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  data is being piped to the program  and if so reading that data in   If it doesn t exist  the processing will take place on the filename given   I have tried using the following the test for size of stdin but I believe since it s a stream and not an actual file  it s not working as I suspected it would and it s always printing -1   I know I could always read the input 1 character at a time while    EOF but I would like a more generic solution so I could end up with either a fd or a FILE  if stdin exists so the rest of the program will function seamlessly   I would also like to be able to know its size  pending the stream has been closed by the previous program   long getSizeOfInput FILE  input     long retvalue   0    fseek input  0L  SEEK END     retvalue   ftell input     fseek input  0L  SEEK SET     return retvalue     int main int argc  char   argv      printf  Size of stdin   ld n   getSizeOfInput stdin      exit 0       Terminal     echo  hi     myprog Size of stdin  -1

User · Answer

You re thinking it wrong   What you are trying to do   If stdin exists use it  else check whether the user supplied a filename   What you should be doing instead   If the user supplies a filename  then use the filename  Else use stdin   You cannot know the total length of an incoming stream unless you read it all and keep it buffered  You just cannot seek backwards into pipes  This is a limitation of how pipes work  Pipes are not suitable for all tasks and sometimes intermediate files are required

User · Answer

You may want to look at how this is done in the cat utility  for example   See code here  If there is no filename as argument  or it is  -   then stdin is used for input  stdin will be there  even if no data is pushed to it  but then  your read call may wait forever

User · Answer

You can just read from stdin unless the user supply a filename    If not  treat the special  filename  - as meaning  read from stdin   The user would have to start the program like cat file   myprogram - if he wants to pipe data to it  and myprogam file if he wants it to read from a file   int main int argc char  argv         FILE  input    if argc    2         usage         return 1          if  strcmp argv 1   -           input   stdin        else         input   fopen argv 1   rb            check for errors         If you re on  nix  you can check whether stdin is a fifo    struct stat st info   if fstat 0  amp st info     0       error       if S ISFIFO st info st mode            stdin is a pipe       Though that won t handle the user doing myprogram  lt file  You can also check if stdin is a terminal console  if isatty 0         stdin is a terminal

User · Answer

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  amp  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  gt  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  lt stdio h gt   include  lt stdlib h gt   include  lt errno h gt   include  lt string h gt   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  gt  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

User · Answer

Just testing for end of file with feof would do  I think

User · Answer

Note that what you want is to know if stdin is connected to a terminal or not  not if it exists  It always exists but when you use the shell to pipe something into it or read a file  it is not connected to a terminal   You can check that a file descriptor is connected to a terminal via the termios h functions    include  lt termios h gt   include  lt stdbool h gt   bool stdin is a pipe void        struct termios t      return  tcgetattr STDIN FILENO   amp t   lt  0       This will try to fetch the terminal attributes of stdin  If it is not connected to a pipe  it is attached to a tty and the tcgetattr function call will succeed  In order to detect a pipe  we check for tcgetattr failure

[c] Read from file or stdin

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