concatenate char array in C

Question

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

User · Answer

include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   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  amp  amp  n2a

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

User · Answer

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

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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

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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    lt 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  lt stdio h gt   include  lt string h gt   include  lt stdlib h gt   int main  int argc  char  argv           char filename 128       char  name    hello       char  extension     txt        if  sizeof filename   lt  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   lt   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

User · Answer

Have a look at the strcat function   In particular  you could try this   const char  name    hello   const char  extension     txt    char  name with extension  name with extension   malloc strlen name  1 4      make space for the new string  should check the return value         strcpy name with extension  name      copy name into the new var    strcat name with extension  extension      add the extension

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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  amp cat    s s   hi  ext

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in rare cases when you can t use strncat  strcat or strcpy  And you don t have access to  lt string h gt  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  quot  s s quot  inputA inputB       return output     It just needs  include  lt stdio h gt  which you will have most likely included already

[c] concatenate char array in C

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