C read file line by line

Question

If your task is not to invent the line-by-line reading function, but just to read the file line-by-line, you may use a typical code snippet involving the getline() function (see the manual page here):

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    FILE * fp;
    char * line = NULL;
    size_t len = 0;
    ssize_t read;

    fp = fopen("/etc/motd", "r");
    if (fp == NULL)
        exit(EXIT_FAILURE);

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

    fclose(fp);
    if (line)
        free(line);
    exit(EXIT_SUCCESS);
}

FILE* filePointer;
int bufferLength = 255;
char buffer[bufferLength];

filePointer = fopen("file.txt", "r");

while(fgets(buffer, bufferLength, filePointer)) {
    printf("%s\n", buffer);
}

fclose(filePointer);

In your readLine function, you return a pointer to the line array (Strictly speaking, a pointer to its first character, but the difference is irrelevant here). Since it's an automatic variable (i.e., it's “on the stack”), the memory is reclaimed when the function returns. You see gibberish because printf has put its own stuff on the stack.

You need to return a dynamically allocated buffer from the function. You already have one, it's lineBuffer; all you have to do is truncate it to the desired length.

    lineBuffer[count] = '\0';
    realloc(lineBuffer, count + 1);
    return lineBuffer;
}

ADDED (response to follow-up question in comment): readLine returns a pointer to the characters that make up the line. This pointer is what you need to work with the contents of the line. It's also what you must pass to free when you've finished using the memory taken by these characters. Here's how you might use the readLine function:

char *line = readLine(file);
printf("LOG: read a line: %s\n", line);
if (strchr(line, 'a')) { puts("The line contains an a"); }
/* etc. */
free(line);
/* After this point, the memory allocated for the line has been reclaimed.
   You can't use the value of `line` again (though you can assign a new value
   to the `line` variable if you want). */

//open and get the file handle
FILE* fh;
fopen_s(&fh, filename, "r");

//check if file exists
if (fh == NULL){
    printf("file does not exists %s", filename);
    return 0;
}


//read line by line
const size_t line_size = 300;
char* line = malloc(line_size);
while (fgets(line, line_size, fh) != NULL)  {
    printf(line);
}
free(line);    // dont forget to free heap memory

readLine() returns pointer to local variable, which causes undefined behaviour.

To get around you can:

1. Create variable in caller function and pass its address to readLine()
2. Allocate memory for line using malloc() - in this case line will be persistent
3. Use global variable, although it is generally a bad practice

Use fgets() to read a line from a file handle.