Allocating unknown char[] in dynamically reading a file in c - c

Question

char* freadline(FILE* fp){
fseek(fp, 0, SEEK_END);
int lSize = ftell(fp);
fseek(fp, 0, SEEK_SET);
char *buffer = malloc(lSize);
fread(buffer, 1, lSize, fp);
fgets(buffer, sizeof(lSize), fp);
return buffer;
}
but it doesn't read line by line any suggestions as to how this would be read line by line

Answer 1

There are couple solutions here.
The first is to get the size of the entire file using fseek and ftell fseek will allow you to go to the end of file and ftell will give you the current position which can be used as a size indicator. You can then allocate enough of a buffer to read the entire file then split them up into lines.
The other solution is to use a temporary buffer of 1000 or so like you're already doing, read a character at a time using fgetc in a loop and feed it into the temporary buffer until you hit a new line indicator , then use the strlen method to get the length and allocate a buffer of that size, copy the temporary buffer then return the allocated buffer.
There is also errors in your code as pointed out in the comments. You're discarding your allocated memory resulting in a leak. And your freadline doesn't actually read a line it just reads whatever size you're telling it to read.

Answer 2

the lines in the file could be of any length.
realloc() is a classic approach, but how about a simple, slow and plodding one:
Read once to find line length, seek, allocate, then read again to save the line.
#include <stdio.h>
char* freadline(FILE *fp) {
int length = 0;
long offset = ftell(fp);
if (offset == -1)
return NULL;
int scan_count = fscanf(fp, "%*[^\n]%n", &length); // Save scan length
if (scan_count == EOF)
return NULL;
if (fseek(fp, offset, SEEK_SET))
return NULL;
size_t n = length + 1u; // +1 for potential \n
char *buf = malloc(n + 1); // + 1 for \0
if (buf == NULL)
return NULL;
size_t len = fread(buf, 1, n, fp);
buf[len] = '\0';
return buf;
}
Test
#include <assert.h>
#include <stdlib.h>
int main() {
FILE *fp = fopen("tmp.txt", "w+");
assert(fp);
for (int i = 0; i < 10; i++) {
int l = i * 7;
for (int j = 0; j < l; j++) {
fputc(rand() % 26 + 'a', fp);
}
fputc('\n', fp);
}
rewind(fp);
char *s;
while ((s = freadline(fp)) != NULL) {
printf("<%s>", s);
free(s);
}
fclose(fp);
return 0;
}
Output
<
><lvqdyoq
><ykfdbxnqdquhyd
><jaeebzqmtblcabwgmscrn
><oiaftlfpcuqffaxozqegxmwgglkh
><vxtdhnzqankyprbwteazdafeqxtijjtkwea
><zqgmplohyxrutojvbzllqgjaidbtqibygdzcxkujvw
><ghwbmjjmbpksnzkgzgiluiggpkzwhaetclrcyxcsixsutjmrm
><vqlybsjnihnfqyfhyszwgpsvnhnngdnjzjypqcflnztrhcfgbkakzxam
><alsuauxxchqjxqaiddtjszgcbullyyjymytioyawpzshhfpqpsatddbcagjgobm
>

Answer 3

If you're ok targeting POSIX, it already has a function that does what you need: getline.
#include <stdio.h>
#include <stdlib.h>
FILE *fh = ...;
char *line = NULL;
size_t buf_size = 0;
while (1) {
ssize_t line_len = getline(&line, &buf_size, fh);
if (line_len == -1)
break;
// ...
}
free(line);
If not, getline can be implemented using using fgets and realloc in a loop. Just start with a arbitrarily-sized buffer.