Formatted Output Functions

This section describes how to call printf and related functions. Prototypes for these functions are in the header file `stdio.h'. Because these functions take a variable number of arguments, you must declare prototypes for them before using them. Of course, the easiest way to make sure you have all the right prototypes is to just include `stdio.h'.

Function: int printf (const char *template, ...)

The printf function prints the optional arguments under the control of the template string template to the stream stdout. It returns the number of characters printed, or a negative value if there was an output error.

Function: int fprintf (FILE *stream, const char *template, ...)

This function is just like printf, except that the output is written to the stream stream instead of stdout.

Function: int sprintf (char *s, const char *template, ...)

This is like printf, except that the output is stored in the character array s instead of written to a stream. A null character is written to mark the end of the string.

The sprintf function returns the number of characters stored in the array s, not including the terminating null character.

The behavior of this function is undefined if copying takes place between objects that overlap--for example, if s is also given as an argument to be printed under control of the `%s' conversion. See section Copying and Concatenation.

Warning: The sprintf function can be dangerous because it can potentially output more characters than can fit in the allocation size of the string s. Remember that the field width given in a conversion specification is only a minimum value.

To avoid this problem, you can use snprintf or asprintf, described below.

Function: int snprintf (char *s, size_t size, const char *template, ...)

The snprintf function is similar to sprintf, except that the size argument specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least size characters for the string s.

The return value is the number of characters stored, not including the terminating null. If this value equals size - 1, then there was not enough space in s for all the output. You should try again with a bigger output string. Here is an example of doing this:

/* Construct a message describing the value of a variable
   whose name is name and whose value is value. */
char *
make_message (char *name, char *value)
{
  /* Guess we need no more than 100 chars of space. */
  int size = 100;
  char *buffer = (char *) xmalloc (size);
  while (1)
    {
      /* Try to print in the allocated space. */
      int nchars = snprintf (buffer, size,
                             "value of %s is %s",
                             name, value);
      /* If that worked, return the string. */
      if (nchars < size)
        return buffer;
      /* Else try again with twice as much space. */
      size *= 2;
      buffer = (char *) xrealloc (size, buffer);
    }
}

In practice, it is often easier just to use asprintf, below.

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