How do I concatenate const literal strings in C

Question

I m working in C  and I have to concatenate a few things   Right now I have this   message   strcat  TEXT    var    message2   strcat strcat  TEXT    foo   strcat   TEXT    bar      Now if you have experience in C I m sure you realize that this gives you a segmentation fault when you try to run it  So how do I work around that

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Strings can also be concatenated at compile time     define SCHEMA  test   define TABLE   data   const char  table   SCHEMA     TABLE      note no   or   or anything const char  qry                    include comments in a string       SELECT                       get all fields       FROM   SCHEMA     TABLE      the table          WHERE x   1                  the filter

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If you have experience in C you will notice that strings are only char arrays where the last character is a null character   Now that is quite inconvenient as you have to find the last character in order to append something  strcat will do that for you   So strcat searches through the first argument for a null character  Then it will replace this with the second argument s content  until that ends in a null    Now let s go through your code   message   strcat  TEXT     var     Here you are adding something to the pointer to the text  TEXT   the type of  TEXT  is const char   A pointer     That will usually not work  Also modifying the  TEXT  array will not work as it is usually placed in a constant segment   message2   strcat strcat  TEXT    foo   strcat   TEXT    bar      That might work better  except that you are again trying to modify static texts  strcat is not allocating new memory for the result   I would propose to do something like this instead   sprintf message2   TEXT  s TEXT  s   foo  bar     Read the documentation of sprintf to check for it s options   And now an important point   Ensure that the buffer has enough space to hold the text AND the null character  There are a couple of functions that can help you  e g   strncat and special versions of printf that allocate the buffer for you  Not ensuring the buffer size will lead to memory corruption and remotely exploitable bugs

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In C   strings  are just plain char arrays  Therefore  you can t directly concatenate them with other  strings     You can use the strcat function  which appends the string pointed to by src to the end of the string pointed to by dest   char  strcat char  dest  const char  src     Here is an example from cplusplus com   char str 80   strcpy str   these     strcat str   strings     strcat str   are     strcat str   concatenated       For the first parameter  you need to provide the destination buffer itself   The destination buffer must be a char array buffer   E g   char buffer 1024    Make sure that the first parameter has enough space to store what you re trying to copy into it   If available to you  it is safer to use functions like  strcpy s and strcat s where you explicitly have to specify the size of the destination buffer    Note  A string literal cannot be used as a buffer  since it is a constant  Thus  you always have to allocate a char array for the buffer   The return value of strcat can simply be ignored  it merely returns the same pointer as was passed in as the first argument  It is there for convenience  and allows you to chain the calls into one line of code   strcat strcat str  foo   bar     So your problem could be solved as follows   char  foo    foo   char  bar    bar   char str 80   strcpy str   TEXT     strcat str  foo   strcat str  bar

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Try something similar to this    include  lt stdio h gt   include  lt string h gt   int main int argc  const char   argv           Insert code here      char firstname 100   secondname 100     printf  Enter First Name        fgets firstname  100  stdin     printf  Enter Second Name        fgets secondname 100 stdin     firstname strlen firstname -1     0     printf  fullname is  s  s   firstname  secondname      return 0

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If you have experience in C you will notice that strings are only char arrays where the last character is a null character   Now that is quite inconvenient as you have to find the last character in order to append something  strcat will do that for you   So strcat searches through the first argument for a null character  Then it will replace this with the second argument s content  until that ends in a null    Now let s go through your code   message   strcat  TEXT     var     Here you are adding something to the pointer to the text  TEXT   the type of  TEXT  is const char   A pointer     That will usually not work  Also modifying the  TEXT  array will not work as it is usually placed in a constant segment   message2   strcat strcat  TEXT    foo   strcat   TEXT    bar      That might work better  except that you are again trying to modify static texts  strcat is not allocating new memory for the result   I would propose to do something like this instead   sprintf message2   TEXT  s TEXT  s   foo  bar     Read the documentation of sprintf to check for it s options   And now an important point   Ensure that the buffer has enough space to hold the text AND the null character  There are a couple of functions that can help you  e g   strncat and special versions of printf that allocate the buffer for you  Not ensuring the buffer size will lead to memory corruption and remotely exploitable bugs

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Assuming you have a char fixed size  rather than a char   you can use a single  creative macro to do it all at once with a  lt  lt cout lt  lt like ordering   rather  s the disjointed  s n    than    printf style format     If you are working with embedded systems  this method will also allow you to leave out malloc and the large  printf family of functions like snprintf    This keeps dietlibc from complaining about  printf too    include  lt unistd h gt    for the write example   note  you should check if offset  sizeof buf  after use  define strcpyALL buf  offset       do        char  bp  char   buf offset     so we can add to the end of a string         const char  s         a         VA ARGS   NULL           ss a        while  s  ss                while   s  amp  amp    offset lt  int sizeof buf                   bp    s          if  offset  sizeof buf   bp 0     while 0   char buf 256   int len 0   strcpyALL buf len       The config file is in  n t  getenv  HOME      config   argv 0    config rc n     if  len lt sizeof buf       write 1 buf len     outputs our message to stdout else     write 2  error n  6      but we can keep adding on because we kept track of the length   this allows printf-like buffering to minimize number of syscalls to write   set len back to 0 if you don t want this behavior strcpyALL buf len  Thanks for using   argv 0     n    if  len lt sizeof buf       write 1 buf len     outputs both messages else     write 2  error n  6      Note 1  you typically wouldn t use argv 0  like this - just an example Note 2  you can use any function that outputs a char   including nonstandard functions like itoa   for converting integers to string types  Note 3  if you are already using printf anywhere in your program there is no reason not to use snprintf    since the compiled code would be larger  but inlined and significantly faster

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You can write your own function that does the same thing as strcat   but that doesn t change anything    define MAX STRING LENGTH 1000 char  strcat const const char  str1 const char  str2       static char buffer MAX STRING LENGTH       strncpy buffer str1 MAX STRING LENGTH       if strlen str1   lt  MAX STRING LENGTH           strncat buffer str2 MAX STRING LENGTH - strlen buffer              buffer MAX STRING LENGTH - 1      0       return buffer     int main int argc char  argv         printf   s  strcat const  Hello    world          Prints  Hello world      return 0      If both strings together are more than 1000 characters long  it will cut the string at 1000 characters  You can change the value of MAX STRING LENGTH to suit your needs

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Assuming you have a char fixed size  rather than a char   you can use a single  creative macro to do it all at once with a  lt  lt cout lt  lt like ordering   rather  s the disjointed  s n    than    printf style format     If you are working with embedded systems  this method will also allow you to leave out malloc and the large  printf family of functions like snprintf    This keeps dietlibc from complaining about  printf too    include  lt unistd h gt    for the write example   note  you should check if offset  sizeof buf  after use  define strcpyALL buf  offset       do        char  bp  char   buf offset     so we can add to the end of a string         const char  s         a         VA ARGS   NULL           ss a        while  s  ss                while   s  amp  amp    offset lt  int sizeof buf                   bp    s          if  offset  sizeof buf   bp 0     while 0   char buf 256   int len 0   strcpyALL buf len       The config file is in  n t  getenv  HOME      config   argv 0    config rc n     if  len lt sizeof buf       write 1 buf len     outputs our message to stdout else     write 2  error n  6      but we can keep adding on because we kept track of the length   this allows printf-like buffering to minimize number of syscalls to write   set len back to 0 if you don t want this behavior strcpyALL buf len  Thanks for using   argv 0     n    if  len lt sizeof buf       write 1 buf len     outputs both messages else     write 2  error n  6      Note 1  you typically wouldn t use argv 0  like this - just an example Note 2  you can use any function that outputs a char   including nonstandard functions like itoa   for converting integers to string types  Note 3  if you are already using printf anywhere in your program there is no reason not to use snprintf    since the compiled code would be larger  but inlined and significantly faster

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Do not forget to initialize the output buffer  The first argument to strcat must be a null terminated string with enough extra space allocated for the resulting string   char out 1024           must be initialized strcat  out  null terminated string        null terminated string has less than 1023 chars

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It is undefined behaviour to attempt to modify string literals  which is what something like   strcat   Hello     name     will attempt to do  It will try to tack on the name string to the end of the string literal  Hello     which is not well defined   Try something this  It achieves what you appear to be trying to do   char message 1000   strcpy  message   TEXT     strcat  message  var     This creates a buffer area that is allowed to be modified and then copies both the string literal and other text to it  Just be careful with buffer overflows  If you control the input data  or check it before-hand   it s fine to use fixed length buffers like I have   Otherwise  you should use mitigation strategies such as allocating enough memory from the heap to ensure you can handle it  In other words  something like   const static char TEXT      TEXT        Make  sure  you have enough space   char  message   malloc  sizeof TEXT    strlen var    1   if  message    NULL       handleOutOfMemoryIntelligently    strcpy  message  TEXT   strcat  message  var       Need to free message at some point after you re done with it

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You can write your own function that does the same thing as strcat   but that doesn t change anything    define MAX STRING LENGTH 1000 char  strcat const const char  str1 const char  str2       static char buffer MAX STRING LENGTH       strncpy buffer str1 MAX STRING LENGTH       if strlen str1   lt  MAX STRING LENGTH           strncat buffer str2 MAX STRING LENGTH - strlen buffer              buffer MAX STRING LENGTH - 1      0       return buffer     int main int argc char  argv         printf   s  strcat const  Hello    world          Prints  Hello world      return 0      If both strings together are more than 1000 characters long  it will cut the string at 1000 characters  You can change the value of MAX STRING LENGTH to suit your needs

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Folks  use strncpy    strncat    or snprintf    Exceeding your buffer space will trash whatever else follows in memory   And remember to allow space for the trailing null   0  character

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As people pointed out string handling improved much  So you may want to learn how to use the C   string library instead of C-style strings  However here is a solution in pure C   include  lt string h gt   include  lt stdio h gt   include  lt stdlib h gt   void appendToHello const char  s        const char  const hello    hello         const size t sLength       strlen s       const size t helloLength   strlen hello       const size t totalLength   sLength   helloLength       char  const strBuf   malloc totalLength   1       if  strBuf    NULL            fprintf stderr   malloc failed n            exit EXIT FAILURE              strcpy strBuf  hello       strcpy strBuf   helloLength  s        puts strBuf        free strBuf       int main  void        appendToHello  blah blah        return 0      I am not sure whether it is correct safe but right now I could not find a better way to do this in ANSI C

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Do not forget to initialize the output buffer  The first argument to strcat must be a null terminated string with enough extra space allocated for the resulting string   char out 1024           must be initialized strcat  out  null terminated string        null terminated string has less than 1023 chars

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Folks  use strncpy    strncat    or snprintf    Exceeding your buffer space will trash whatever else follows in memory   And remember to allow space for the trailing null   0  character

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The first argument of strcat   needs to be able to hold enough space for the concatenated string  So allocate a buffer with enough space to receive the result   char bigEnough 64         strcat bigEnough   TEXT    strcat bigEnough  foo       and so on      strcat   will concatenate the second argument with the first argument  and store the result in the first argument  the returned char  is simply this first argument  and only for your convenience   You do not get a newly allocated string with the first and second argument concatenated  which I d guess you expected based on your code

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You are trying to copy a string into an address that is statically allocated  You need to cat into a buffer   Specifically      snip     destination  Pointer to the destination array  which should contain a C string  and be large enough to contain the concatenated resulting string       snip     http   www cplusplus com reference clibrary cstring strcat html  There s an example here as well

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Avoid using strcat in C code  The cleanest and  most importantly  the safest way is to use snprintf   char buf 256   snprintf buf  sizeof buf    s s s s   str1  str2  str3  str4     Some commenters raised an issue that the number of arguments may not match the format string and the code will still compile  but most compilers already issue a warning if this is the case

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If you have experience in C you will notice that strings are only char arrays where the last character is a null character   Now that is quite inconvenient as you have to find the last character in order to append something  strcat will do that for you   So strcat searches through the first argument for a null character  Then it will replace this with the second argument s content  until that ends in a null    Now let s go through your code   message   strcat  TEXT     var     Here you are adding something to the pointer to the text  TEXT   the type of  TEXT  is const char   A pointer     That will usually not work  Also modifying the  TEXT  array will not work as it is usually placed in a constant segment   message2   strcat strcat  TEXT    foo   strcat   TEXT    bar      That might work better  except that you are again trying to modify static texts  strcat is not allocating new memory for the result   I would propose to do something like this instead   sprintf message2   TEXT  s TEXT  s   foo  bar     Read the documentation of sprintf to check for it s options   And now an important point   Ensure that the buffer has enough space to hold the text AND the null character  There are a couple of functions that can help you  e g   strncat and special versions of printf that allocate the buffer for you  Not ensuring the buffer size will lead to memory corruption and remotely exploitable bugs

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In C   strings  are just plain char arrays  Therefore  you can t directly concatenate them with other  strings     You can use the strcat function  which appends the string pointed to by src to the end of the string pointed to by dest   char  strcat char  dest  const char  src     Here is an example from cplusplus com   char str 80   strcpy str   these     strcat str   strings     strcat str   are     strcat str   concatenated       For the first parameter  you need to provide the destination buffer itself   The destination buffer must be a char array buffer   E g   char buffer 1024    Make sure that the first parameter has enough space to store what you re trying to copy into it   If available to you  it is safer to use functions like  strcpy s and strcat s where you explicitly have to specify the size of the destination buffer    Note  A string literal cannot be used as a buffer  since it is a constant  Thus  you always have to allocate a char array for the buffer   The return value of strcat can simply be ignored  it merely returns the same pointer as was passed in as the first argument  It is there for convenience  and allows you to chain the calls into one line of code   strcat strcat str  foo   bar     So your problem could be solved as follows   char  foo    foo   char  bar    bar   char str 80   strcpy str   TEXT     strcat str  foo   strcat str  bar

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Also malloc and realloc are useful if you don t know ahead of time how many strings are being concatenated    include  lt stdio h gt   include  lt string h gt   void example const char  header  const char   words  size t num words        size t message len   strlen header    1       1 for terminating NULL        char  message    char   malloc message len       strncat message  header  message len        for int i   0  i  lt  num words    i               message len    1   strlen words i       1   for separator               message    char   realloc message  message len          strncat strncat message       message len   words i   message len              puts message        free message

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Folks  use strncpy    strncat    or snprintf    Exceeding your buffer space will trash whatever else follows in memory   And remember to allow space for the trailing null   0  character

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Avoid using strcat in C code  The cleanest and  most importantly  the safest way is to use snprintf   char buf 256   snprintf buf  sizeof buf    s s s s   str1  str2  str3  str4     Some commenters raised an issue that the number of arguments may not match the format string and the code will still compile  but most compilers already issue a warning if this is the case

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The first argument of strcat   needs to be able to hold enough space for the concatenated string  So allocate a buffer with enough space to receive the result   char bigEnough 64         strcat bigEnough   TEXT    strcat bigEnough  foo       and so on      strcat   will concatenate the second argument with the first argument  and store the result in the first argument  the returned char  is simply this first argument  and only for your convenience   You do not get a newly allocated string with the first and second argument concatenated  which I d guess you expected based on your code

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If you have experience in C you will notice that strings are only char arrays where the last character is a null character   Now that is quite inconvenient as you have to find the last character in order to append something  strcat will do that for you   So strcat searches through the first argument for a null character  Then it will replace this with the second argument s content  until that ends in a null    Now let s go through your code   message   strcat  TEXT     var     Here you are adding something to the pointer to the text  TEXT   the type of  TEXT  is const char   A pointer     That will usually not work  Also modifying the  TEXT  array will not work as it is usually placed in a constant segment   message2   strcat strcat  TEXT    foo   strcat   TEXT    bar      That might work better  except that you are again trying to modify static texts  strcat is not allocating new memory for the result   I would propose to do something like this instead   sprintf message2   TEXT  s TEXT  s   foo  bar     Read the documentation of sprintf to check for it s options   And now an important point   Ensure that the buffer has enough space to hold the text AND the null character  There are a couple of functions that can help you  e g   strncat and special versions of printf that allocate the buffer for you  Not ensuring the buffer size will lead to memory corruption and remotely exploitable bugs

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Also malloc and realloc are useful if you don t know ahead of time how many strings are being concatenated    include  lt stdio h gt   include  lt string h gt   void example const char  header  const char   words  size t num words        size t message len   strlen header    1       1 for terminating NULL        char  message    char   malloc message len       strncat message  header  message len        for int i   0  i  lt  num words    i               message len    1   strlen words i       1   for separator               message    char   realloc message  message len          strncat strncat message       message len   words i   message len              puts message        free message

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The first argument of strcat   needs to be able to hold enough space for the concatenated string  So allocate a buffer with enough space to receive the result   char bigEnough 64         strcat bigEnough   TEXT    strcat bigEnough  foo       and so on      strcat   will concatenate the second argument with the first argument  and store the result in the first argument  the returned char  is simply this first argument  and only for your convenience   You do not get a newly allocated string with the first and second argument concatenated  which I d guess you expected based on your code

User · Answer

Avoid using strcat in C code  The cleanest and  most importantly  the safest way is to use snprintf   char buf 256   snprintf buf  sizeof buf    s s s s   str1  str2  str3  str4     Some commenters raised an issue that the number of arguments may not match the format string and the code will still compile  but most compilers already issue a warning if this is the case

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int main         char input 100       gets input        char str 101       strcpy str            strcat str  input        char  p   str       while  p           if  p         amp  amp  isalpha   p 1      0             printf   c    p 1           p               return 0

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The first argument of strcat   needs to be able to hold enough space for the concatenated string  So allocate a buffer with enough space to receive the result   char bigEnough 64         strcat bigEnough   TEXT    strcat bigEnough  foo       and so on      strcat   will concatenate the second argument with the first argument  and store the result in the first argument  the returned char  is simply this first argument  and only for your convenience   You do not get a newly allocated string with the first and second argument concatenated  which I d guess you expected based on your code

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In C   strings  are just plain char arrays  Therefore  you can t directly concatenate them with other  strings     You can use the strcat function  which appends the string pointed to by src to the end of the string pointed to by dest   char  strcat char  dest  const char  src     Here is an example from cplusplus com   char str 80   strcpy str   these     strcat str   strings     strcat str   are     strcat str   concatenated       For the first parameter  you need to provide the destination buffer itself   The destination buffer must be a char array buffer   E g   char buffer 1024    Make sure that the first parameter has enough space to store what you re trying to copy into it   If available to you  it is safer to use functions like  strcpy s and strcat s where you explicitly have to specify the size of the destination buffer    Note  A string literal cannot be used as a buffer  since it is a constant  Thus  you always have to allocate a char array for the buffer   The return value of strcat can simply be ignored  it merely returns the same pointer as was passed in as the first argument  It is there for convenience  and allows you to chain the calls into one line of code   strcat strcat str  foo   bar     So your problem could be solved as follows   char  foo    foo   char  bar    bar   char str 80   strcpy str   TEXT     strcat str  foo   strcat str  bar

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Also malloc and realloc are useful if you don t know ahead of time how many strings are being concatenated    include  lt stdio h gt   include  lt string h gt   void example const char  header  const char   words  size t num words        size t message len   strlen header    1       1 for terminating NULL        char  message    char   malloc message len       strncat message  header  message len        for int i   0  i  lt  num words    i               message len    1   strlen words i       1   for separator               message    char   realloc message  message len          strncat strncat message       message len   words i   message len              puts message        free message

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It is undefined behaviour to attempt to modify string literals  which is what something like   strcat   Hello     name     will attempt to do  It will try to tack on the name string to the end of the string literal  Hello     which is not well defined   Try something this  It achieves what you appear to be trying to do   char message 1000   strcpy  message   TEXT     strcat  message  var     This creates a buffer area that is allowed to be modified and then copies both the string literal and other text to it  Just be careful with buffer overflows  If you control the input data  or check it before-hand   it s fine to use fixed length buffers like I have   Otherwise  you should use mitigation strategies such as allocating enough memory from the heap to ensure you can handle it  In other words  something like   const static char TEXT      TEXT        Make  sure  you have enough space   char  message   malloc  sizeof TEXT    strlen var    1   if  message    NULL       handleOutOfMemoryIntelligently    strcpy  message  TEXT   strcat  message  var       Need to free message at some point after you re done with it

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Best way to do it without having a limited buffer size is by using asprintf    char  concat const char  str1  const char  str2        char  result      asprintf  amp result    s s   str1  str2       return result

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int main         char input 100       gets input        char str 101       strcpy str            strcat str  input        char  p   str       while  p           if  p         amp  amp  isalpha   p 1      0             printf   c    p 1           p               return 0

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Folks  use strncpy    strncat    or snprintf    Exceeding your buffer space will trash whatever else follows in memory   And remember to allow space for the trailing null   0  character

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Strings can also be concatenated at compile time     define SCHEMA  test   define TABLE   data   const char  table   SCHEMA     TABLE      note no   or   or anything const char  qry                    include comments in a string       SELECT                       get all fields       FROM   SCHEMA     TABLE      the table          WHERE x   1                  the filter

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This was my solution    include  lt stdlib h gt   include  lt stdarg h gt   char  strconcat int num args             int strsize   0      va list ap      va start ap  num args       for  int i   0  i  lt  num args  i             strsize    strlen va arg ap  char          char  res   malloc strsize 1       strsize   0      va start ap  num args       for  int i   0  i  lt  num args  i              char  s   va arg ap  char            strcpy res strsize  s           strsize    strlen s             va end ap       res strsize      0        return res      but you need to specify how many strings you re going to concatenate  char  str   strconcat 3   testing     this     thing

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As people pointed out string handling improved much  So you may want to learn how to use the C   string library instead of C-style strings  However here is a solution in pure C   include  lt string h gt   include  lt stdio h gt   include  lt stdlib h gt   void appendToHello const char  s        const char  const hello    hello         const size t sLength       strlen s       const size t helloLength   strlen hello       const size t totalLength   sLength   helloLength       char  const strBuf   malloc totalLength   1       if  strBuf    NULL            fprintf stderr   malloc failed n            exit EXIT FAILURE              strcpy strBuf  hello       strcpy strBuf   helloLength  s        puts strBuf        free strBuf       int main  void        appendToHello  blah blah        return 0      I am not sure whether it is correct safe but right now I could not find a better way to do this in ANSI C

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You are trying to copy a string into an address that is statically allocated  You need to cat into a buffer   Specifically      snip     destination  Pointer to the destination array  which should contain a C string  and be large enough to contain the concatenated resulting string       snip     http   www cplusplus com reference clibrary cstring strcat html  There s an example here as well

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You are trying to copy a string into an address that is statically allocated  You need to cat into a buffer   Specifically      snip     destination  Pointer to the destination array  which should contain a C string  and be large enough to contain the concatenated resulting string       snip     http   www cplusplus com reference clibrary cstring strcat html  There s an example here as well

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You are trying to copy a string into an address that is statically allocated  You need to cat into a buffer   Specifically      snip     destination  Pointer to the destination array  which should contain a C string  and be large enough to contain the concatenated resulting string       snip     http   www cplusplus com reference clibrary cstring strcat html  There s an example here as well

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Also malloc and realloc are useful if you don t know ahead of time how many strings are being concatenated    include  lt stdio h gt   include  lt string h gt   void example const char  header  const char   words  size t num words        size t message len   strlen header    1       1 for terminating NULL        char  message    char   malloc message len       strncat message  header  message len        for int i   0  i  lt  num words    i               message len    1   strlen words i       1   for separator               message    char   realloc message  message len          strncat strncat message       message len   words i   message len              puts message        free message

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Try something similar to this    include  lt stdio h gt   include  lt string h gt   int main int argc  const char   argv           Insert code here      char firstname 100   secondname 100     printf  Enter First Name        fgets firstname  100  stdin     printf  Enter Second Name        fgets secondname 100 stdin     firstname strlen firstname -1     0     printf  fullname is  s  s   firstname  secondname      return 0

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This was my solution    include  lt stdlib h gt   include  lt stdarg h gt   char  strconcat int num args             int strsize   0      va list ap      va start ap  num args       for  int i   0  i  lt  num args  i             strsize    strlen va arg ap  char          char  res   malloc strsize 1       strsize   0      va start ap  num args       for  int i   0  i  lt  num args  i              char  s   va arg ap  char            strcpy res strsize  s           strsize    strlen s             va end ap       res strsize      0        return res      but you need to specify how many strings you re going to concatenate  char  str   strconcat 3   testing     this     thing

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Best way to do it without having a limited buffer size is by using asprintf    char  concat const char  str1  const char  str2        char  result      asprintf  amp result    s s   str1  str2       return result

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It is undefined behaviour to attempt to modify string literals  which is what something like   strcat   Hello     name     will attempt to do  It will try to tack on the name string to the end of the string literal  Hello     which is not well defined   Try something this  It achieves what you appear to be trying to do   char message 1000   strcpy  message   TEXT     strcat  message  var     This creates a buffer area that is allowed to be modified and then copies both the string literal and other text to it  Just be careful with buffer overflows  If you control the input data  or check it before-hand   it s fine to use fixed length buffers like I have   Otherwise  you should use mitigation strategies such as allocating enough memory from the heap to ensure you can handle it  In other words  something like   const static char TEXT      TEXT        Make  sure  you have enough space   char  message   malloc  sizeof TEXT    strlen var    1   if  message    NULL       handleOutOfMemoryIntelligently    strcpy  message  TEXT   strcat  message  var       Need to free message at some point after you re done with it

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Do not forget to initialize the output buffer  The first argument to strcat must be a null terminated string with enough extra space allocated for the resulting string   char out 1024           must be initialized strcat  out  null terminated string        null terminated string has less than 1023 chars

User · Answer

Avoid using strcat in C code  The cleanest and  most importantly  the safest way is to use snprintf   char buf 256   snprintf buf  sizeof buf    s s s s   str1  str2  str3  str4     Some commenters raised an issue that the number of arguments may not match the format string and the code will still compile  but most compilers already issue a warning if this is the case

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It is undefined behaviour to attempt to modify string literals  which is what something like   strcat   Hello     name     will attempt to do  It will try to tack on the name string to the end of the string literal  Hello     which is not well defined   Try something this  It achieves what you appear to be trying to do   char message 1000   strcpy  message   TEXT     strcat  message  var     This creates a buffer area that is allowed to be modified and then copies both the string literal and other text to it  Just be careful with buffer overflows  If you control the input data  or check it before-hand   it s fine to use fixed length buffers like I have   Otherwise  you should use mitigation strategies such as allocating enough memory from the heap to ensure you can handle it  In other words  something like   const static char TEXT      TEXT        Make  sure  you have enough space   char  message   malloc  sizeof TEXT    strlen var    1   if  message    NULL       handleOutOfMemoryIntelligently    strcpy  message  TEXT   strcat  message  var       Need to free message at some point after you re done with it

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Do not forget to initialize the output buffer  The first argument to strcat must be a null terminated string with enough extra space allocated for the resulting string   char out 1024           must be initialized strcat  out  null terminated string        null terminated string has less than 1023 chars

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In C   strings  are just plain char arrays  Therefore  you can t directly concatenate them with other  strings     You can use the strcat function  which appends the string pointed to by src to the end of the string pointed to by dest   char  strcat char  dest  const char  src     Here is an example from cplusplus com   char str 80   strcpy str   these     strcat str   strings     strcat str   are     strcat str   concatenated       For the first parameter  you need to provide the destination buffer itself   The destination buffer must be a char array buffer   E g   char buffer 1024    Make sure that the first parameter has enough space to store what you re trying to copy into it   If available to you  it is safer to use functions like  strcpy s and strcat s where you explicitly have to specify the size of the destination buffer    Note  A string literal cannot be used as a buffer  since it is a constant  Thus  you always have to allocate a char array for the buffer   The return value of strcat can simply be ignored  it merely returns the same pointer as was passed in as the first argument  It is there for convenience  and allows you to chain the calls into one line of code   strcat strcat str  foo   bar     So your problem could be solved as follows   char  foo    foo   char  bar    bar   char str 80   strcpy str   TEXT     strcat str  foo   strcat str  bar

[c] How do I concatenate const/literal strings in C?

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