gdb not printing string value correctly - c

Question

I have a multithreaded (pthreads) program in which main() calls a function omp_file_open_all() and passes in a string as char* alongwith other arguments. I was debugging something using gdb and saw that gdb does not print out the string value correctly, whereas a printf inside the function prints it out correcly.
Breakpoint 1, omp_file_open_all (fd=0x423bb950, filename=0x7f605df078e0 "", mode=-16843009) at pthread_coll_file_open.c:29
29 if(omp_get_thread_num() == MASTER)
(gdb) print filename
$1 = 0x7f605df078e0 ""
So gdb shows filename as empty, whereas a printf inside the function outputs the correct value as "/tmp/test.out". The function being called (omp_file_open_all) is defined as follows (not in the same file as main()):
int omp_file_open_all (int fd, char* filename, int mode);
I cant post my program here as this is a part of a larger code thats approx. 1500 lines of code. 'filename' is a global variable and is set in main() by the main thread before newer threads are spawned.
So this is not an error, and I merely stumbled across it, but I am interested in finding out why gdb does not display the correct value.
OS: 64bit OpenSUSE,
gdb 6.8
Thanks for your help.

Answer 1

There might be some thing going wrong in your code. With the following code snippet, I am getting the string printed by gdb correctly.
#include <stdio.h>
#include <stdlib.h>
void checkString( char* fileName )
{
printf("%s", fileName);
}
int main()
{
char* name = "temp";
checkString(name);
return 0;
}
mahesh-babu-vattiguntas-macbook-pro:Desktop mahesh$ gdb gdb.out
GNU gdb 6.3.50-20050815 (Apple version gdb-1469) (Wed May 5 04:36:56 UTC 2010)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin"...Reading symbols for shared libraries .. done
(gdb) b gdb.c:6
Breakpoint 1 at 0x100000ebc: file gdb.c, line 6.
(gdb) run gdb.out
Starting program: /Users/mahesh/Desktop/gdb.out gdb.out
Reading symbols for shared libraries +. done
Breakpoint 1, checkString (fileName=0x100000f05 "temp") at gdb.c:6
6 printf("%s", fileName);
(gdb) p fileName
$1 = 0x100000f05 "temp"
(gdb)

Answer 2

Try stepping forward one line (gdb "s" command) after you hit the breakpoint, then try printing it again. I've sometimes seen gdb have trouble displaying parameter values correctly when breaking at the beginning of a function.

Answer 3

My first guess was that there is a scoping issue, since the name of the function parameter and your global variable is identical. However, this does not seem to be the case for the following very small program:
#include <cstdio>
static char const* filename = "something";
int foobar(char const* filename)
{
printf("%s\n", filename);
}
int main(int argc, char** argv)
{
return foobar("somethingelse");
}
Compiled with:
g++ -ggdb -g3 -O0 test.cpp -o test
GDB (7.2, also on x64 but Ubuntu) gives:
Breakpoint 1, foobar (filename=0x400706 "somethingelse") at test.cpp:7
7 printf("%s\n", filename);
(gdb) p filename
$1 = 0x400706 "somethingelse"
So it's not about scoping per-se. Also, the output suggests that the parameter is indeed an empty string at execution time. Could you please provide us with the output of bt at the same time you break into the debugger? Last two stack frames are sufficient.