Save string pointed by pointer to array

Question

I'm trying to save string from pointer to array. But my code outputs segmentation fault. Here's my code:

  char timelog[maxline];
  matchescount = 0;
  while ((read = getline(&line, &len, fp)) != -1) {
    struct matches matched = check_match(line,lgd,grd);
    if (matched.status==1)
    {
      strcpy(timelog[matchescount],matched.timelog);
      matchescount++;
    }
  }

Here: matched.timelog="10:24:12" like string. And i want to save it to timelog[matchescount] . So i want this from timelog array:

timelog[0]="10:24:12" 

timelog[1]="10:24:13"

UPDATE: Can i store 2d array of strings ?

char timelog[maxline][255]

creates

[0][0]="1" [0][1]="0" [0][2]=":" [0][3]="2" [0][4]="4" [0][5]=":" [0][6]="1" [0][7]="2"

[1][0]="1" .......

right ?

Can i i store like this ?

[0][0]="10:24:12" [0][1]="10:24:13"

[1][0]="10:24:14" [1][1]="10:24:15"

timelog is an array of chars not an array of strings. Use strncpy to avoid buffer overruns. Tag either C or C++ (they're different languages). — legends2k
– legends2k, Commented Dec 21, 2014 at 5:09
Which is it, C or C++? If it's C++, use std::vector<std::string> — PaulMcKenzie
– PaulMcKenzie, Commented Dec 21, 2014 at 5:11
Try char timelog[maxline][20];. Your current timelog can hold 1 string not many strings. — Spikatrix
– Spikatrix, Commented Dec 21, 2014 at 5:26

varun · Accepted Answer · 2014-12-21 10:21:08Z

just make your array 2D. The error was because of your array is 1D so you ccan store only one string int that array.To Store multiple make the following changes.

 char timelog[maxline][10];
  matchescount = 0;
  while ((read = getline(&line, &len, fp)) != -1) {
    struct matches matched = check_match(line,lgd,grd);
    if (matched.status==1)
    {
      strcpy(timelog[matchescount],matched.timelog);
      matchescount++;
    }
  }

UPDATE:

char timelog[maxline][255]

creates a 2d array of char.As String is an array of chars you can store only si1D array of string in 2d array of char

timelog[0][0]='a';
timelog[0][1]='b';
timelog[0][2]='c';
timelog[0][3]='d';
timelog[0][4]='e';

this indicates you have a string "abcde" at timelog[0];

to store 2d array of strings you need a 3D char array

timelog[maxline][noOfStrings][maxLengthOfEachString];

now you can store 2D array of strings.

strcpy(timelog[0][0],"abcd");
strcpy(timelog[0][1],"efgh");
strcpy(timelog[1][0],"ijkl");
strcpy(timelog[1][1],"xyz");

maartenb64 · Accepted Answer · 2014-12-21 05:32:59Z

2

Assuming your timelog string always look like "hh:mm:ss", you can do

#define MAXTIMELOG 9
#define MAXENTRIES 1000


char timelog[MAXENTRIES][MAXTIMELOG];
matchescount = 0;
while ((read = getline(&line, &len, fp)) != -1) {
    struct matches matched = check_match(line, lgd, grd);
    if (matched.status==1)
    {
        strncpy(timelog[matchescount], matched.timelog, MAXTIMELOG);
        timelog[matchescount][MAXTIMELOG-1] = 0;
        if (++matchescount == MAXENTRIES) {
            ... deal with full array ...
        }
    }
}

answered Dec 21, 2014 at 5:32

maartenb64

335 bronze badges

Comments

cmaster - reinstate monica · Accepted Answer · 2014-12-21 08:52:06Z

0

Actually, you shouldn't copy the strings to a preallocated array. Use dynamic arrays instead:

size_t timelogSize = 8, matchescount = 0;
char** timelog = malloc(timelogSize*sizeof(*timelog));
while ((read = getline(&line, &len, fp)) != -1) {
    struct matches matched = check_match(line,lgd,grd);
    if (matched.status==1) {
        if(matchescount == timelogSize) {
            timelogSize *= 2;
            timelog = realloc(timelog, timelogSize*sizeof(*timelog));
        }
        timelog[matchescount++] = strdup(matched.timelog);
    }
}

This has the big advantage that you can handle input strings of arbitrary size and count. By doing this everywhere you need an array, you avoid a lot of bugs that are just waiting to happen.

edited Dec 21, 2014 at 8:52

answered Dec 21, 2014 at 8:45

cmaster - reinstate monica

41.1k9 gold badges69 silver badges110 bronze badges

8 Comments

varun Over a year ago

Instead of multiplying timelogSize with 2,shouldnt you be adding 8 to it? timelogSize += 8;

Odko Over a year ago

Thanks ;). It only work when matched.timelog's size is 8 right ?. What if it's size always changes ?

cmaster - reinstate monica Over a year ago

@varun: No, it should not be timelogSize += 8;. A fixed increment would give the algorithm a quadratic order, a fixed factor ensures that the algorithm stays O(N).

cmaster - reinstate monica Over a year ago

@akiD I have not looked at struct matches, so I didn't change how it's used. However, if matched.timelog is an array (which I suspect), I would want to replace it with a char* as well.

Odko Over a year ago

@cmaster How do i free timelog ? when i try free(timelog) it shows double free or corruption (!prev)

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