[c] concatenate char array in C

I have a a char array:

char* name = "hello";

I want to add an extension to that name to make it

hello.txt

How can I do this?

name += ".txt" won't work

asprintf is not 100% standard, but it's available via the GNU and BSD standard C libraries, so you probably have it. It allocates the output, so you don't have to sit there and count characters.

char *hi="Hello";
char *ext = ".txt";
char *cat;

asprintf(&cat, "%s%s", hi, ext);

in rare cases when you can't use strncat, strcat or strcpy. And you don't have access to <string.h> so you can't use strlen. Also you maybe don't even know the size of the char arrays and you still want to concatenate because you got only pointers. Well, you can do old school malloc and count characters yourself like..

char *combineStrings(char* inputA, char* inputB) {
    size_t len = 0, lenB = 0;
    while(inputA[len] != '\0') len++;
    while(inputB[lenB] != '\0') lenB++;
    char* output = malloc(len+lenB);
    sprintf((char*)output,"%s%s",inputA,inputB);
    return output;
}

It just needs #include <stdio.h> which you will have most likely included already

I have a char array:

char* name = "hello";

No, you have a character pointer to a string literal. In many usages you could add the const modifier, depending on whether you are more interested in what name points to, or the string value, "hello". You shouldn't attempt to modify the literal ("hello"), because bad things can happen.

The major thing to convey is that C does not have a proper (or first-class) string type. "Strings" are typically arrays of chars (characters) with a terminating null ('\0' or decimal 0) character to signify end of a string, or pointers to arrays of characters.

I would suggest reading Character Arrays, section 1.9 in The C Programming Language (page 28 second edition). I strongly recommend reading this small book ( <300 pages), in order to learn C.

Further to your question, sections 6 - Arrays and Pointers and section 8 - Characters and Strings of the C FAQ might help. Question 6.5, and 8.4 might be good places to start.

I hope that helps you to understand why your excerpt doesn't work. Others have outlined what changes are needed to make it work. Basically you need an char array (an array of characters) big enough to store the entire string with a terminating (ending) '\0' character. Then you can use the standard C library function strcpy (or better yet strncpy) to copy the "Hello" into it, and then you want to concatenate using the standard C library strcat (or better yet strncat) function. You will want to include the string.h header file to declare the functions declarations.

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

int main( int argc, char *argv[] )
{
    char filename[128];
    char* name = "hello";
    char* extension = ".txt";

    if (sizeof(filename) < strlen(name) + 1 ) { /* +1 is for null character */
        fprintf(stderr, "Name '%s' is too long\n", name);
        return EXIT_FAILURE;
    }
    strncpy(filename, name, sizeof(filename));

    if (sizeof(filename) < (strlen(filename) + strlen(extension) + 1) ) {
        fprintf(stderr, "Final size of filename is too long!\n");
        return EXIT_FAILURE;
    }
    strncat(filename, extension, (sizeof(filename) - strlen(filename)) );
    printf("Filename is %s\n", filename);

    return EXIT_SUCCESS;
}

First copy the current string to a larger array with strcpy, then use strcat.

For example you can do:

char* str = "Hello";
char dest[12];

strcpy( dest, str );
strcat( dest, ".txt" );

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

char *name = "hello";

int main(void) {
  char *ext = ".txt";
  int len   = strlen(name) + strlen(ext) + 1;
  char *n2  = malloc(len);
  char *n2a = malloc(len);

  if (n2 == NULL || n2a == NULL)
    abort();

  strlcpy(n2, name, len);
  strlcat(n2, ext, len);
  printf("%s\n", n2);

  /* or for conforming C99 ...  */
  strncpy(n2a, name, len);
  strncat(n2a, ext, len - strlen(n2a));
  printf("%s\n", n2a);

  return 0; // this exits, otherwise free n2 && n2a
}

You can concatenate strings by using the sprintf() function. In your case, for example:

char file[80];
sprintf(file,"%s%s",name,extension);

And you'll end having the concatenated string in "file".

You could copy and paste an answer here, or you could go read what our host Joel has to say about strcat.