing a non-zero sized file
  print_file_size(argv[0]);
  //Testing for a non-existing file
  print_file_size("some file that does not exist...");
  const char * filename = "/tmp/some_empty_file";
  //Creating an empty file
  FILE * fout = fopen(filename, "w");
  fclose(fout);
  //Testing for an empty file
  print_file_size(filename);
 
  return 0;
}

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