How to append strings using sprintf

Question

I am facing a serious issue with sprintf  Suppose my code snippet is  sprintf Buffer  quot Hello World quot    sprintf Buffer  quot Good Morning quot    sprintf Buffer  quot Good Afternoon quot           Some hundred sprints     If I do like this  it s getting overwritten  How can I avoid overwriting using sprintf  If I give a printf at the end I want to see all the lines

User · Answer

Why do you want to use sprintf for string concatenation when there are methods intended specifically for what you need such as strcat and strncat

User · Answer

For safety  buffer overflow  I recommend to use snprintf     const int MAX BUF   1000  char  Buffer   malloc MAX BUF    int length   0  length    snprintf Buffer length  MAX BUF-length   Hello World    length    snprintf Buffer length  MAX BUF-length   Good Morning    length    snprintf Buffer length  MAX BUF-length   Good Afternoon

User · Answer

A snprintfcat   wrapper for snprintf     size t  snprintfcat      char  buf      size t bufSize      char const  fmt                 size t result      va list args      size t len   strnlen  buf  bufSize        va start  args  fmt       result   vsnprintf  buf   len  bufSize - len  fmt  args       va end  args        return result   len

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int length   0  length    sprintf Buffer length   Hello World    length    sprintf Buffer length   Good Morning    length    sprintf Buffer length   Good Afternoon      Here is a version with some resistance to errors   It is useful if you do not care when errors happen so long as you can continue along your merry way when they do   int bytes added  int result of sprintf         return  result of sprintf  gt  0    result of sprintf   0     int length   0  length    bytes added sprintf Buffer length   Hello World     length    bytes added sprintf Buffer length   Good Morning     length    bytes added sprintf Buffer length   Good Afternoon

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I think you are looking for fmemopen 3     include  lt assert h gt   include  lt stdio h gt   int main void        char buf 128      0        FILE  fp   fmemopen buf  sizeof buf    w         assert fp        fprintf fp   Hello World  n        fprintf fp    s also work  of course  n    Format specifiers        fclose fp        puts buf       return 0      If dynamic storage is more suitable for you use-case you could follow Liam s excellent suggestion about using open memstream 3     include  lt assert h gt   include  lt stdio h gt   include  lt stdlib h gt   int main void        char  buf      size t size      FILE  fp   open memstream  amp buf   amp size        assert fp        fprintf fp   Hello World  n        fprintf fp    s also work  of course  n    Format specifiers        fclose fp        puts buf       free buf       return 0

User · Answer

I find the following method works nicely   sprintf Buffer  Hello World    sprintf  amp Buffer strlen Buffer    Good Morning    sprintf  amp Buffer strlen Buffer    Good Afternoon

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Small full code example  Using flat plain stdio standard library only   include  lt stdio h gt  int main             char c 1024       int  i 0       i  sprintf c i  We            i  sprintf c i  Love             sprintf c i  Coding         printf   s  c           OUTPUT  We Love Coding

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You need   sprintf Buffer  Hello World    sprintf Buffer   strlen Buffer   Good Morning    sprintf Buffer   strlen Buffer   Good Afternoon      and of course you need your buffer to be big enough

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Are you simply appending string literals   Or are you going to be appending various data types  ints  floats  etc       It might be easier to abstract this out into its own function  the following assumes C99     include  lt stdio h gt   include  lt stdarg h gt   include  lt string h gt   int appendToStr char  target  size t targetSize  const char   restrict format           va list args    char temp targetSize     int result     va start args  format     result   vsnprintf temp  targetSize  format  args     if  result    EOF          if  strlen temp    strlen target   gt  targetSize              fprintf stderr   appendToStr  target buffer not large enough to hold additional string          return 0            strcat target  temp         va end args     return result      And you would use it like so   char target 100     0       appendToStr target  sizeof target    s  d  f n    This is a test   42  3 14159   appendToStr target  sizeof target   blah blah blah      etc   The function returns the value from vsprintf  which in most implementations is the number of bytes written to the destination   There are a few holes in this implementation  but it should give you some ideas

User · Answer

Using strcat buffer  Your new string   here    as an option

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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  100  i              str append  amp json   id- d   i              printf   s n   json        if  json  free json        return 0

User · Answer

What about   char s 100         sprintf s    s s   s   s1     sprintf s    s s   s   s2     sprintf s    s s   s   s3     printf   s   s     But take into account possible buffer ovewflows

User · Answer

Use the return value of sprintf    Buffer    sprintf Buffer  Hello World    Buffer    sprintf Buffer  Good Morning    Buffer    sprintf Buffer  Good Afternoon

User · Answer

Use strcat http   www cplusplus com reference cstring strcat   int main                char str 80         strcpy  str  these           strcat  str  strings           strcat  str  are           strcat  str  concatenated           puts  str         return 0                Output        these strings are concatenated

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char string1      test   char string2      string   int len   sizeof string1    sizeof string2   char totalString len   sprintf totalString    s s  string1 string2

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You can use the simple line shown below to append strings in one buffer   sprintf Buffer   s  s  s   Hello World   Good Morning   Good Afternoon

[c] How to append strings using sprintf?

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