c - Saving regex matched strings to an array of strings -

so im starting pick c end goal write function searches string regular expression , returns array of matches.

the biggest problem i'm having saving strings memory can return or referenced in pointer passed in parameter.

i'd way tell how many matches there c# equivalent; if(matches.count() > 0) { /* have match! */ } resulting string of each match group depending on pattern i'll pass in.

i know isn't correct , has other errors in practice here code walked away trying figure out reading on pointers, structs, char arrays..etc

typedef struct {     char *match; } matches;  int main() {     regex_t regex;     int reti;     char msgbuf[100];     int max_matches = 10;     regmatch_t m[max_matches];      char str[] = "hello world";      reti = regcomp(&regex, "(hello) (world)", reg_extended);     if( reti )     {         fprintf(stderr, "could not compile regex\n");         exit(1);     }      reti = regexec(&regex, str, (size_t) max_matches, m, 0);     if( !reti )     {         puts("match");     }     else if( reti == reg_nomatch )     {         puts("no match");     }     else     {         regerror(reti, &regex, msgbuf, sizeof(msgbuf));         fprintf(stderr, "regex match failed: %s\n", msgbuf);         exit(1);     }      char *p = str;     int num_of_matches = 0;      matches *matches;      int = 0;     for(i = 0; < max_matches; i++)     {         if (m[i].rm_so == -1) break;          int start = m[i].rm_so + (p - str);         int finish = m[i].rm_eo + (p - str);          if (i == 0)             printf ("$& ");         else             printf ("$%d ", i);          char match[finish - start + 1];         memcpy(match, str + start, finish - start);         match[sizeof(match)] = 0;          matches[i].match = match; //need access string in array outside of loop          printf ("'%.*s' (bytes %d:%d)\n", (finish - start), str + start, start, finish);          num_of_matches++;     }     p += m[0].rm_eo;      for(i = 0; < num_of_matches; i++)     {         printf("'%s'\n", matches[i].match);     }      /* free compiled regular expression if want use regex_t again */     regfree(&regex);      return 0; } 

just when thought got when matching "world" noticed when commented out printf statements last printf statement returning empty chars or random chars.

your problems memory issues c strings.

first, define array matches:

matches *matches; 

this defines pointer match structure, pointer uninitialised , doesn't point anywhere sensible. instead, should define array of matches:

matches matches[max_matches]; 

this give 10 (local) matches can access.

next, define local string hold match variable-length array (vla):

char match[finish - start + 1]; 

this time, have allocated enough space hold substring. char buffer local , gone when reach closing brace of for loop body. next pass through loop might use same memory. illegal access memory after loop.

one solution allocate memory on heap malloc:

char *match = malloc(finish - start + 1); 

note have release resources again later explicitly free.

you copy substring , end null character. however, when so, don't location of null character right:

match[sizeof(match)] = 0; 

sizeof compile-time operand tells how many bytes type of given expression occupies in memory. when used vla, sizeof(match) 1 after end of thatb buffer. use pointer allocated memory, sizeof size of pointer.

often sizeof confused strlen, here can't use strlen, because match not yet null-terminated strlen requires. know size of string, of yourse:

match[finish - start] = 0; 

you don't need pointer p, define:

int start = m[i].rm_so; int finish = m[i].rm_eo; 

so:

• make sure allocate memory when want store things.
• take care local memory isn't invalidated before access it. (the egregious example of return address of local array function. case less offensive, less visible.)
• long-lived memory can allocated malloc. such memory isn't garbage collected, must explicitly freed free.
• sizeof compile-time operand. crutch needed raw memory functions malloc. (i've omitted sizeof here, because sizeof(char) guaranteed 1.)

isn't working strings in c fun?