Reading a file with variable line lengths line by line in c

Question

In C, is there a way to read a text file line by line without knowing how much space to allocate for it?

here's an example of what I mean:

fgets(line, <dynamic line size>, fileHandle);

Thanks for the help!

Answer 1

Nothing automatic. You need to keep growing your buffer and calling fgets until you get the newline or the EOF.

// NOTE: not production ready as does not handle memory allocation failures
size_t alloced = 128;
char *p = malloc(alloced);
char *walk = p;
size_t to_read = alloced;

for (;;) {
    if (fgets(walk, to_read, fp) == NULL)
        break;

    if (walk[strlen(walk) - 1] == '\n')
        break;

    to_read = alloced;
    alloced *= 2;

    p = realloc(p, allocated);
    walk = p + to_read;
}

Answer 2

If you have glibc or another libc that supports POSIX (2008), you can use getline:

ssize_t getline(char **lineptr, size_t *n, FILE *stream);

getline() reads an entire line from stream, storing the address of the buffer containing the text into *lineptr. The buffer is null-terminated and includes the newline character, if one was found.

If *lineptr is NULL, then getline() will allocate a buffer for storing the line, which should be freed by the user program. (The value in *n is ignored.)