newbie c string question

nomb · 03-15-2007, 10:23 AM

Hey guys,

I'm very stuck. I am trying to put together some strings to make a command to run with system. Here is what I have:

Declarations:

gchar *method[0]

GtkWidget *dmcal_new_txt_Path = NULL;
GtkWidget *dmcal_new_txt_Size = NULL;
gchar *file_size = NULL;
gchar *file_path = NULL;
gchar *command = NULL;
gchar *compart = NULL;

Code:

g_print("dd if=%s of=%s bs=1024 count=%s\n",method[0],file_path,file_size);
compart = "dd if=";
command = strcat (compart, method[0]);
g_print(command)

Bisically I want to build what is in the first g_print into the variable command. The first g_print prints everything out great with no problems. Its just when it gets to the strcat it errors and pops up the bug buddy.

Thanks a lot for your help.

nomb

weibullguy · 03-15-2007, 10:35 AM

See if this works

Code:

g_print("dd if=%s of=%s bs=1024 count=%s\n",method[0],file_path,file_size);
compart = "dd if=";
command = (char *)calloc(strlen(compart) + strlen(method[0]) + 1, sizeof(char));
command = strcat(compart,method[0]);
g_print(command);

nomb · 03-15-2007, 10:41 AM

Im afraid that still errored on me and popped up the bug reporting tool.

weibullguy · 03-15-2007, 10:45 AM

Are you running the program from the command line or using a launcher? Try running it from the command line. Try using gdb to trace the program.

nomb · 03-15-2007, 10:49 AM

I am running it from the command line. Im not sure how to use gdb. I have it but not sure how to use it.

nomb · 03-15-2007, 10:56 AM

Oh wait this is what it says on the error:

Startng program: /home/nomb/Projects/DMCaL/src/dmcal
[Thread debugging using libthread_db enabled]
[New Thread -1208641312 (LWP 5993)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1208641312 (LWP 5993)]
0x45155c95 in strcat () from /lib/libc.so.6

I'm not sure what that means tho.

weibullguy · 03-15-2007, 10:59 AM

Try a slightly different approach

Code:

g_print("dd if=%s of=%s bs=1024 count=%s\n",method[0],file_path,file_size);
command = (char *)calloc(strlen("dd if=") + strlen(method[0]) + 1, sizeof(char));
command = "dd if=";
strcat(command,method[0]);
g_print(command);

To run gdb, just execute

Code:

gdb ./foo-bar

. This starts gdb and loads the foo-bar program. Then run the program in gdb by executing run. Here's the on-line manual http://sourceware.org/gdb/current/on...s/gdb_toc.html for more in-depth help with gdb.

nomb · 03-15-2007, 11:15 AM

I got:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1208915744 (LWP 6259)]
0x45155c95 in strcat () from /lib/libc.so.6

graemef · 03-16-2007, 12:39 AM

The strcat is expecting a valid memory address with which to place the string data. So you have declared compart and command as pointers to a character (correct) Next you assign some memory into command with the calloc function (correct), then comes the curious line

Code:

compart = "dd if=";

compart is a pointer so this assignment will not work, compart is expecting an address to memory not actual data.

Next check out the details of strcat (man strcat) and you will see that strcat changes the details of the first argument. So build up the details of command in two steps, both using strcat the first adding the literal "dd if=" the next call appending the value of method[0].

weibullguy · 03-16-2007, 12:25 PM

Code:

gchar *method[0]

GtkWidget *dmcal_new_txt_Path = NULL;
GtkWidget *dmcal_new_txt_Size = NULL;
gchar *file_size = NULL;
gchar *file_path = NULL;
gchar *command;
gchar *compart="dd if=";

g_print("dd if=%s of=%s bs=1024 count=%s\n",method[0],file_path,file_size);
command = (char *)calloc(strlen("dd if=") + strlen(method[0]) + 1, sizeof(char));
strcat(command, compart);
strcat(command, method[0]);
g_print(command)