sscanf a string containing variable length strings and numbers in C

Question

I have a string such as: "1.5 3 aaa 0 bbbbbbbbbb" which I need to sscanf (I know exactly how many space-separated elements I expect and what their data type is, but I don't know the length of the two strings aaa and bbbbbbbbbb).

I would like to have something like this:

sscanf(rx_buffer, "%f %d %s %d %s", &a, &b, string1, &c, string2);

What is a simple, nice and effective way of storing the two strings into string1 and string2 (two char *)? Is maybe realloc involved?

Thank you in advance, every tip is greatly appreciated.

A "simple, nice and effective" way is to use strtod, strtol, and strchr. — William Pursell
– William Pursell, Commented Jun 25, 2020 at 13:27

Stephan Lechner · Accepted Answer · 2020-06-25 11:58:41Z

1

You cannot interrupt sscanf while reading in a string, so you have no chance to perform a realloc once you exceed some buffer.

One thing you can do is providing a buffer that is large enough to hold the longest string you assume; you can limit the number of characters you accept in order to avoid overflows; and you can check how many items sscanf successfully scanned:

char string1[100];
char string2[100];
int elementsScanned = sscanf(rx_buffer, "%f %d %99s %d %99s", &a, &b, string1, &c, string2);
if (elementsScanned != 5) ... // do some error handling here

Another thing you can do is to assume that neither string1 nor string2 can ever get larger than rx_buffer itself. So you could allocate memory accordingly. This is "wasting" some memory, but you are on the save side:

size_t len = strlen(rx_buffer);
char* string1 = malloc(len+1);
char* string2 = malloc(len+1);
int elementsScanned = sscanf(rx_buffer, "%f %d %s %d %s", &a, &b, string1, &c, string2);

edited Jun 25, 2020 at 11:58

answered Jun 25, 2020 at 11:53

Stephan Lechner

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1 Comment

godo Over a year ago

Thanks, but that is exactly what I would like to avoid, i.e.: having to define many strings (two was just an example for simplicity) considering the worst case scenario. I wonder if there is a nice sscanf alternative which could output the desired result.

chqrlie · Accepted Answer · 2020-06-25 20:59:02Z

If you can assume a maximum length for the strings to parse from rx_buffer, sscanf() is a simple an effective solution.

If rx_buffer can have an arbitrary length, you could parse it using library functions, such as strtod(), strtol(), strspn(), strcspn() and strndup():

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

struct context {
    float a;
    int b;
    char *string1;
    int c;
    char *string2;
};

// return the number of fields parsed
int parse_response(struct context *cp, const char *rx_buffer) {
    //sscanf(rx_buffer, "%f %d %s %d %s", &a, &b, string1, &c, string2);
    const char *src = rx_buffer;
    char *p;
    size_t n;
    cp->a = strtod(src, &p);
    if (p == src)
        return 0;
    cp->b = strtol(src = p, &p, 10);
    if (p == src)
        return 1;
    src = p + strspn(p, " \t\r\n\f\v");
    n = strcspn(src, " \t\r\n\f\v");
    if (n == 0)
        return 2;
    if ((cp->string1 = strndup(src, n)) == NULL)
        return -1;
    src += n;
    src += strspn(src, " \t\r\n\f\v");
    cp->c = strtol(src, &p, 10);
    if (p == src)
        return 3;
    src = p + strspn(p, " \t\r\n\f\v");
    n = strcspn(src, " \t\r\n\f\v");
    if (n == 0)
        return 4;
    if ((cp->string2 = strndup(src, n)) == NULL)
        return -1;
    return 5;
}

This is obviously neither simple nor nice.

If your target is a linux system with the GNU libc, you could use an extension of their sscanf() implementation: the m prefix between % and s provides a simple, nice and effective solution:

#include <stdio.h>

// return the number of fields parsed
int parse_response(const char *rx_buffer) {
    float a;
    int b, c;
    char *string1 = NULL, *string2 = NULL;
    int n = sscanf(rx_buffer, "%f %d %ms %d %ms", &a, &b, &string1, &c, &string2);
    if (n == 5) {
        // all fields parsed correctly, handle values
        ...
    }
    free(string1);
    free(string2);
    return n;
}

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