Read from file or stdin

Question

I am writing a utility which accepts either a filename, or reads from stdin.

I would like to know the most robust / fastest way of checking to see if stdin exists (d

Answer 1

First, ask the program to tell you what is wrong by checking the errno, which is set on failure, such as during fseek or ftell.

Others (tonio & LatinSuD) have explained the mistake with handling stdin versus checking for a filename. Namely, first check argc (argument count) to see if there are any command line parameters specified if (argc > 1), treating - as a special case meaning stdin.

If no parameters are specified, then assume input is (going) to come from stdin, which is a stream not file, and the fseek function fails on it.

In the case of a stream, where you cannot use file-on-disk oriented library functions (i.e. fseek and ftell), you simply have to count the number of bytes read (including trailing newline characters) until receiving EOF (end-of-file).

For usage with large files you could speed it up by using fgets to a char array for more efficient reading of the bytes in a (text) file. For a binary file you need to use fopen(const char* filename, "rb") and use fread instead of fgetc/fgets.

You could also check the for feof(stdin) / ferror(stdin) when using the byte-counting method to detect any errors when reading from a stream.

The sample below should be C99 compliant and portable.

#include 
#include 
#include 
#include 

long getSizeOfInput(FILE *input){
   long retvalue = 0;
   int c;

   if (input != stdin) {
      if (-1 == fseek(input, 0L, SEEK_END)) {
         fprintf(stderr, "Error seek end: %s\n", strerror(errno));
         exit(EXIT_FAILURE);
      }
      if (-1 == (retvalue = ftell(input))) {
         fprintf(stderr, "ftell failed: %s\n", strerror(errno));
         exit(EXIT_FAILURE);
      }
      if (-1 == fseek(input, 0L, SEEK_SET)) {
         fprintf(stderr, "Error seek start: %s\n", strerror(errno));
         exit(EXIT_FAILURE);
      }
   } else {
      /* for stdin, we need to read in the entire stream until EOF */
      while (EOF != (c = fgetc(input))) {
         retvalue++;
      }
   }

   return retvalue;
}

int main(int argc, char **argv) {
   FILE *input;

   if (argc > 1) {
      if(!strcmp(argv[1],"-")) {
         input = stdin;
      } else {
         input = fopen(argv[1],"r");
         if (NULL == input) {
            fprintf(stderr, "Unable to open '%s': %s\n",
                  argv[1], strerror(errno));
            exit(EXIT_FAILURE);
         }
      }
   } else {
      input = stdin;
   }

   printf("Size of file: %ld\n", getSizeOfInput(input));

   return EXIT_SUCCESS;
}