Getting first string from string array in c

Question

I am trying to create processes for my project. I will pas arguments to child process from parent and the argument will change in time, so i wanted to make a try first with passing 1 to the child. The string format should be like this "childname.exe c" where c represents random character (in this case it is 1 for just trial).

I created a childname array and and all i wanted was concatenate the new string with childname string and copy it to another string array(lpCommandLine variable). When i debugged the code below i saw that child_name[0] (when i is equal to 0) returns only 'C' although i expected it to return "ChildProj1.exe". Is there a point that i missed or how to do it in c?

here there is a image of what i getin debugger: here stored values of in variables

#define NO_OF_PROCESS 3

char *child_names[]= {"ChildProj1.exe", "ChildProj2.exe", "ChildProj3.exe" };
char* lpCommandLine[NO_OF_PROCESS];
int i;

    for (i = 0; i < NO_OF_PROCESS; i++)
        lpCommandLine[i] = (char *)malloc(sizeof(char) * 16);


    for (i = 0; i < NO_OF_PROCESS; i++)
    {
        strcat_s(child_names[i], strlen(child_names[i]), " 1");
        strcpy_s(lpCommandLine[i], strlen(lpCommandLine[i]), child_names[i]);
    }

where do you think the new string "<childproces> 1" is going to be stored. strcat_s failed becuase you tried to add characters to the buffer - check the return value of srcat_s — pm100
– pm100, Commented Nov 16, 2016 at 23:30
@Y.E.S. It is unclear what you are going to get in the array lpCommandLine. Show its result content. — Vlad from Moscow
– Vlad from Moscow, Commented Nov 16, 2016 at 23:44
I want it to be stored in lpCommandLine. In child_names array 0th string is "ChildProj1.exe" and i want lpCommandLine[0] to be "ChildProj1.exe 1". So do you suggest to allocate memory for each child_names indexes for 16 chars which means ChildProj1.exe + 3 (for blank and 1 and \0) — Y.E.S.
– Y.E.S., Commented Nov 16, 2016 at 23:46
@vlad-from-moscow it is not necessary to know what i am trying to do with lpCommandLine array. All i want is concatenate " 1" with strings of array child_names and copy each string to lpCommandLine array. — Y.E.S.
– Y.E.S., Commented Nov 16, 2016 at 23:48
@Y.E.S. It seems I have understood what you want. See my answer.:) — Vlad from Moscow
– Vlad from Moscow, Commented Nov 17, 2016 at 0:21

Vlad from Moscow · Accepted Answer · 2016-11-23 11:34:11Z

From your description it follows that you want to get strings like this

"childname.exe c"

However this loop

for (i = 0; i < NO_OF_PROCESS; i++)
{
    strcat_s(child_names[i], strlen(child_names[i]), " 1");
    strcpy_s(lpCommandLine[i], strlen(lpCommandLine[i]), child_names[i]);
}

does not do what you want.

This loop has undefined behavior because in this statement

    strcat_s(child_names[i], strlen(child_names[i]), " 1");

there is an attempt to modify a string literal. You may not change string literals neither in C nor in C++.

Moreover in this statement

    strcpy_s(lpCommandLine[i], strlen(lpCommandLine[i]), child_names[i]);

this call

strlen(lpCommandLine[i])

also has undefined behavior because array pointed to by this pointer lpCommandLine[i] does not has the terminating zero.

There is no any need to use the functions strcat_s and strcpy_s. It is much better to use standard function strcat and strcpy.

What you is the following that is shown in this demonstrative program.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define NO_OF_PROCESS   3

int main(void) 
{
    const char * child_names[]= 
    {
        "ChildProj1.exe", 
        "ChildProj2.exe", 
        "ChildProj3.exe" 
    };

    const char *s = " 1";
    size_t n = strlen( s );

    char* lpCommandLine[NO_OF_PROCESS];

    for ( int i = 0; i < NO_OF_PROCESS; i++ )
    {
        lpCommandLine[i] = ( char * )malloc( strlen( child_names[i] ) + n + 1 );
    }

    for ( int i = 0; i < NO_OF_PROCESS; i++ )
    {
        strcpy( lpCommandLine[i], child_names[i] );
        strcat( lpCommandLine[i],  s );
    }

    for ( int i = 0; i < NO_OF_PROCESS; i++ ) puts( lpCommandLine[i] );

for ( int i = 0; i < NO_OF_PROCESS; i++ ) free( lpCommandLine[i] );

    return 0;
}

The program output is

ChildProj1.exe 1
ChildProj2.exe 1
ChildProj3.exe 1

Eric M. · Accepted Answer · 2016-11-16 23:28:40Z

0

Instead of char * child_names[] did you mean something like char[][] child_names, char[] * child_names, or char ** child_names?

answered Nov 16, 2016 at 23:28

Eric M.

6625 silver badges16 bronze badges

2 Comments

Y.E.S. Over a year ago

I want to store strings in that array, already tried char[ ][ ], char** but didn't work.

Y.E.S. Over a year ago

with char* child_names[ ], i tried to print each element and it printed correctly with this code printf("%s", child_names[i]);. It prints each string like this but when i want to do concatenating things go crazy.

pm100 · Accepted Answer · 2016-11-16 23:33:42Z

0

to do the string concat do

size_t sz = strlen(child_names[i]) + 3; // space, '1' and \0
char *buff = malloc(sz); 
strcat_s(buff,sz,child_names[i]);
strcat_s(buff,sz," 1");

answered Nov 16, 2016 at 23:33

pm100

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2 Comments

pm100 Over a year ago

your issue about childname[0] = 'C' is a red herring. YOu just need to display it as a string rather than a char in the debugger

Y.E.S. Over a year ago

I just tried it now and in buff there is only "\0" it says. If you have time you can try it with your compiler, i wonder if there is a problem with my compiler now.

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