How to compare substrings and parse numbers from a C string - c

Question

I'm new to low level programming and have been stuck with the following problem:
I get a string in the following format:
?cmd=ASET&<hour>&<minute>
where the hour and minute values always consist of 2 decimal numbers.
So an example of the string that can be received is:
"?cmd=ASET&08&30"
I am trying to write an if statement that recognizes that the string starts with "?cmd=ASET" and changes two global variables called minute and hour to the values in the String. I've been trying to do this with strtok(), but have not had any luck so far. So the global layout of my if statement would be:
if (String starts with "?cmd=ASET") {
minute = value found in the string;
hour = value found in the string;
}
I would really appreciate any help.

Answer 1

Try something like this, where cmd is a char * or char [] type variable. Note, strncmp() is safer than strcmp(). In general, in C programming, you want to use the variants of functions that limit the length, to avoid stack overflow attacks and other security risks. And since string to numeric functions can fail if given bad input it is better to use a form where you can check their status, which is why atoi() and atol() are not recommended. sscanf() allows status to be checked as does strtol() so they are both acceptable.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int
main()
{
char *string = "?cmd=ASET&12&30";
#define ASET_CMD "?cmd=ASET"
int hour = 0, minute = 0;
if (strncmp(string, ASET_CMD, strlen(ASET_CMD)) == 0) {
if (sscanf(string + strlen(ASET_CMD), "&%d&%d", &hour, &minute) != 2) {
printf("Couldn't parse input string");
exit(EXIT_FAILURE);
}
}
printf("hour: %d, minute: %d\n", hour, minute);
return(EXIT_SUCCESS);
}
$ cc -o prog prog.c
$ ./prog
hour: 12, minute: 30

Answer 2

If sscanf() is available for OP, consider:
unsigned hour, minute;
int n;
int cnt = sscanf(buffer, "?cmd=ASET&%2u&%2u%n", &hour, &minute, &n);
if (cnt == 2 && buffer[n] == 0) Success();
else Failure();
cnt will have the value of 2 if the prefix matches and 2 numbers where found.
The n detects if any addtional characters exist in the string.

Answer 3

there is a function atoi() which returns integer from string.
You can parse string like that
char string[16];
strcpy(string, "?cmd=ASET&08&30");
if (String starts with "?cmd=ASET") {
hour = atoi(string+10);
minute = atoi(string+13);
}
where +10 is position for hour and +13 for minute.
here is a working example:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main ()
{
int hour, minute;
char string[16];
strcpy(string, "?cmd=ASET&08&30");
/* Check if string contains ?cmd=ASET */
if (strstr(string, "?cmd=ASET") != NULL)
{
/* Parse minute and hour */
hour = atoi(string+10);
minute = atoi(string+13);
/* Print output */
printf("Minute: %02d \nHour: %02d\n", minute, hour);
}
}