How to read the content of a file to a string in C?

What is the simplest way (least error-prone, least lines of code, however you want to interpret it) to open a file in C and read its contents into a string (char*, char[], whatever)?

I tend to just load the entire buffer as a raw memory chunk into memory and do the parsing on my own. That way I have best control over what the standard lib does on multiple platforms.

This is a stub I use for this. you may also want to check the error-codes for fseek, ftell and fread. (omitted for clarity).

char * buffer = 0;
long length;
FILE * f = fopen (filename, "rb");

if (f)
{
  fseek (f, 0, SEEK_END);
  length = ftell (f);
  fseek (f, 0, SEEK_SET);
  buffer = malloc (length);
  if (buffer)
  {
    fread (buffer, 1, length, f);
  }
  fclose (f);
}

if (buffer)
{
  // start to process your data / extract strings here...
}

Another, unfortunately highly OS-dependent, solution is memory mapping the file. The benefits generally include performance of the read, and reduced memory use as the applications view and operating systems file cache can actually share the physical memory.

POSIX code would look like this:

int fd = open("filename", O_RDONLY);
int len = lseek(fd, 0, SEEK_END);
void *data = mmap(0, len, PROT_READ, MAP_PRIVATE, fd, 0);

Windows on the other hand is little more tricky, and unfortunately I don't have a compiler in front of me to test, but the functionality is provided by CreateFileMapping() and MapViewOfFile().

If "read its contents into a string" means that the file does not contain characters with code 0, you can also use getdelim() function, that either accepts a block of memory and reallocates it if necessary, or just allocates the entire buffer for you, and reads the file into it until it encounters a specified delimiter or end of file. Just pass '\0' as the delimiter to read the entire file.

This function is available in the GNU C Library, http://www.gnu.org/software/libc/manual/html_mono/libc.html#index-getdelim-994

The sample code might look as simple as

char* buffer = NULL;
size_t len;
ssize_t bytes_read = getdelim( &buffer, &len, '\0', fp);
if ( bytes_read != -1) {
  /* Success, now the entire file is in the buffer */

If you are reading special files like stdin or a pipe, you are not going to be able to use fstat to get the file size beforehand. Also, if you are reading a binary file fgets is going to lose the string size information because of embedded '\0' characters. Best way to read a file then is to use read and realloc:

#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

int main () {
    char buf[4096];
    ssize_t n;
    char *str = NULL;
    size_t len = 0;
    while (n = read(STDIN_FILENO, buf, sizeof buf)) {
        if (n < 0) {
            if (errno == EAGAIN)
                continue;
            perror("read");
            break;
        }
        str = realloc(str, len + n + 1);
        memcpy(str + len, buf, n);
        len += n;
        str[len] = '\0';
    }
    printf("%.*s\n", len, str);
    return 0;
}

Note: This is a modification of the accepted answer above.

Here's a way to do it, complete with error checking.

I've added a size checker to quit when file was bigger than 1 GiB. I did this because the program puts the whole file into a string which may use too much ram and crash a computer. However, if you don't care about that you could just remove it from the code.

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

#define FILE_OK 0
#define FILE_NOT_EXIST 1
#define FILE_TOO_LARGE 2
#define FILE_READ_ERROR 3

char * c_read_file(const char * f_name, int * err, size_t * f_size) {
	char * buffer;
	size_t length;
	FILE * f = fopen(f_name, "rb");
	size_t read_length;
	
	if (f) {
		fseek(f, 0, SEEK_END);
		length = ftell(f);
		fseek(f, 0, SEEK_SET);
		
		// 1 GiB; best not to load a whole large file in one string
		if (length > 1073741824) {
			*err = FILE_TOO_LARGE;
			
			return NULL;
		}
		
		buffer = (char *)malloc(length + 1);
		
		if (length) {
			read_length = fread(buffer, 1, length, f);
			
			if (length != read_length) {
                 free(buffer);
			     *err = FILE_READ_ERROR;

                 return NULL;
			}
		}
		
		fclose(f);
		
		*err = FILE_OK;
		buffer[length] = '\0';
		*f_size = length;
	}
	else {
		*err = FILE_NOT_EXIST;
		
		return NULL;
	}
	
	return buffer;
}

And to check for errors:

int err;
size_t f_size;
char * f_data;

f_data = c_read_file("test.txt", &err, &f_size);

if (err) {
	// process error
}
else {
    // process data
    free(f_data);
}

If the file is text, and you want to get the text line by line, the easiest way is to use fgets().

char buffer[100];
FILE *fp = fopen("filename", "r");                 // do not use "rb"
while (fgets(buffer, sizeof(buffer), fp)) {
... do something
}
fclose(fp);

If you're using glib, then you can use g_file_get_contents;

gchar *contents;
GError *err = NULL;

g_file_get_contents ("foo.txt", &contents, NULL, &err);
g_assert ((contents == NULL && err != NULL) || (contents != NULL && err == NULL));
if (err != NULL)
  {
	// Report error to user, and free error
	g_assert (contents == NULL);
	fprintf (stderr, "Unable to read file: %s\n", err->message);
	g_error_free (err);
  }
else
  {
	// Use file contents
	g_assert (contents != NULL);
  }
}

Just modified from the accepted answer above.

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

char *readFile(char *filename) {
    FILE *f = fopen(filename, "rt");
    assert(f);
    fseek(f, 0, SEEK_END);
    long length = ftell(f);
    fseek(f, 0, SEEK_SET);
    char *buffer = (char *) malloc(length + 1);
    buffer[length] = '\0';
    fread(buffer, 1, length, f);
    fclose(f);
    return buffer;
}

int main() {
    char *content = readFile("../hello.txt");
    printf("%s", content);
}

// Assumes the file exists and will seg. fault otherwise.
const GLchar *load_shader_source(char *filename) {
  FILE *file = fopen(filename, "r");             // open 
  fseek(file, 0L, SEEK_END);                     // find the end
  size_t size = ftell(file);                     // get the size in bytes
  GLchar *shaderSource = calloc(1, size);        // allocate enough bytes
  rewind(file);                                  // go back to file beginning
  fread(shaderSource, size, sizeof(char), file); // read each char into ourblock
  fclose(file);                                  // close the stream
  return shaderSource;
}

This is a pretty crude solution because nothing is checked against null.

> What is the simplest way (least error-prone, least lines of code, however you want to interpret it) to open a file in C and read its contents into a string ...?

Sadly, answers, after years are error prone and many lack proper string formation.

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

// Read the file into allocated memory.
// Return NULL on error.
char* readfile(FILE *f) {
  // f invalid? fseek() fail?
  if (f == NULL || fseek(f, 0, SEEK_END)) {
    return NULL;
  }

  long length = ftell(f);
  rewind(f);
  // Did ftell() fail?  Is the length too long?
  if (length == -1 || (unsigned long) length >= SIZE_MAX) {
    return NULL;
  }

  // Convert from long to size_t
  size_t ulength = (size_t) length;
  char *buffer = malloc(ulength + 1);
  // Allocation failed? Read incomplete?
  if (buffer == NULL || fread(buffer, 1, ulength, f) != ulength) {
    free(buffer);
    return NULL;
  }
  buffer[ulength] = '\0'; // Now buffer points to a string

  return buffer;
}

Note that if the text file contains null characters, the allocated data will contain all the file data, yet the string will appear to be short. Better code would also return the length information so the caller can handle that.

char* readfile(FILE *f, size_t *ulength_ptr) {
  ...
  if (ulength_ptr) *ulength_ptr == *ulength;
  ...
} 

I will add my own version, based on the answers here, just for reference. My code takes into consideration sizeof(char) and adds a few comments to it.

// Open the file in read mode.
FILE *file = fopen(file_name, "r");
// Check if there was an error.
if (file == NULL) {
    fprintf(stderr, "Error: Can't open file '%s'.", file_name);
    exit(EXIT_FAILURE);
}
// Get the file length
fseek(file, 0, SEEK_END);
long length = ftell(file);
fseek(file, 0, SEEK_SET);
// Create the string for the file contents.
char *buffer = malloc(sizeof(char) * (length + 1));
buffer[length] = '\0';
// Set the contents of the string.
fread(buffer, sizeof(char), length, file);
// Close the file.
fclose(file);
// Do something with the data.
// ...
// Free the allocated string space.
free(buffer);

easy and neat(assuming contents in the file are less than 10000):

void read_whole_file(char fileName[1000], char buffer[10000])
{
	FILE * file = fopen(fileName, "r");
	if(file == NULL)
	{
		puts("File not found");
		exit(1);
	}
	char  c;
	int idx=0;
	while (fscanf(file , "%c" ,&c) == 1)
	{
		buffer[idx] = c;
		idx++;
	}
	buffer[idx] = 0;
}