[c] C string append

I want to append two strings. I used the following command:

new_str = strcat(str1, str2);

This command changes the value of str1. I want new_str to be the concatanation of str1 and str2 and at the same time str1 is not to be changed.

You'll have to strncpy str1 into new_string first then.

Consider using the great but unknown open_memstream() function.

FILE *open_memstream(char **ptr, size_t *sizeloc);

Example of usage :

// open the stream
FILE *stream;
char *buf;
size_t len;
stream = open_memstream(&buf, &len);

// write what you want with fprintf() into the stream
fprintf(stream, "Hello");
fprintf(stream, " ");
fprintf(stream, "%s\n", "world");

// close the stream, the buffer is allocated and the size is set !
fclose(stream);
printf ("the result is '%s' (%d characters)\n", buf, len);
free(buf);

If you don't know in advance the length of what you want to append, this is convenient and safer than managing buffers yourself.

You can try something like this:

strncpy(new_str, str1, strlen(str1));
strcat(new_str, str2);

More info on strncpy: http://www.cplusplus.com/reference/clibrary/cstring/strncpy/

You could use asprintf to concatenate both into a new string:

char *new_str;
asprintf(&new_str,"%s%s",str1,str2);

man page of strcat says that arg1 and arg2 are appended to arg1.. and returns the pointer of s1. If you dont want disturb str1,str2 then you have write your own function.

char * my_strcat(const char * str1, const char * str2)
{
   char * ret = malloc(strlen(str1)+strlen(str2));

   if(ret!=NULL)
   {
     sprintf(ret, "%s%s", str1, str2);
     return ret;
   }
   return NULL;    
}

Hope this solves your purpose

strcpy(str1+strlen(str1), str2);

I write a function support dynamic variable string append, like PHP str append: str + str + ... etc.

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

int str_append(char **json, const char *format, ...)
{
    char *str = NULL;
    char *old_json = NULL, *new_json = NULL;

    va_list arg_ptr;
    va_start(arg_ptr, format);
    vasprintf(&str, format, arg_ptr);

    // save old json
    asprintf(&old_json, "%s", (*json == NULL ? "" : *json));

    // calloc new json memory
    new_json = (char *)calloc(strlen(old_json) + strlen(str) + 1, sizeof(char));

    strcat(new_json, old_json);
    strcat(new_json, str);

    if (*json) free(*json);
    *json = new_json;

    free(old_json);
    free(str);

    return 0;
}

int main(int argc, char *argv[])
{
    char *json = NULL;

    str_append(&json, "name: %d, %d, %d", 1, 2, 3);
    str_append(&json, "sex: %s", "male");
    str_append(&json, "end");
    str_append(&json, "");
    str_append(&json, "{\"ret\":true}");

    int i;
    for (i = 0; i < 10; i++) {
        str_append(&json, "id-%d", i);
    }

    printf("%s\n", json);

    if (json) free(json);

    return 0;
}

I needed to append substrings to create an ssh command, I solved with sprintf (Visual Studio 2013)

char gStrSshCommand[SSH_COMMAND_MAX_LEN]; // declare ssh command string

strcpy(gStrSshCommand, ""); // empty string

void appendSshCommand(const char *substring) // append substring
{
  sprintf(gStrSshCommand, "%s %s", gStrSshCommand, substring);
}

do the following:

strcat(new_str,str1);
strcat(new_str,str2);