fgets function doesn't wait for input

Asked Jan 16 '17 at 22:49

Active Jan 16 '17 at 22:51

Viewed 297 times

I am trying to initiaze an m*n matrix line by line parsing the data with sscanf. The problem is that at the second iteration of the outer loop fgets doesn't wait for any input.

for(i = 0; i < m; ++i){
        if(fgets(line, sizeof(line), fp)){
            for(j = 0; j < n; ++j){
                printf("j = %d i = %d n = %d line = %s\n", j, i, n, line);//DEBUG
                if(sscanf(line+j*2, "%g", &mtx[i][j]) != 1){
                    /*Manage the error*/
                }
            }
        }
    }

Here it's the debug output:

j = 0 i = 0 n = 4 line = 1 7 3 0
j = 1 i = 0 n = 4 line = 1 7 3 0
j = 2 i = 0 n = 4 line = 1 7 3 0
j = 3 i = 0 n = 4 line = 1 7 3 0
j = 0 i = 1 n = 4 line =

And the corresponding input:

asked Jan 16 '17 at 22:49

sworwitz

1

What input should it wait for? It is reading from a file. – Eugene Sh. Jan 16 '17 at 22:50
4

How exactly is `line` defined? – EDD Jan 16 '17 at 22:56
@EugeneSh. Actually the program can read either from file or stdin. But instead of reading the second line it pass by – sworwitz Jan 16 '17 at 22:58
2

Specifically, how big is `line`? If it's not big enough to store the entire line, then `fgets` will leave input behind in `fp` (which would be read on a subsequent call). – jamesdlin Jan 16 '17 at 22:58
Works OK for me: http://ideone.com/2aOvVk – Barmar Jan 16 '17 at 22:58
2

@sworwitz If `line` is a pointer rather than an array, `sizeof(line)` is probably just 4 bytes, not the size of the array it points to. – Barmar Jan 16 '17 at 22:59
@Barmar If it points to any at all.... – Eugene Sh. Jan 16 '17 at 23:00
@Barmar yes sorry I just deleted the comment. It's an array of size 256 – sworwitz Jan 16 '17 at 23:01
3

Post more of the code. What you've written should work, since it works for me. – Barmar Jan 16 '17 at 23:02
1

@Barmar: Actually you know `sizeof(line)` is at least 8 bytes since it read 4 digits, 3 spaces, and had room for NUL. – jamesdlin Jan 16 '17 at 23:02
@sworwitz The input lines don't have hundreds of extraneous spaces at the end of any line, right? – jamesdlin Jan 16 '17 at 23:04
No, I'll post more code – sworwitz Jan 16 '17 at 23:06
@chux there is a \n each four numbers – sworwitz Jan 16 '17 at 23:07
1

@sworwitz If `fgets(line, sizeof(line), fp)` read a `'\n'`, I would expect `printf("j = %d i = %d n = %d line = %s\n", j, i, n, line);` to display 2 `'\n'`, one from `line`, one from the format. Post only shows 1. Suggest `... line = <%s>\n"` to demarcate `line`. – chux - Reinstate Monica Jan 16 '17 at 23:10
1

@sworwitz Post the code that supports "It's an array of size 256". Code problem is readily explained if `line `is instead an 8-byte pointer. – chux - Reinstate Monica Jan 16 '17 at 23:16
You need to post enough code so that it's possible to reproduce the problem. See [minimal, complete, and verifiable example](https://stackoverflow.com/help/mcve) in the [help center](https://stackoverflow.com/help). – user3386109 Jan 16 '17 at 23:16
OK I found the error. It was quite stupid... line is an array but I pass it to the function that contains the code above, so sizeof is actually just 8. Now I use a macro – sworwitz Jan 16 '17 at 23:32

fgets function doesn't wait for input

0 Answers0