sprintf to a list of char pointers in c? Segmentations errors

Question

I have this code that works (it's a bit stripped down):

char *parmList[6];

parmList[0] ="/root/ssl_send";
parmList[1] ="-m 1";            
...etc...
parmList[5] = NULL;
execvp(parmList[0], parmList);

Now I want to write something to one string in this list with sprintf (it's more correct to say that I want that one pointer of *parmList[6] points to a char array constructed with sprintf). I am getting "Segmentation errors" all the time. I have tried:

using malloc,
declaring a double array so the memory space is reserved,
using snprintf,....

I am obviously doing something wrong. The problem is similliar to Sprintf Segmentation Fault, just that I need to have a list of pointers/char_arrays for execvp.

Why don't you show us what you've tried instead of telling? — oh whatever
– oh whatever, Commented Apr 19, 2012 at 7:36
Most likely you are trying to modify a string literal through the pointer causing an Undefined Behavior and the crash. — Alok Save
– Alok Save, Commented Apr 19, 2012 at 7:37
String literals are typically read-only and it's UB to try and write to them. If you want to modify these then you need to allocate memory for them and copy the string literal data. — Paul R
– Paul R, Commented Apr 19, 2012 at 7:37

Ivo Bosticky · Accepted Answer · 2012-04-19 08:09:17Z

1

Here is code that uses sprintf to create a string and uses that string in your argument list. Make sure you allocate enough space for the sprintf output.

char *parmList[6];
parmList[0] = "/bin/ls";
char arg1[10];
sprintf(arg1, "%s", "-l");
parmList[1] = arg1;
parmList[2] = NULL;
execvp(parmList[0], parmList);

edited Apr 19, 2012 at 8:09

answered Apr 19, 2012 at 7:54

Ivo Bosticky

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1 Comment

Ivo Bosticky Over a year ago

You're right, I didn't read the question carefully enough the first time. Sample code updated.

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