I was asked how to let a gdb controlled slave read from stdin, and couldn’t remember how it was done, so I wrote the following little test program, figuring that muscle memory would remind me once I had gdb running.

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#include <stdio.h>
 
int main()
{
   size_t rc = 0;
   char buf[2];
 
   do {
      rc = fread( buf, 1, 1, stdin );
      if ( rc == 1 ) {
         printf( "%c\n", buf[0] );
      }
   } while ( rc );
 
   return 0;
}

Turns out the answer is just the gdb run command, which can specify stdin for the program. Here’s a demo session that shows this in action

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(gdb) b main
Breakpoint 1 at 0x40061e: file x.c, line 5.
(gdb) run < x.c
Starting program: /home/pjoot/tmp/readit/a.out < x.c
 
Breakpoint 1, main () at x.c:5
5          size_t rc = 0;
Missing separate debuginfos, use: yum debuginfo-install glibc-2.28-225.0.4.el8_8.6.x86_64
(gdb) n
9             rc = fread( buf, 1, 1, stdin );
(gdb) n
10            if ( rc == 1 ) {
(gdb) p rc
$1 = 1
(gdb) p buf[0]
$2 = 35 '#'
(gdb) n
11               printf( "%c\n", buf[0] );
(gdb)
#
13         } while ( rc );
(gdb)
9             rc = fread( buf, 1, 1, stdin );
(gdb)
10            if ( rc == 1 ) {
(gdb) p rc
$3 = 1
(gdb) p buf[0]
$4 = 105 'i'

The x.c input that I passed to run, was the source of the program that I debugging.