I am getting these unexpected chars when I try to print the string inside of a file. How can I deal with this situation?
This is my code:
#include <stdlib.h>
#include <stdio.h>
char *readFile(char *filename)
{
char * buffer;
long length;
FILE * f = fopen (filename, "r");
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);
}
return buffer;
}
int main() {
char *firstS = readFile("file.txt");
printf("%s \n", firstS);
return 0;
}
You're not NUL terminating your string. Instead, do
buffer = malloc(length+1);
and then after you read in the file
buffer[length] = '\0';
When printf reads past the allocated memory for buffer, it invokes undefined behavior, printf happily reads along until it finds a '\0'. Also note you should initialize buffer to NULL:
char * buffer = NULL;
and check for that back in main before you print. If there's a problem opening the file for instance, you return an uninitialized buffer to main, and trying to read from that is another UB invoker:
char *firstS = readFile("file.txt");
if (firstS != NULL)
{
printf("%s \n", firstS);
}
else
{
// print error message to your liking
}
I am trying to read a RSA key file into a buffer with the code below
char* readkeyBuffer(){
char *buffer;
long length;
FILE *f;
f= fopen ("file.p12", "rb");
if (f)
{
fseek (f, 0, SEEK_END);
length = ftell (f);
rewind(f);
//fseek (f, 0, SEEK_SET);
buffer = (char *) malloc (sizeof(char) * (length +1 ));
lr_message("Length = %d",length);
lr_message("sizeof Buffer = %d",sizeof(buffer));
if (buffer)
{
fread (buffer, 1, length, f);
//strcat(buffer,'\0');
}
lr_message("sizeof Buffer = %d",sizeof(buffer));
lr_message("buffer len = %d",strlen(buffer));
//lr_message("%s",buffer);
fclose (f);
}
return buffer;
}
The file size is 4476 , but somehow the file being read into the buffer has only 59 chars. Can you please suggest what is the issue?
Output ---
Length = 4476
sizeof Buffer = 4
sizeof Buffer = 4
buffer len = 59
I'm trying to read text from a file and store it into a string so that I can then encrypt and decrypt it using openssl. How can I do this?
You could use dynamic memory. My "skeleton" code for reading any type of file is this:
// Assumes: the file # file name is a text file; ASCII or UTF-8 (no BOM)
void readwholefile(char *filename)
{
FILE *fp;
char *buffer = NULL;
size_t i, len;
fp = fopen(filename, "rb");
fseek(fp, 0, SEEK_END);
len = ftell(fp);
rewind(fp);
buffer = malloc(len + 1);
if (NULL == buffer)
{
// Handle malloc failure & exit
exit(-1);
}
fread(buffer, 1, len, fp);
fclose(fp);
buffer[len] = '\0';
// buffer now contains the content of your file; do what you want with it
free(buffer);
buffer = NULL;
}
If you are using a POSIX system, you can use getline:
char *line = nullptr;
size_t line_size = 0
ssize_t len = getline(&line, &line_size, fp);
This will read until a newline and malloc enough space for the resulting line. You can use getdelim to read up to some delimiter other than a newline.
I'm trying to dynamically realloc memory for a file being read one character at a time. It is not printing the buffer character by character. It looks like the fread function is not advancing 1 character at a time.
int main() {
FILE *fp;
char *newBuffer;
char *buffer = malloc(sizeof(char));
int count = 0;
/* Open file for both reading and writing */
fp = fopen("test.txt", "r");
if (!fp) {
exit(99);
}
/* Seek to the beginning of the file */
fseek(fp, SEEK_SET, 0);
/* Read into memory and display the buffer as its read */
while (1) {
newBuffer = (char*)realloc(buffer, (sizeof(char) * (++count)));
if (newBuffer) {
buffer = newBuffer;
buffer += (count - 1);
fread(buffer, sizeof(char), 1, fp);
if (feof(fp)) {
buffer = newBuffer;
break;
}
buffer = newBuffer;
printf(" %s\n", buffer);
} else {
// realloc failed
free(buffer);
exit(1);
}
}
fclose(fp);
free(newBuffer);
return(0);
}
You do not null terminate the buffer before using it as a string in printf, this is a problem.
Note that you can simplify or improve the code in various ways:
no need to fseek(fp, SEEK_SET, 0); after fopen, the FILE is already at the starting position. Note that you interverted the arguments to fseek: it should be fseek(fp, 0L, SEEK_SET); but you are lucky SEEK_SET is #defined as 0.
reading one byte from the file is much simpler with getc than fread(buffer, sizeof(char), 1, fp);. It allows for a simpler and better test for end of file. Using feof() only works in your example because you only attempt to read a single byte.
no need for the initial malloc, set buffer toNULL.reallocacceptsNULLand behaves likemallocwith such as argument,freeaccepts aNULL` argument and does nothing.
do not cast the return value of malloc, nor realloc.
sizeof(char) is 1 by definition: either use sizeof(*buffer) or elide the sizeof completely.
do not parenthesize the return expression.
the prototype for main without arguments is int main(void)
Here is a simpler version:
int main(void) {
FILE *fp;
char *newBuffer;
char *buffer = NULL;
int count = 0, c;
/* Open file for both reading */
fp = fopen("test.txt", "r");
if (!fp) {
exit(99);
}
/* Read into memory and display the buffer read */
while ((c = getc(fp)) != EOF) {
newBuffer = realloc(buffer, count + 2);
if (newBuffer) {
buffer = newBuffer;
buffer[count++] = c;
buffer[count] = '\0';
printf(" %s\n", buffer);
} else {
// realloc failed
fclose(fp);
free(buffer);
exit(1);
}
}
fclose(fp);
free(buffer);
return 0;
}
Your printf(" %s\n", buffer); expects buffer to end with a '\0' (null) character. Your code doesn't provide the required 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 (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);
}
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, even after years, answers are error prone and many lack proper string formation and error checking.
#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;
...
}
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.
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;
}