How do I make sscanf ignore white spaces? C

scanf ignores leading whitespace for most conversions, but what you want is the opposite of ignoring whitespace.

You cannot tell scanf to error out on a whitespace, but you can detect how many whitespace characters were consumed. For example:

  #include <stdio.h>

  int main()
  {
      int first, second, before, after;
      int nread;

      nread = scanf("%d,%n %n%d", &first, &before, &after, &second);
      if (nread != 2) {
          printf ("Error in the input stream, 2 items expected, %d matched\n", nread);
      }
      else if (before != after) {
          printf ("Error in the input stream, %d whitespace characters detected\n", 
                      after-before);
      }
      else {
          printf ("Got numbers %d %d\n", first, second);
      }
  }

This detects whitespace before the second input.

Having said that, erroring out on whitespace is probably not a good idea.

The %d conversion format ignores leading white space as well as an optional + sign. If you want a stricter validation, you can:

• use an extra validation phase before or after converting the values.
• use a hand coded conversion that rejects malformed input.

Here are some propositions:

#include <stdio.h>
#include <string.h>

int main() {
    char input[100];
    char verif[100];
    int a, b, c;

    if (fgets(input, sizeof input, stdin)) {
        if (sscanf(input, "%d,%d,%d", &a, &b, &c) == 3) {
            snprintf(verif, sizeof verif, "%d,%d,%d\n", a, b, c);
            if (strcmp(input, verif) == 0) {
                printf("input is valid, a=%d, b=%d, c=%d\n", a, b, c);
                return 0;
            } else {
                printf("input contains extra characters\n");
                return 1;
            }
        } else {
            printf("input does not match the pattern\n");
            return 1;
        }
    } else {
       printf("no input\n");
       return 1;
    }
}

You could just check if the input string contains any white space:

    if (input[strcspn(input, " \t")] != '\0') {
        printf("input string contains spaces or TABs\n");
    }

But negative values, redundant + signs and leading zeroes will pass the test (eg: "-1,+1,+0001").

If all values must be positive, you could use scanf() to perform a poor man's pattern matching:

    if (fgets(input, sizeof input, stdin)) {
        char c1, c2;
        if (sscanf(input, "%*[0-9],%*[0-9],%*[0-9]%c%c", &c1, &c2) == 1 && c1 == '\n') {
            /* input line contains exactly 3 numbers separated by a `,` */
        } else {
            printf("invalid format\n");
        }
    }

Note however these remarks:

• redundant leading zeroes would still be accepted by this validation phase (eg: "00,01,02").
• overlong numbers will technically cause undefined behavior in all of the above methods (eg: "0,0,999999999999999999999999999999"), But the first approach will detect this problem if the conversion just truncates or maximises the converted value.
• an overlong input line (longer than 98 bytes plus a newline) will be truncated by fgets(), leaving the rest of the input for the next read operation. This may be a problem too.

The solution to your problem depends on how strict and concise the validation must be.

One idea to validate the input is to use the regular expression to match three integers delimited by commas without including other characters.

#include <stdio.h>
#include <regex.h>
#include <string.h>

int main()
{
    regex_t preg;                               // regex pattern
    int ret;
    int n1, n2, n3;
    char *p;                                    // pointer to a character th the string
    char buffer[BUFSIZ];                        // input buffer

    // compile regex to match three integers delimited by commas
    ret = regcomp(&preg, "^([+-]?[[:digit:]]+,){2}[+-]?[[:digit:]]+$", REG_EXTENDED);
    if (ret) {
        fprintf(stderr, "Compile error of regex\n");
        exit(1);
    }

    while (fgets(buffer, BUFSIZ, stdin)) {
        if ((p = rindex(buffer, '\n'))) *p = 0; // remove trailing newline
        ret = regexec(&preg, buffer, 0, NULL, 0);
        if (!ret) {                             // input matches the regex pattern
            sscanf(buffer, "%d,%d,%d", &n1, &n2, &n3);
            printf("=> %d,%d,%d\n", n1, n2, n3);
        } else {
            printf("Error in the input: %s\n", buffer);
        }
    }

    return 0;
}

where ^([+-]?[[:digit:]]+,){2}[+-]?[[:digit:]]+$ is the regex:

• ^ anchors the start of the input string.
• [+-]? matches zero or one leading plus or minus sign.
• [[:digit:]]+, matches the sequence of digits followed by a comma.
• The parentheses ( pattern ) makes a group of the pattern.
• The quantifier {2} specifies the number of repetition of the previous atom (including group).
• $ anchors the end of the input string.