Thread: trying to understand a program via GDB

I'm working through C programming: Modern Approach by K.N. King

justify is a program that can correctly format (spacing) any sort of text. So if you write something like "hello space space space space space world" it will output "hello world". (cannot put multiple spaces as this forum is fixing it )

The program contains three .c files:
justify.c
line.c
word.c

In order to run this program, the following is done
./justify <quote

Quote is a text file containing sentences that are not formatted correctly. When you redirect the quote file into the justify program, the output is formatted with correct spacing.

I want to analyse the program via GDB but I'm having difficulties bringing the file quote into gdb.

What I tried:

Code:

gcc -g -o justify.c line.c word.c justify
gdb ./justify
run < quote

after I type run < quote, program doesn't actually run, I just see this:

(gdb) run < quote
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /Users/prakagan/c_king/writing_large_programs/justify < quote
[New Thread 0x2303 of process 17155]
warning: unhandled dyld version (15)

> In order to run this program, the following is done
> ./justify <quote
TBH, I would add a second option to justify, namely to open the file supplied as a command line argument.

As in
./justify quote

Code:

int main ( int argc, char *argv[] ) {
  FILE *in = stdin;  // by default
  if ( argc > 1 ) {
    in = fopen(argv[1],"r");
  }

  // now you read from stdin or a file
  char buff[BUFSIZ];
  while ( fgets( buff, BUFSIZ, in ) ) {
  }
}

To do what you want, you would have to keep editing your quote file to intermix all the gdb commands with all the actual program input.

As soon as you hit a breakpoint, gdb is just going to try to interpret the rest of the input file as gdb commands, and that just isn't going to work.

It's fine to share stdin between the program and gdb in an interactive session, because you know at each keystroke whether you're talking to the program or gdb.

No problems here:

Code:

$ cc -g -o test test.c
$ echo 'test' > input
$ gdb test
Reading symbols from test...done.
(gdb) r < input
Starting program ./test < input
test
[Inferior 1 (process 8995) exited normally]

Test program:

Code:

#include <stdio.h>

int main( void )
{
  char buffer[128];

  scanf( "%s", buffer );
  puts( buffer );
}