How to debug a program that uses input from an auxiliary txt file in gdb?

Question

I want to debug a file, say file.c, and this file reads information from a txt file, say input.txt. So normally, to compile and run the program I would do the following:

gcc -std=c99 -g file.c -o file.exe

and to debug I would try the following:

gdb ./file.exe input.txt

However this doesn't work and fails with No such file exits. '(null)': Bad address when the program, file.c, attempts to open the file specified in argv[1] which input.txt is.

I have tried the following methods:

1. gdb ./file.exe
   (gdb) run < input.txt

2. gdb ./file.exe
   b main
   (gdb) r
   (gdb) call (int)dup2(open("input.txt",0),0)
   $1 = 0

All with the same outcome as described above...No such file exits. '(null)': Bad address

The code is merely:

FILE *input = fopen(argv[1],"r");

Your file is expected as a parameter, not redirected stream. — Eugene Sh.
– Eugene Sh., Commented Apr 29, 2019 at 21:04
regarding: FILE *input = fopen(argv[1],"r"); This will not compile. It needs: 1) #include <stdio.h> 2) int main( void ){ FILE *input = fopen(argv[1],"r"); .... etc — user3629249
– user3629249, Commented May 1, 2019 at 5:43

Jasen · Accepted Answer · 2019-04-29 21:23:39Z

1

Do

gdb --args ./file.exe file.txt
b main
run

or do

gdb ./file.exe
b main
run file.txt

answered Apr 29, 2019 at 21:23

Jasen

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