Concatenation of strings as char pointers

haven't used C in a while....so this might be really simple for most of you....

can someone please tell me how to concatenate two char * strings?

eg.
char * c1 = "SYSTEM PATH = "
char *c2 = getenv("PATH");

how do I concatenate c1 + c2 and put the result into a third char * variable?

Thanks very much.

strcat()

that doesn't work here cos I guess the getenv function returns a pointer to the whole PATH but the PATH does not have a defined '\0' character at the end....

Code:

---

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

int main(void)
{
  char *a = "SYSTEM PATH = ";
  char *b = getenv("PATH");
  if ( b != NULL )
  {
      char *c = malloc(strlen(a) + strlen(b) + 1);
      if ( c != NULL )
      {
        strcpy(c, a);
        strcat(c, b);
        puts(c);
        free(c);
      }
  }
  return 0;
}

---

thanx a lot for your reply....

however, i ran the code that you listed in a MS vc++ environment, and it gives me an error:

error C2440: 'initializing' : cannot convert from 'void *' to 'char *'
Conversion from 'void*' to pointer to non-'void' requires an explicit cast

thanks,
sameer

Compile the C code as C? If you have the file saved as .cpp, try it again as a .c file.

tried but did not work...

i think it's cos c2 does not have an explicit '\0' character.....
so it crashes when it encounters the strcat() function...

-sameer

Quote:

---

7.20.4.5 The getenv function

Synopsis

1 #include <stdlib.h>
char *getenv(const char *name);

Description

2 The getenv function searches an environment list, provided by the host environment, for a string that matches the string pointed to by name. The set of environment names and the method for altering the environment list are implementation-defined.

3 The implementation shall behave as if no library function calls the getenv function.

Returns

4 The getenv function returns a pointer to a string associated with the matched list member. The string pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the getenv function. If the specified name cannot be found, a null pointer is returned.

---

Using "string" in the C standard sense implies null termination.

Were you dynamically allocating memory for the right amount of space? Or were you trying to strcat more than the string could hold?

hmm...then i guess i thought up the wrong reason....

i did allocate a lot of space....

let me paste some of my actual code... (you will see there's hardly any difference between the example that i discussed earlier)

Code:

---

char * classpath = new char[65536];

if (classpath == NULL)
{
        fprintf(stderr,"\nMemory allocation failed!");
        exit(1);
}

char * javapath = "-Djava.class.path=";

char * env_var = getenv("JNI_CLASSPATH");


if ( classpath != NULL )
{
        strcpy(classpath, javapath);
        strcat(classpath, env_var);
}

---

[Ew. C/C++. (As opposed to one language or the other.)]

Is getenv successful? You're not checking it's return value. Dereferencing a null pointer will generally do bad things.

[edit]http://www.parashift.com/c++-faq-lit....html#faq-16.6

thanx dave....

but this finally seemed to work....i guess it was the (char *) before the malloc that was needed

Code:

---

c3 = (char *)malloc(strlen(c1) + strlen(c2) +1);
strcpy(c3,c1);
strcat(c3,c2);

---

Not in C it isn't. In C++ yes. But not in C. You never need to typecast the return of malloc, and in fact you shouldn't. Search the FAQ or forum for why.

Quzah.