C string append

Question

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

User · Accepted Answer

You need to allocate new space as well   Consider this code fragment   char   new str   if  new str   malloc strlen str1  strlen str2  1      NULL       new str 0      0        ensures the memory is an empty string     strcat new str str1       strcat new str str2     else       fprintf STDERR  malloc failed  n           exit      You might want to consider strnlen 3  which is slightly safer   Updated  see above   In some versions of the C runtime  the memory returned by malloc isn t initialized to 0  Setting the first byte of new str to zero ensures that it looks like an empty string to strcat

User · Answer

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  amp buf   amp 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

User · Answer

strcpy str1 strlen str1   str2

User · Answer

You ll have to strncpy str1 into new string first then

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

I write a function support dynamic variable string append  like PHP str append  str   str       etc    include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   include  lt stdarg h gt   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  amp str  format  arg ptr           save old json     asprintf  amp 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  amp json   name   d   d   d   1  2  3       str append  amp json   sex   s    male        str append  amp json   end        str append  amp json           str append  amp json      ret   true          int i      for  i   0  i  lt  10  i              str append  amp json   id- d   i              printf   s n   json        if  json  free json        return 0

User · Answer

do the following    strcat new str str1   strcat new str str2

User · Answer

You could use asprintf to concatenate both into a new string   char  new str  asprintf  amp new str   s s  str1 str2

[c] C string append

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