How to read a line from a file in C

Question

I want to read lines from a file line-by-line, but it's not working for me.

Here is what I tried to do:

FILE *file;
char *line = NULL;
int len = 0;
char read;
file=fopen(argv[1], "r");

if (file == NULL)
    return 1;

while ((read = getline(&line, len, file)) != -1) {
    printf("Retrieved line of length %s :\n", &read);
    printf("%s", line);
}

if (line)
    free(line);

return 0;

Any suggestions why that isn't working?

2 mistakes ... 1:- line is just a pointer, that pointer needs to point to some malloced memory .... 2:- len = 0, why do want to read 0 bytes???? .. google for a "simple file operation program in c" ... — Deepthought, Nov 06 '13 at 14:00
@Deepthought No, that's not how `getline()` works. It allocates memory for the line itself. — , Nov 06 '13 at 14:01
Straight example at [man getline](http://linux.die.net/man/3/getline). — Sunil Bojanapally, Nov 06 '13 at 14:01
Possible duplicate of [C read file line by line](https://stackoverflow.com/questions/3501338/c-read-file-line-by-line) — Ciro Santilli Путлер Капут 六四事, Nov 24 '17 at 16:38
Your compiler should have told you that the second argument to `getline` is wrong. It expects a pointer and not an integer. — Jens Gustedt, Nov 24 '17 at 21:31

score 5 · Accepted Answer · answered Nov 06 '13 at 14:06

5

To get it to work correctly, there's a few changes.

Change int len to size_t len for the correct type.

getline() syntax is incorrect. It should be:

while ((read = getline(&line, &len, file)) != -1) {

And the printf line should also be modified, to print the number returned instead of a char and string interpretation:

printf("Retrieved line of length %d:\n", read);

answered Nov 06 '13 at 14:06

Leigh

11,344
4
27
35

1

Note `read` should be type `ssize_t` and not `char`. 2 problems:`char` may be `unsigned char` and never == -1 and `char` is not the same range as `ssize_t`. The `printf()` format specifier is a bit of a problem too. Suggest: `printf("Retrieved line of length %lld:\n", (long long) read);` – chux - Reinstate Monica Apr 06 '15 at 21:44

score 4 · Answer 2 · answered Apr 01 '15 at 17:21

4

Alternatively you can also use this code. It will read the whole file line by line and print those lines.

char buf[1000];

ptr_file =fopen("input3.txt","r");
if (!ptr_file)
    return 1;

while (fgets(buf,1000, ptr_file)!=NULL)
    printf("%s",buf);

answered Apr 01 '15 at 17:21

user3651854

124
1
8

unwind · Answer 3 · 2013-11-06T14:09:21.107

3

Your second argument to getline() is (very) wrong.

It should be size_t *, you're passing int. You should have received compiler warnings for this problem.

Make it:

size_t len;

and in the call:

getline(&line, &len, file)

Also the return value is of type ssize_t, not char.

You should really read the manual page for getline() and make sure you understand it, before writing code to use the function.

edited Nov 06 '13 at 14:09

answered Nov 06 '13 at 13:57

unwind

378,987
63
458
590

How to read a line from a file in C

3 Answers3