Easiest way to get file's contents in C

Question

What is the simplest way (least error-prone, least lines of code, however you want to interpret it) to open a file in C and read its contents into a string (char*, char[], whatever)?

Answer 1

I tend to just load the entire buffer as a raw memory chunk into memory and do the parsing on my own. That way I have best control over what the standard lib does on multiple platforms.

This is a stub I use for this. you may also want to check the error-codes for fseek, ftell and fread. (omitted for clarity).

char * buffer = 0;
long length;
FILE * f = fopen (filename, "rb");

if (f)
{
  fseek (f, 0, SEEK_END);
  length = ftell (f);
  fseek (f, 0, SEEK_SET);
  buffer = malloc (length);
  if (buffer)
  {
    fread (buffer, 1, length, f);
  }
  fclose (f);
}

if (buffer)
{
  // start to process your data / extract strings here...
}

Answer 2

"simplest way" and "least error-prone" are often opposites of each other.

Answer 3

If "read its contents into a string" means that the file does not contain characters with code 0, you can also use getdelim() function, that either accepts a block of memory and reallocates it if necessary, or just allocates the entire buffer for you, and reads the file into it until it encounters a specified delimiter or end of file. Just pass '\0' as the delimiter to read the entire file.

This function is available in the GNU C Library, http://www.gnu.org/software/libc/manual/html_mono/libc.html#index-getdelim-994

The sample code might look as simple as

char* buffer = NULL;
ssize_t bytes_read = getdelim( &buffer, 0, '\0', fp);
if ( bytes_read != -1) {
  /* Success, now the entire file is in the buffer */

Answer 4

Another, unfortunately highly OS-dependent, solution is memory mapping the file. The benefits generally include performance of the read, and reduced memory use as the applications view and operating systems file cache can actually share the physical memory.

POSIX code would look like:

    int fd = open("filename", O_RDONLY);
    int len = lseek(fd, 0, SEEK_END);
    void *data = mmap(0, len, PROT_READ, MAP_PRIVATE, fd, 0);

Windows on the other hand is little more tricky, and unfortunately I don't have a compiler in front of me to test, but the functionality is provided by CreateFileMapping() and MapViewOfFile().

Answer 5

If the file is text, and you want to get the text line by line, the easiest way is to use fgets().

char buffer[100];
FILE *fp = fopen("filename", "r");                 // do not use "rb"
while (fgets(buffer, sizeof(buffer), fp)) {
... do something
}
fclose(fp);