sscanf parse response with whitespaces

Question

I need to parse a response from a device (SIM900) like this:

\r\n+CIPRXGET:1

+CIPRXGET: 2,1,3
DATA COMPOSED BY A WHITESPACE AND MAYBE OTHER
OK

so i use sscanf twice: first to remove the final string "OK" and second to parse data.

char buffer[256] = sim900.getResponse();
char data[256];
int bytesRead, bytesToRead;
sscanf(buffer, "%[^OK]", buffer);
sscanf(buffer, "%*s,%d,%d\r\n%[^\\0]", &bytesRead, &bytesToRead, data);

my response start with a whitespace (character 0x20) and i got a dirty output, that is "\r\n \r\n" (or in hex representation "0x0D 0x0A 0x20 0x0D 0x0A").

I tried everything but i can't parse correctly only the whitespace character into the output buffer.

You should check the return value of sscanf to better understand what is going wrong. Can you please state more clearly what output are you getting now and what you would like to have. — terence hill, Dec 16 '15 at 16:49
C11 draft standard, `7.21.6.7 The sscanf function, Section 2 [...] If copying takes place between objects that overlap, the behavior is undefined.` — EOF, Dec 16 '15 at 16:50
now i'm getting " ", where and are \r and \n. I want to obtain only the whitespace beetween s. — Francesco S., Dec 16 '15 at 16:52
@EOF i tried to use different buffers and i obtained the same result. — Francesco S., Dec 16 '15 at 16:54
What part of *undefined* do you not understand? Whether or not you "obtained the same result" is immaterial. You cannot reason about the program until it does not exhibit undefined behavior. — EOF, Dec 16 '15 at 16:56

score 0 · Accepted Answer · edited May 23 '17 at 12:31

Problems:

sscanf(buffer, "%[^OK]", buffer); attempts to read and write to the same buffer. This is undefined behavior. Use different buffers. @EOF
"%[^OK]" Looks for all char that is not 'O' and is not 'K', so it stops at \r\n+CIPRX ... DATA C
"%*s" in sscanf("%*s,%d..." does 2 things 1) scan and not save all leading white-space characters. 2) scan and not save (because of '*') all non-white-space characters. There will never be a ',' following "all non-white-space characters", so sccanf() stops.

When using sscanf() and having troubles, the first thing to code is a check of the return values of sscanf().

Unclear as to OP overall goal, but perhaps the following will help.

#include <stdio.h>

char *text = 
"\r\n+CIPRXGET:1\r\n+CIPRXGET: 2,1,3\r\nDATA COMPOSED BY A WHITESPACE AND MAYBE OTHER\r\nOK";

int main(void) {
  char data[256];
  int bytesRead, bytesToRead;
  if (sscanf(text, "%*[^,],%d,%d %255[^\r\n]", &bytesRead, &bytesToRead, data) == 3) {
    printf("bytesRead:%d\nbytesToRead:%d\ndata:'%s'\n",bytesRead, bytesToRead, data);
  }
  return 0;
}

Output

bytesRead:1
bytesToRead:3
data:'DATA COMPOSED BY A WHITESPACE AND MAYBE OTHER'

sscanf parse response with whitespaces

1 Answers1