CODEWITHSUNDEEP
cgdb
user1464460
user1464460

Reputation: 59

Unable to initialize c array

I was writing some low level c code on fedora 14, but going nuts over this piece of code. The first array is not initialized to '0', the second is. Gone through gdb several times, but it's like magic. What is happening?

const int maxsize=100000;
char num[7];
char tnum[6];
int pos=0;
while(pos<(maxsize+1)) {
  int c=0;
  int j=0;

  int myindex;
  int tindex;
  for(myindex=0;myindex<7;myindex++) num[myindex]='0';
  for(tindex=0;tindex<6;tindex++) tnum[tindex]='0';
//....
}

I printed the array values inside gdb, both as p num , p tnum and as p num[0] and p tnum[0]. I also tried to initialize as plain 0, same thing also happens.

Here is the debugger output

Temporary breakpoint 1, main () at inversionscount.c:3
3   int main() {
Missing separate debuginfos, use: debuginfo-install glibc-2.13-1.i686
(gdb) s
5   const int maxsize=100000;
(gdb) s
6   int startarray[maxsize];
(gdb) s
14  int pos=0;
(gdb) s
15  while(pos<(maxsize+1)) {
(gdb) s
19      int c=0;
(gdb) s
20      int j=0;
(gdb) s
24      for(myindex=0;myindex<7;myindex++) num[myindex]='0';
(gdb) s
25      for(tindex=0;tindex<6;tindex++) tnum[tindex]='0';
(gdb) s
27      while( c=getchar()!="\n") {
(gdb) p num
$1 = "\370\377\277\270\367\377"
(gdb) p tnum
$2 = "000000"
(gdb)

Upvotes: 2

Views: 313

Answers (3)

blearn
blearn

Reputation: 1208

You should use memset for both of your arrays:

#include <string.h>
char num[7];
memset(num, '\0', sizeof(num));

Upvotes: 0

ouah
ouah

Reputation: 145899

How do you check you array are initialized with the '0' character literal? If you print them as strings, remember string are null terminated and your arrays are not.

Add this:

 num[sizeof num - 1] = '\0';
 tnum[sizeof tnum - 1] = '\0';

before printing them:

printf("%s\n", num);
printf("%s\n", tnum);

Also notice '\0' is the int value 0 while '0' is the printable character 0.

Upvotes: 0

unwind
unwind

Reputation: 400009

What is maxsize? Make sure the code follows the execution path you think it does, by single-stepping through with a debugger.

Also, you shouldn't repeat magical constants, the for loops are better written as:

size_t i;

for(i = 0; i < sizeof num; ++i)
  num[i] = '0';
for(i = 0; i < sizeof tnum; ++i)
  tnum[i] = '0';

or, since these are char arrays, just use memset():

#include <string.h>

memset(num, '0', sizeof num);
memset(tnum, '0', sizeof tnum);

Upvotes: 1

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