Why is 'n' parameter of snprintf ignored?

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I have found that the n parameter of snprintf() seems to be ignored in my code.



char asdf[10];
Serial1.println(snprintf(asdf, 2, "hello"));


This prints 5 when I would expect it to print 2. What is happening?










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    up vote
    1
    down vote

    favorite












    I have found that the n parameter of snprintf() seems to be ignored in my code.



    char asdf[10];
    Serial1.println(snprintf(asdf, 2, "hello"));


    This prints 5 when I would expect it to print 2. What is happening?










    share|improve this question









    New contributor




    Westin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
    Check out our Code of Conduct.





















      up vote
      1
      down vote

      favorite









      up vote
      1
      down vote

      favorite











      I have found that the n parameter of snprintf() seems to be ignored in my code.



      char asdf[10];
      Serial1.println(snprintf(asdf, 2, "hello"));


      This prints 5 when I would expect it to print 2. What is happening?










      share|improve this question









      New contributor




      Westin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      I have found that the n parameter of snprintf() seems to be ignored in my code.



      char asdf[10];
      Serial1.println(snprintf(asdf, 2, "hello"));


      This prints 5 when I would expect it to print 2. What is happening?







      string






      share|improve this question









      New contributor




      Westin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      share|improve this question









      New contributor




      Westin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.









      share|improve this question




      share|improve this question








      edited 23 mins ago









      Greenonline

      1,92241639




      1,92241639






      New contributor




      Westin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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      asked 55 mins ago









      Westin

      82




      82




      New contributor




      Westin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.





      New contributor





      Westin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






      Westin is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.




















          3 Answers
          3






          active

          oldest

          votes

















          up vote
          2
          down vote



          accepted










          snprintf() will not write more than <size> (snprintf's 2d argument) characters to your buffer, but it does count (and discard the extra) characters it would have written, had there been space enough, and that is the number it returns. Yeah, it can be confusing!



          See this snprintf() reference.






          share|improve this answer



























            up vote
            3
            down vote













            A test sketch for the Arduino Uno:



            char buffer[10];

            void setup()
            Serial.begin(9600);
            int n = snprintf(buffer, 2, "hello");
            Serial.println(n);
            Serial.println(buffer);


            void loop()



            As @JRobert wrote, the "would have" is the key. As far as I know only the snprintf and the vsnprintf return a "would have" number.



            I think the reason is to be able to tell if the string was truncated. Suppose the 'size' parameter is 25 and the format string is very long, then the return value can be tested against 25. If the return value was 26 (the "would have" number of bytes), then the string was truncated.

            This information was not possible to retrieve when the "would have" number was not available.






            share|improve this answer





























              up vote
              0
              down vote













              For completion, the man page for fprintf states:




              The snprintf() function shall be equivalent to sprintf(), with the
              addition of the n argument which states the size of the buffer
              referred to by s. If n is zero, nothing shall be written and s may
              be a null pointer. Otherwise, output bytes beyond the n‐1st shall be
              discarded instead of being written to the array, and a null byte is
              written at the end of the bytes actually written into the array.




              and, more relevant:




              Upon successful completion, the snprintf() function shall return the
              number of bytes that would be written to s had n been sufficiently
              large excluding the terminating null byte.







              share|improve this answer




















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                3 Answers
                3






                active

                oldest

                votes








                3 Answers
                3






                active

                oldest

                votes









                active

                oldest

                votes






                active

                oldest

                votes








                up vote
                2
                down vote



                accepted










                snprintf() will not write more than <size> (snprintf's 2d argument) characters to your buffer, but it does count (and discard the extra) characters it would have written, had there been space enough, and that is the number it returns. Yeah, it can be confusing!



                See this snprintf() reference.






                share|improve this answer
























                  up vote
                  2
                  down vote



                  accepted










                  snprintf() will not write more than <size> (snprintf's 2d argument) characters to your buffer, but it does count (and discard the extra) characters it would have written, had there been space enough, and that is the number it returns. Yeah, it can be confusing!



                  See this snprintf() reference.






                  share|improve this answer






















                    up vote
                    2
                    down vote



                    accepted







                    up vote
                    2
                    down vote



                    accepted






                    snprintf() will not write more than <size> (snprintf's 2d argument) characters to your buffer, but it does count (and discard the extra) characters it would have written, had there been space enough, and that is the number it returns. Yeah, it can be confusing!



                    See this snprintf() reference.






                    share|improve this answer












                    snprintf() will not write more than <size> (snprintf's 2d argument) characters to your buffer, but it does count (and discard the extra) characters it would have written, had there been space enough, and that is the number it returns. Yeah, it can be confusing!



                    See this snprintf() reference.







                    share|improve this answer












                    share|improve this answer



                    share|improve this answer










                    answered 39 mins ago









                    JRobert

                    9,13811035




                    9,13811035




















                        up vote
                        3
                        down vote













                        A test sketch for the Arduino Uno:



                        char buffer[10];

                        void setup()
                        Serial.begin(9600);
                        int n = snprintf(buffer, 2, "hello");
                        Serial.println(n);
                        Serial.println(buffer);


                        void loop()



                        As @JRobert wrote, the "would have" is the key. As far as I know only the snprintf and the vsnprintf return a "would have" number.



                        I think the reason is to be able to tell if the string was truncated. Suppose the 'size' parameter is 25 and the format string is very long, then the return value can be tested against 25. If the return value was 26 (the "would have" number of bytes), then the string was truncated.

                        This information was not possible to retrieve when the "would have" number was not available.






                        share|improve this answer


























                          up vote
                          3
                          down vote













                          A test sketch for the Arduino Uno:



                          char buffer[10];

                          void setup()
                          Serial.begin(9600);
                          int n = snprintf(buffer, 2, "hello");
                          Serial.println(n);
                          Serial.println(buffer);


                          void loop()



                          As @JRobert wrote, the "would have" is the key. As far as I know only the snprintf and the vsnprintf return a "would have" number.



                          I think the reason is to be able to tell if the string was truncated. Suppose the 'size' parameter is 25 and the format string is very long, then the return value can be tested against 25. If the return value was 26 (the "would have" number of bytes), then the string was truncated.

                          This information was not possible to retrieve when the "would have" number was not available.






                          share|improve this answer
























                            up vote
                            3
                            down vote










                            up vote
                            3
                            down vote









                            A test sketch for the Arduino Uno:



                            char buffer[10];

                            void setup()
                            Serial.begin(9600);
                            int n = snprintf(buffer, 2, "hello");
                            Serial.println(n);
                            Serial.println(buffer);


                            void loop()



                            As @JRobert wrote, the "would have" is the key. As far as I know only the snprintf and the vsnprintf return a "would have" number.



                            I think the reason is to be able to tell if the string was truncated. Suppose the 'size' parameter is 25 and the format string is very long, then the return value can be tested against 25. If the return value was 26 (the "would have" number of bytes), then the string was truncated.

                            This information was not possible to retrieve when the "would have" number was not available.






                            share|improve this answer














                            A test sketch for the Arduino Uno:



                            char buffer[10];

                            void setup()
                            Serial.begin(9600);
                            int n = snprintf(buffer, 2, "hello");
                            Serial.println(n);
                            Serial.println(buffer);


                            void loop()



                            As @JRobert wrote, the "would have" is the key. As far as I know only the snprintf and the vsnprintf return a "would have" number.



                            I think the reason is to be able to tell if the string was truncated. Suppose the 'size' parameter is 25 and the format string is very long, then the return value can be tested against 25. If the return value was 26 (the "would have" number of bytes), then the string was truncated.

                            This information was not possible to retrieve when the "would have" number was not available.







                            share|improve this answer














                            share|improve this answer



                            share|improve this answer








                            edited 26 mins ago

























                            answered 32 mins ago









                            Jot

                            1,771416




                            1,771416




















                                up vote
                                0
                                down vote













                                For completion, the man page for fprintf states:




                                The snprintf() function shall be equivalent to sprintf(), with the
                                addition of the n argument which states the size of the buffer
                                referred to by s. If n is zero, nothing shall be written and s may
                                be a null pointer. Otherwise, output bytes beyond the n‐1st shall be
                                discarded instead of being written to the array, and a null byte is
                                written at the end of the bytes actually written into the array.




                                and, more relevant:




                                Upon successful completion, the snprintf() function shall return the
                                number of bytes that would be written to s had n been sufficiently
                                large excluding the terminating null byte.







                                share|improve this answer
























                                  up vote
                                  0
                                  down vote













                                  For completion, the man page for fprintf states:




                                  The snprintf() function shall be equivalent to sprintf(), with the
                                  addition of the n argument which states the size of the buffer
                                  referred to by s. If n is zero, nothing shall be written and s may
                                  be a null pointer. Otherwise, output bytes beyond the n‐1st shall be
                                  discarded instead of being written to the array, and a null byte is
                                  written at the end of the bytes actually written into the array.




                                  and, more relevant:




                                  Upon successful completion, the snprintf() function shall return the
                                  number of bytes that would be written to s had n been sufficiently
                                  large excluding the terminating null byte.







                                  share|improve this answer






















                                    up vote
                                    0
                                    down vote










                                    up vote
                                    0
                                    down vote









                                    For completion, the man page for fprintf states:




                                    The snprintf() function shall be equivalent to sprintf(), with the
                                    addition of the n argument which states the size of the buffer
                                    referred to by s. If n is zero, nothing shall be written and s may
                                    be a null pointer. Otherwise, output bytes beyond the n‐1st shall be
                                    discarded instead of being written to the array, and a null byte is
                                    written at the end of the bytes actually written into the array.




                                    and, more relevant:




                                    Upon successful completion, the snprintf() function shall return the
                                    number of bytes that would be written to s had n been sufficiently
                                    large excluding the terminating null byte.







                                    share|improve this answer












                                    For completion, the man page for fprintf states:




                                    The snprintf() function shall be equivalent to sprintf(), with the
                                    addition of the n argument which states the size of the buffer
                                    referred to by s. If n is zero, nothing shall be written and s may
                                    be a null pointer. Otherwise, output bytes beyond the n‐1st shall be
                                    discarded instead of being written to the array, and a null byte is
                                    written at the end of the bytes actually written into the array.




                                    and, more relevant:




                                    Upon successful completion, the snprintf() function shall return the
                                    number of bytes that would be written to s had n been sufficiently
                                    large excluding the terminating null byte.








                                    share|improve this answer












                                    share|improve this answer



                                    share|improve this answer










                                    answered 14 mins ago









                                    Greenonline

                                    1,92241639




                                    1,92241639




















                                        Westin is a new contributor. Be nice, and check out our Code of Conduct.









                                         

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