Creating C formatted strings not printing them

Question

I have a function that accepts a string  that is   void log out char       In calling it  I need to create a formatted string on the fly like   int i   1  log out  some text  d   i     How do I do this in ANSI C     Only  since sprintf   returns a int  this means that I have to write at least 3 commands  like   char  s  sprintf s    d t d   ix  iy   log out s     Any way to shorten this

User · Answer

If you have a POSIX-2008 compliant system (any modern Linux), you can use the safe and convenient asprintf() function: It will malloc() enough memory for you, you don't need to worry about the maximum string size. Use it like this:

char* string;
if(0 > asprintf(&string, "Formatting a number: %d\n", 42)) return error;
log_out(string);
free(string);

This is the minimum effort you can get to construct the string in a secure fashion. The sprintf() code you gave in the question is deeply flawed:

There is no allocated memory behind the pointer. You are writing the string to a random location in memory!
Even if you had written
```
char s[42];
```
you would be in deep trouble, because you can't know what number to put into the brackets.
Even if you had used the "safe" variant snprintf(), you would still run the danger that your strings gets truncated. When writing to a log file, that is a relatively minor concern, but it has the potential to cut off precisely the information that would have been useful. Also, it'll cut off the trailing endline character, gluing the next log line to the end of your unsuccessfully written line.
If you try to use a combination of malloc() and snprintf() to produce correct behavior in all cases, you end up with roughly twice as much code than I have given for asprintf(), and basically reprogram the functionality of asprintf().

If you are looking at providing a wrapper of log_out() that can take a printf() style parameter list itself, you can use the variant vasprintf() which takes a va_list as an argument. Here is a perfectly safe implementation of such a wrapper:

//Tell gcc that we are defining a printf-style function so that it can do type checking.
//Obviously, this should go into a header.
void log_out_wrapper(const char *format, ...) __attribute__ ((format (printf, 1, 2)));

void log_out_wrapper(const char *format, ...) {
    char* string;
    va_list args;

    va_start(args, format);
    if(0 > vasprintf(&string, format, args)) string = NULL;    //this is for logging, so failed allocation is not fatal
    va_end(args);

    if(string) {
        log_out(string);
        free(string);
    } else {
        log_out("Error while logging a message: Memory allocation failed.\n");
    }
}

User · Answer

http   www gnu org software hello manual libc Variable-Arguments-Output html gives the following example to print to stderr  You can modify it to use your log function instead     include  lt stdio h gt    include  lt stdarg h gt    void  eprintf  const char  template             va list ap     extern char  program invocation short name      fprintf  stderr    s     program invocation short name      va start  ap  template      vfprintf  stderr  template  ap      va end  ap        Instead of vfprintf you will need to use vsprintf where you need to provide an adequate buffer to print into

User · Answer

If you have the code to log out    rewrite it   Most likely  you can do   static FILE  logfp         void log out const char  fmt             va list args       va start args  fmt       vfprintf logfp  fmt  args       va end args       If there is extra logging information needed  that can be printed before or after the message shown   This saves memory allocation and dubious buffer sizes and so on and so forth   You probably need to initialize logfp to zero  null pointer  and check whether it is null and open the log file as appropriate - but the code in the existing log out   should be dealing with that anyway   The advantage to this solution is that you can simply call it as if it was a variant of printf    indeed  it is a minor variant on printf     If you don t have the code to log out    consider whether you can replace it with a variant such as the one outlined above   Whether you can use the same name will depend on your application framework and the ultimate source of the current log out   function   If it is in the same object file as another indispensable function  you would have to use a new name   If you cannot work out how to replicate it exactly  you will have to use some variant like those given in other answers that allocates an appropriate amount of memory   void log out wrapper const char  fmt             va list args      size t  len      char    space       va start args  fmt       len   vsnprintf 0  0  fmt  args       va end args       if   space   malloc len   1      0                 va start args  fmt            vsnprintf space  len 1  fmt  args            va end args            log out space            free space                else - what to do if memory allocation fails         Obviously  you now call the log out wrapper   instead of log out   - but the memory allocation and so on is done once   I reserve the right to be over-allocating space by one unnecessary byte - I ve not double-checked whether the length returned by vsnprintf   includes the terminating null or not

User · Answer

Don t use sprintf  It will overflow your String-Buffer and crash your Program  Always use snprintf

User · Answer

Use sprintf  int sprintf   char   str  const char   format           Write formatted data to string Composes a string with the same text that would be printed if format was used on printf  but instead of being printed  the content is stored as a C string in the buffer pointed by str  The size of the buffer should be large enough to contain the entire resulting string  see snprintf for a safer version   A terminating null character is automatically appended after the content  After the format parameter  the function expects at least as many additional arguments as needed for format   Parameters  str   Pointer to a buffer where the resulting C-string is stored  The buffer should be large enough to contain the resulting string   format   C string that contains a format string that follows the same specifications as format in printf  see printf for details         additional arguments    Depending on the format string  the function may expect a sequence of additional arguments  each containing a value to be used to replace a format specifier in the format string  or a pointer to a storage location  for n   There should be at least as many of these arguments as the number of values specified in the format specifiers  Additional arguments are ignored by the function   Example     Allocates storage char  hello world    char  malloc 13   sizeof char       Prints  quot Hello world  quot  on hello world sprintf hello world   quot  s  s  quot    quot Hello quot    quot world quot

User · Answer

I haven t done this  so I m just going to point at the right answer   C has provisions for functions that take unspecified numbers of operands  using the  lt stdarg h gt  header   You can define your function as void log out const char  fmt         and get the va list inside the function   Then you can allocate memory and call vsprintf   with the allocated memory  format  and va list   Alternately  you could use this to write a function analogous to sprintf   that would allocate memory and return the formatted string  generating it more or less as above   It would be a memory leak  but if you re just logging out it may not matter

User · Answer

It sounds to me like you want to be able to easily pass a string created using printf-style formatting to the function you already have that takes a simple string  You can create a wrapper function using stdarg h facilities and vsnprintf    which may not be readily available  depending on your compiler platform     include  lt stdarg h gt   include  lt stdio h gt      a function that accepts a string   void foo  char  s       You d like to call a function that takes a format string      and then calls foo     void foofmt  char  fmt             char buf 100          this should really be sized appropriately                           possibly in response to a call to vsnprintf       va list vl      va start vl  fmt        vsnprintf  buf  sizeof  buf   fmt  vl        va end  vl        foo  buf        int main         int val   42       foofmt   Some value   d n   val       return 0      For platforms that don t provide a good implementation  or any implementation  of the snprintf   family of routines  I ve successfully used a nearly public domain snprintf   from Holger Weiss

[c] Creating C formatted strings (not printing them)

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