How to set the environment variable of child same as parent to use execve in C?

Question

I was trying to set the environment of child process similar to the parent process and I need to fill the array called envp with string as follows:

char *envp = malloc(sizeof(args) / sizeof(char *));
strcpy(envp[0], strcat("HOME=", getenv("HOME")));
envp[1] = strcat("PATH=", getenv("PATH"));
envp[2] = strcat("TZ=", getenv("TZ"));
envp[3] = strcat("USER=", getenv("USER"));
envp[4] = strcat("LOGNAME=", getenv("LOGNAME"));
envp[5] = 0;

Inside if(fork() ==0)

setenv("parent", cwd, 1);
if((execve(args[0], &args[0], envp)) < 0 ){
            perror(*args);
            exit(EXIT_FAILURE);
}

I'm doing this because I don't know the values of these environment variables so I want to copy the parent variables in order to use them in execve() that will replace the child process! I'm using execve() instead of the execvp() because I want to handle searching the cwd before executing the command and search the directories of the shell path name of the cwd is not found.

So my question is: How to set the values of the array in a correct way? Plus, am I misunderstanding any concept?

I looked at many posts here, but it's obvious that I'm lost! Thanks in advance,

Answer 1

You are not allocating memory for the strings that envp [x]is pointing to. Instead you are trying to strcat or strcpy to constant or non-allocated strings which is a no-no (your compiler has most probably warned you)

The general concept is

1. You need to allocate memory for the array of pointers to environment strings (you did that).
2. You also need to allocate memory for each of the strings those pointers point at (you didn't do that). Note that memory goes into the ownership of the process environment in most OSs, so you cannot use local variables and you may not free this memory.
3. You need to save the pointers retrieved from (2) into the array positions you allocated at (1) (You did that)

Your code should look similar to (example for "HOME" only, length of envbuff might need to be adjusted/checked):

char envBuff [100];

strcpy (envBuff, "HOME=");
strcat (envBuff, getenv ("HOME));
envp [0] = strdup (envBuff);

strdupactually allocates the memory it uses, strcator a simple assignment as you do for envp[0] don't - strcat assumes there is enough room for both the original and the appended part and the memory is writable, which is both not true in your case.

Answer 2

• strcat() is a terrible function
• separate strdup()s for each "Name=value" pair could also be an option.
• having all the strings in one block of memory is nicer
• snprintf() will never write beyond the allowed size; instead it gives you the size that you'll need
• getenv() can return NULL, this MUST be handled

Note: I omitted the checks for realloc() returning NULL, you should add these.

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

#define COUNTOF(a) (sizeof a / sizeof a[0])
char * names[] = {"HOME", "PATH", "TZ", "USER", "LOGNAME", NULL};

int main(void)
{
char *all=NULL;
char *array[COUNTOF(names) ];
size_t size, used, idx, envc, len;

for (idx=envc=size=used= 0; names[idx] ; idx++ ) {
        char *cp;
        cp = getenv(names[idx] );
        if (!cp) {
                fprintf(stderr, "%s not found\n", names[idx] );
                continue;
                }
        fprintf(stderr, "%s found: \"%s\"\n", names[idx], cp );
        while(1) {
                len = snprintf(all+used, size-used, "%s=%s", names[idx], cp);
                        /* enough space for the "NAME=val\0" pair */
                if (len+1 < size-used) break;
                fprintf(stderr, "Len=%zu; realloc %zu += %zu\n" , len, size, 2*len+1 );
                all = realloc(all, size += 2*len+1);
                }
        array[envc++] = all+used; used += len+1;
        }

fprintf(stderr, "Final realloc %zu -->> %zu\n" , size, used+1 );
all = realloc(all, used+1);
array[envc] = NULL;

for (idx=0; idx < envc; idx++) {
        printf("[%zu] -> %s\n", idx, array[idx] );
        }

return 0;
}

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BTW: if your OS/environment allows it you can define main() as

int main(int argc, char **argv, char **envp) { ... }

, and use the existing env pointer of the parent process.