Related
I wrote this function to read a line from a file:
const char *readLine(FILE *file) {
if (file == NULL) {
printf("Error: file pointer is null.");
exit(1);
}
int maximumLineLength = 128;
char *lineBuffer = (char *)malloc(sizeof(char) * maximumLineLength);
if (lineBuffer == NULL) {
printf("Error allocating memory for line buffer.");
exit(1);
}
char ch = getc(file);
int count = 0;
while ((ch != '\n') && (ch != EOF)) {
if (count == maximumLineLength) {
maximumLineLength += 128;
lineBuffer = realloc(lineBuffer, maximumLineLength);
if (lineBuffer == NULL) {
printf("Error reallocating space for line buffer.");
exit(1);
}
}
lineBuffer[count] = ch;
count++;
ch = getc(file);
}
lineBuffer[count] = '\0';
char line[count + 1];
strncpy(line, lineBuffer, (count + 1));
free(lineBuffer);
const char *constLine = line;
return constLine;
}
The function reads the file correctly, and using printf I see that the constLine string did get read correctly as well.
However, if I use the function e.g. like this:
while (!feof(myFile)) {
const char *line = readLine(myFile);
printf("%s\n", line);
}
printf outputs gibberish. Why?
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]; /* not ISO 90 compatible */
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
A complete, fgets() solution:
#include <stdio.h>
#include <string.h>
#define MAX_LEN 256
int main(void)
{
FILE* fp;
fp = fopen("file.txt", "r");
if (fp == NULL) {
perror("Failed: ");
return 1;
}
char buffer[MAX_LEN];
while (fgets(buffer, MAX_LEN, fp))
{
// Remove trailing newline
buffer[strcspn(buffer, "\n")] = 0;
printf("%s\n", buffer);
}
fclose(fp);
return 0;
}
Output:
First line of file
Second line of file
Third (and also last) line of file
Remember, if you want to read from Standard Input (rather than a file as in this case), then all you have to do is pass stdin as the third parameter of fgets() method, like this:
while(fgets(buffer, MAX_LEN, stdin))
Appendix
Removing trailing newline character from fgets() input
how to detect a file is opened or not in c
readLine() returns pointer to local variable, which causes undefined behaviour.
To get around you can:
Create variable in caller function and pass its address to readLine()
Allocate memory for line using malloc() - in this case line will be persistent
Use global variable, although it is generally a bad practice
Use fgets() to read a line from a file handle.
Some things wrong with the example:
you forgot to add \n to your printfs. Also error messages should go to stderr i.e. fprintf(stderr, ....
(not a biggy but) consider using fgetc() rather than getc(). getc() is a macro, fgetc() is a proper function
getc() returns an int so ch should be declared as an int. This is important since the comparison with EOF will be handled correctly. Some 8 bit character sets use 0xFF as a valid character (ISO-LATIN-1 would be an example) and EOF which is -1, will be 0xFF if assigned to a char.
There is a potential buffer overflow at the line
lineBuffer[count] = '\0';
If the line is exactly 128 characters long, count is 128 at the point that gets executed.
As others have pointed out, line is a locally declared array. You can't return a pointer to it.
strncpy(count + 1) will copy at most count + 1 characters but will terminate if it hits '\0' Because you set lineBuffer[count] to '\0' you know it will never get to count + 1. However, if it did, it would not put a terminating '\0' on, so you need to do it. You often see something like the following:
char buffer [BUFFER_SIZE];
strncpy(buffer, sourceString, BUFFER_SIZE - 1);
buffer[BUFFER_SIZE - 1] = '\0';
if you malloc() a line to return (in place of your local char array), your return type should be char* - drop the const.
Here is my several hours... Reading whole file line by line.
char * readline(FILE *fp, char *buffer)
{
int ch;
int i = 0;
size_t buff_len = 0;
buffer = malloc(buff_len + 1);
if (!buffer) return NULL; // Out of memory
while ((ch = fgetc(fp)) != '\n' && ch != EOF)
{
buff_len++;
void *tmp = realloc(buffer, buff_len + 1);
if (tmp == NULL)
{
free(buffer);
return NULL; // Out of memory
}
buffer = tmp;
buffer[i] = (char) ch;
i++;
}
buffer[i] = '\0';
// Detect end
if (ch == EOF && (i == 0 || ferror(fp)))
{
free(buffer);
return NULL;
}
return buffer;
}
void lineByline(FILE * file){
char *s;
while ((s = readline(file, 0)) != NULL)
{
puts(s);
free(s);
printf("\n");
}
}
int main()
{
char *fileName = "input-1.txt";
FILE* file = fopen(fileName, "r");
lineByline(file);
return 0;
}
const char *readLine(FILE *file, char* line) {
if (file == NULL) {
printf("Error: file pointer is null.");
exit(1);
}
int maximumLineLength = 128;
char *lineBuffer = (char *)malloc(sizeof(char) * maximumLineLength);
if (lineBuffer == NULL) {
printf("Error allocating memory for line buffer.");
exit(1);
}
char ch = getc(file);
int count = 0;
while ((ch != '\n') && (ch != EOF)) {
if (count == maximumLineLength) {
maximumLineLength += 128;
lineBuffer = realloc(lineBuffer, maximumLineLength);
if (lineBuffer == NULL) {
printf("Error reallocating space for line buffer.");
exit(1);
}
}
lineBuffer[count] = ch;
count++;
ch = getc(file);
}
lineBuffer[count] = '\0';
char line[count + 1];
strncpy(line, lineBuffer, (count + 1));
free(lineBuffer);
return line;
}
char linebuffer[256];
while (!feof(myFile)) {
const char *line = readLine(myFile, linebuffer);
printf("%s\n", line);
}
note that the 'line' variable is declared in calling function and then passed, so your readLine function fills predefined buffer and just returns it. This is the way most of C libraries work.
There are other ways, which I'm aware of:
defining the char line[] as static
(static char line[MAX_LINE_LENGTH]
-> it will hold it's value AFTER returning from the function). -> bad,
the function is not reentrant, and
race condition can occur -> if you
call it twice from two threads, it
will overwrite it's results
malloc()ing the char line[], and
freeing it in calling functions ->
too many expensive mallocs, and,
delegating the responsibility to free the buffer to another function (the most elegant solution is to call malloc and free on any buffers in same function)
btw, 'explicit' casting from char* to const char* is redundant.
btw2, there is no need to malloc() the lineBuffer, just define it char lineBuffer[128], so you don't need to free it
btw3 do not use 'dynamic sized stack arrays' (defining the array as char arrayName[some_nonconstant_variable]), if you don't exactly know what are you doing, it works only in C99.
void readLine(FILE* file, char* line, int limit)
{
int i;
int read;
read = fread(line, sizeof(char), limit, file);
line[read] = '\0';
for(i = 0; i <= read;i++)
{
if('\0' == line[i] || '\n' == line[i] || '\r' == line[i])
{
line[i] = '\0';
break;
}
}
if(i != read)
{
fseek(file, i - read + 1, SEEK_CUR);
}
}
what about this one?
Implement method to read, and get content from a file (input1.txt)
#include <stdio.h>
#include <stdlib.h>
void testGetFile() {
// open file
FILE *fp = fopen("input1.txt", "r");
size_t len = 255;
// need malloc memory for line, if not, segmentation fault error will occurred.
char *line = malloc(sizeof(char) * len);
// check if file exist (and you can open it) or not
if (fp == NULL) {
printf("can open file input1.txt!");
return;
}
while(fgets(line, len, fp) != NULL) {
printf("%s\n", line);
}
free(line);
}
Hope this help. Happy coding!
You should use the ANSI functions for reading a line, eg. fgets. After calling you need free() in calling context, eg:
...
const char *entirecontent=readLine(myFile);
puts(entirecontent);
free(entirecontent);
...
const char *readLine(FILE *file)
{
char *lineBuffer=calloc(1,1), line[128];
if ( !file || !lineBuffer )
{
fprintf(stderr,"an ErrorNo 1: ...");
exit(1);
}
for(; fgets(line,sizeof line,file) ; strcat(lineBuffer,line) )
{
if( strchr(line,'\n') ) *strchr(line,'\n')=0;
lineBuffer=realloc(lineBuffer,strlen(lineBuffer)+strlen(line)+1);
if( !lineBuffer )
{
fprintf(stderr,"an ErrorNo 2: ...");
exit(2);
}
}
return lineBuffer;
}
My implement from scratch:
FILE *pFile = fopen(your_file_path, "r");
int nbytes = 1024;
char *line = (char *) malloc(nbytes);
char *buf = (char *) malloc(nbytes);
size_t bytes_read;
int linesize = 0;
while (fgets(buf, nbytes, pFile) != NULL) {
bytes_read = strlen(buf);
// if line length larger than size of line buffer
if (linesize + bytes_read > nbytes) {
char *tmp = line;
nbytes += nbytes / 2;
line = (char *) malloc(nbytes);
memcpy(line, tmp, linesize);
free(tmp);
}
memcpy(line + linesize, buf, bytes_read);
linesize += bytes_read;
if (feof(pFile) || buf[bytes_read-1] == '\n') {
handle_line(line);
linesize = 0;
memset(line, '\0', nbytes);
}
}
free(buf);
free(line);
Provide a portable and generic getdelim function, test passed via msvc, clang, gcc.
/*
* An implementation conform IEEE Std 1003.1-2017:
* https://pubs.opengroup.org/onlinepubs/9699919799/functions/getdelim.html
*
* <nio.h>:
* https://github.com/junjiemars/c/blob/c425bd0e49df35a2649327664d3f6cd610791996/src/posix/nio.h
* <nio.c>:
* https://github.com/junjiemars/c/blob/c425bd0e49df35a2649327664d3f6cd610791996/src/posix/nio.c
*
*/
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <limits.h>
/*
* LINE_MAX dependents on OS' implementations so check it first.
* https://github.com/junjiemars/c/blob/c425bd0e49df35a2649327664d3f6cd610791996/src/posix/nlim_auto_check
*/
#define NM_LINE_MAX 4096 /* Linux */
#if (MSVC)
typedef SSIZE_T ssize_t;
# if !defined(SSIZE_MAX)
# define SSIZE_MAX ((ssize_t)((size_t)((ssize_t)-1) >> 1))
# endif
#endif
ssize_t getdelim(char **restrict lineptr, size_t *restrict n, int delimiter,
FILE *restrict stream);
#if defined(getline)
# undef getline
#endif
#define getline(lp, n, f) getdelim((lp), (n), 0x0a, (f))
ssize_t
getdelim(char **restrict lineptr, size_t *restrict n, int delimiter,
FILE *restrict stream)
{
int c;
char *p, *p1;
ssize_t len;
if (NULL == lineptr || NULL == n || NULL == stream
|| (UCHAR_MAX < delimiter || delimiter < 0))
{
errno = EINVAL;
return EOF;
}
if (feof(stream) || ferror(stream))
{
return EOF;
}
if (0 == *lineptr)
{
if (0 == *n)
{
*n = NM_LINE_MAX;
}
*lineptr = malloc(*n);
if (0 == *lineptr)
{
return EOF;
}
}
p = *lineptr;
len = 0;
while (EOF != (c = fgetc(stream)))
{
if (SSIZE_MAX == (ssize_t) len)
{
errno = EOVERFLOW;
return EOF;
}
if ((size_t) len == (*n - 1))
{
*n <<= 1;
p1 = realloc(*lineptr, *n);
if (0 == p1)
{
return EOF;
}
*lineptr = p1;
p = p1 + len;
}
*p++ = (char) c;
len++;
if (c == delimiter)
{
break;
}
}
if (ferror(stream))
{
return EOF;
}
*p = 0;
return len;
}
int
main(void)
{
FILE *fp;
char *line = NULL;
size_t len = 0;
ssize_t read;
fp = fopen("/some-file", "r");
if (fp == NULL)
exit(1);
while ((read = getline(&line, &len, fp)) != -1) {
printf("Retrieved line of length %zu :\n", read);
printf("%s", line);
}
if (ferror(fp)) {
/* handle error */
}
free(line);
fclose(fp);
return 0;
}
You make the mistake of returning a pointer to an automatic variable.
The variable line is allocated in the stack and only lives as long as the function lives.
You are not allowed to return a pointer to it, because as soon as it returns the memory will be given elsewhere.
const char* func x(){
char line[100];
return (const char*) line; //illegal
}
To avoid this, you either return a pointer to memory which resides on the heap eg. lineBuffer
and it should be the user's responsibility to call free() when he is done with it.
Alternatively you can ask the user to pass you as an argument a memory address on which to write the line contents at.
I want a code from ground 0 so i did this to read the content of dictionary's word line by line.
char temp_str[20]; // you can change the buffer size according to your requirements And A single line's length in a File.
Note I've initialized the buffer With Null character each time I read line.This function can be Automated But Since I need A proof of Concept and want to design a programme Byte By Byte
#include<stdio.h>
int main()
{
int i;
char temp_ch;
FILE *fp=fopen("data.txt","r");
while(temp_ch!=EOF)
{
i=0;
char temp_str[20]={'\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0'};
while(temp_ch!='\n')
{
temp_ch=fgetc(fp);
temp_str[i]=temp_ch;
i++;
}
if(temp_ch=='\n')
{
temp_ch=fgetc(fp);
temp_str[i]=temp_ch;
}
printf("%s",temp_str);
}
return 0;
}
I am trying to make a program in C, that reads a text file and replace \r\n with \n to the same file converting the line ending from DOS to UNIX. I use fgetc and treat the file as a binary file. Thanks in advance.
#include <stdio.h>
int main()
{
FILE *fptr = fopen("textfile.txt", "rb+");
if (fptr == NULL)
{
printf("erro ficheiro \n");
return 0;
}
while((ch = fgetc(fptr)) != EOF) {
if(ch == '\r') {
fprintf(fptr,"%c", '\n');
} else {
fprintf(fptr,"%c", ch);
}
}
fclose(fptr);
}
If we assume the file uses a single byte character set, we just need to ignore all the '\r' characters when converting a text file form DOS to UNIX.
We also assume that the size of the file is less than the highest unsigned integer.
The reason we do these assumptions, is to keep the example short.
Be aware that the example below overwrites the original file, as you asked. Normally you shouldn't do this, as you can lose the contents of the original file, if an error occurs.
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
// Return a negative number on failure and 0 on success.
int main()
{
const char* filename = "textfile.txt";
// Get the file size. We assume the filesize is not bigger than UINT_MAX.
struct stat info;
if (stat(filename, &info) != 0)
return -1;
size_t filesize = (size_t)info.st_size;
// Allocate memory for reading the file
char* content = (char*)malloc(filesize);
if (content == NULL)
return -2;
// Open the file for reading
FILE* fptr = fopen(filename, "rb");
if (fptr == NULL)
return -3;
// Read the file and close it - we assume the filesize is not bigger than UINT_MAX.
size_t count = fread(content, filesize, 1, fptr);
fclose(fptr);
if (count != 1)
return -4;
// Remove all '\r' characters
size_t newsize = 0;
for (long i = 0; i < filesize; ++i) {
char ch = content[i];
if (ch != '\r') {
content[newsize] = ch;
++newsize;
}
}
// Test if we found any
if (newsize != filesize) {
// Open the file for writing and truncate it.
FILE* fptr = fopen(filename, "wb");
if (fptr == NULL)
return -5;
// Write the new output to the file. Note that if an error occurs,
// then we will lose the original contents of the file.
if (newsize > 0)
count = fwrite(content, newsize, 1, fptr);
fclose(fptr);
if (newsize > 0 && count != 1)
return -6;
}
// For a console application, we don't need to free the memory allocated
// with malloc(), but normally we should free it.
// Success
return 0;
} // main()
To only remove '\r' followed by '\n' replace the loop with this loop:
// Remove all '\r' characters followed by a '\n' character
size_t newsize = 0;
for (long i = 0; i < filesize; ++i) {
char ch = content[i];
char ch2 = (i < filesize - 1) ? content[i + 1] : 0;
if (ch == '\r' && ch2 == '\n') {
ch = '\n';
++i;
}
content[newsize++] = ch;
}
My goal is to implement a function that calculates the number of lines in a file. And empty file is considered to have no lines. If the last line of the given file is not empty, it should be counted as a line despite not ending with a newline character.
I've come up with the following code:
int linecount(const char *filename)
{
FILE *f = fopen(filename, "r");
if(!f)
return -1;
int lines = 0;
int c = 0;
int n = 0;
while((c = fgetc(f)) != EOF){
if(c == '\n')
lines++;
n++;
}
if(n==0)
return 0; //return 0 if the file is empty
if(c!='\n' && !isspace(c))
lines++; //count the last line if it's not empty
fclose(f);
return lines;
}
However, even after playing with it for over an hour I can't figure out why its return value lines is one too large in some cases...
You were close, here how you could do it:
int linecount(const char *filename) {
FILE *f = fopen(filename, "r");
if (!f)
return -1;
int lines = 0;
int c = 0;
int n = 0;
int read_line = 0;
while ((c = fgetc(f)) != EOF) {
read_line = 1;
if (c == '\n') {
lines++;
read_line = 0;
}
n++;
}
if (n == 0)
return 0; //return 0 if the file is empty
if(read_line)
lines++;
fclose(f);
return lines;
}
The idea is that we want to know if we started reading a line AND if we met a newline, at end of this line. So, we use another variable, called read_line and we use it as a flag.
We set it to 1 (true) if we just started reading a line and we set it to 0 (false) if we just met a newline (end of the line).
Now, if we have something like:
1[newline]
2[newline]
3
we will be OK, since we need to check if read_line after we read the file. Is so, we have to increment our line counter by one.
This is also OK:
1[newline]
2[newline]
3[newline]
since we saw three newlines and the read_line is 0 after we read the file.
Same goes for this case:
1[newline]
2[newline]
3[newline]
[nothing here]
since our flag is going to be equal to 0 after reading the file, since the 3rd newline should set it to 0 and we never actually enter the 4th line in our loop, since there is nothing to read.
With your previous implementation, as stated in the comments, this line:
if(c!='\n' && !isspace(c))
would be executed with c being equal to EOF.
Or you could just use fgets() and you are done. Check the example:
#include <stdio.h>
#include <string.h>
#define bufSize 1024
int main(int argc, char *argv[])
{
FILE *fp;
char buf[bufSize];
if ((fp = fopen("test.txt", "rb")) == NULL)
{ /* Open source file. */
perror("fopen source-file");
return 1;
}
int lines = 0;
while (fgets(buf, sizeof(buf), fp) != NULL)
{ /* While we don't reach the end of source. */
/* Read characters from source file to fill buffer. */
/* fgets will stop when it finds a newline. */
lines++;
}
printf("lines = %d\n", lines);
fclose(fp);
return 0;
}
Modify sample
int linecount(const char *filename)
{
FILE *f = fopen(filename, "r");
if(!f)
return -1;
int lines = 0;
int c = 0;
int flag = 1;
while((c = fgetc(f)) != EOF){
if(flag = (c == '\n'))
lines++;
}
if(!flag)
lines++; //count the last line if it's not empty
fclose(f);
return lines;
}
A simple solution can be
int linecount(const char *filename)
{
FILE *stream;
char *line = NULL;
size_t len = 0;
ssize_t read;
int numOfLines = 0;
stream = fopen(filename, "r");
if (stream == NULL)
exit(EXIT_FAILURE);
while ((read = getline(&line, &len, stream)) != -1) {
numOfLines++;
}
free(line);
fclose(stream);
return numOfLines;
}
I'm quite new to C. I'm trying to write a code that finds a string in a I/O stream, and I don't understand what I'm doing wrong. I know the error is probably in the large while loop (in the code below).
I want the function to return the location in bytes from the beginning of the stream and -1 if it fails for some reason. It just keeps returning -1 for any file I try it on.
long find_string(const char *str, const char *filename, long offset)
{
FILE *f = fopen(filename, "r");
if (!f){
return -1;
}
int s=0,c;
c = fgetc(f);
if(c == EOF){
return -1;
}
char *check = malloc(sizeof(char));
fseek(f, 0L, SEEK_END); // Sees and stores how long the file is
long sz = ftell(f);
fseek(f, 0L, SEEK_SET);
if(fseek(f, offset,SEEK_SET) != 0){ // finds the position of offset
return -1;
}
while(fgetc(f) != EOF){
c = fgetc(f);
if(c == str[0] && ftell(f) < sz){
check[0] = c;
offset = ftell(f);
}
s++;
for (unsigned int r=1; r < (strlen(str));r++){
c = fgetc(f);
if(c == str[s]){
check = realloc(check, sizeof(char)*s);
check[s] = c;
s++;
}
}
if(strcmp(check, str)==0){
free(check);
fclose(f);
break;
}
else{
check = realloc(check, sizeof(char));
offset = -1;
}
}
return offset;}
Any help is greatly appreciated
This would be much easier if you simply memory-mapped the entire file and ran a standard string searching algorithm on it.
For memory mapping, see: Linux - Memory Mapped File
For string searching code, see: strstr() for a string that is NOT null-terminated
Please check the lines with comment updated
long find_string(const char *str, const char *filename, long offset)
{
FILE *f = fopen(filename, "r");
if (!f){
return -1;
}
int s=0,c;
c = fgetc(f);
if(c == EOF){
return -1;
}
char *check = malloc(sizeof(char));
fseek(f, 0L, SEEK_END); // Sees and stores how long the file is
long sz = ftell(f);
fseek(f, 0L, SEEK_SET);
if(fseek(f, offset,SEEK_SET) != 0){ // finds the position of offset
return -1;
}
c = fgetc(f); // Updated
while(c != EOF){ // Updated
if(c == str[0] && ftell(f) < sz){
check[0] = c;
offset = ftell(f);
}
s++;
for (unsigned int r=1; r < (strlen(str));r++){
c = fgetc(f);
if(c == str[s]){
check = realloc(check, sizeof(char)*s);
check[s] = c;
s++;
}
}
if(strcmp(check, str)==0){
free(check);
fclose(f);
break;
}
else{
check = realloc(check, sizeof(char));
offset = -1;
}
c = fgetc(f); //Updated
}
return offset;}
since you are using fgetc at the condition and start of the look, you actually comparing the second char of file with first char of str. update and check.
I wrote this function to read a line from a file:
const char *readLine(FILE *file) {
if (file == NULL) {
printf("Error: file pointer is null.");
exit(1);
}
int maximumLineLength = 128;
char *lineBuffer = (char *)malloc(sizeof(char) * maximumLineLength);
if (lineBuffer == NULL) {
printf("Error allocating memory for line buffer.");
exit(1);
}
char ch = getc(file);
int count = 0;
while ((ch != '\n') && (ch != EOF)) {
if (count == maximumLineLength) {
maximumLineLength += 128;
lineBuffer = realloc(lineBuffer, maximumLineLength);
if (lineBuffer == NULL) {
printf("Error reallocating space for line buffer.");
exit(1);
}
}
lineBuffer[count] = ch;
count++;
ch = getc(file);
}
lineBuffer[count] = '\0';
char line[count + 1];
strncpy(line, lineBuffer, (count + 1));
free(lineBuffer);
const char *constLine = line;
return constLine;
}
The function reads the file correctly, and using printf I see that the constLine string did get read correctly as well.
However, if I use the function e.g. like this:
while (!feof(myFile)) {
const char *line = readLine(myFile);
printf("%s\n", line);
}
printf outputs gibberish. Why?
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]; /* not ISO 90 compatible */
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
A complete, fgets() solution:
#include <stdio.h>
#include <string.h>
#define MAX_LEN 256
int main(void)
{
FILE* fp;
fp = fopen("file.txt", "r");
if (fp == NULL) {
perror("Failed: ");
return 1;
}
char buffer[MAX_LEN];
while (fgets(buffer, MAX_LEN, fp))
{
// Remove trailing newline
buffer[strcspn(buffer, "\n")] = 0;
printf("%s\n", buffer);
}
fclose(fp);
return 0;
}
Output:
First line of file
Second line of file
Third (and also last) line of file
Remember, if you want to read from Standard Input (rather than a file as in this case), then all you have to do is pass stdin as the third parameter of fgets() method, like this:
while(fgets(buffer, MAX_LEN, stdin))
Appendix
Removing trailing newline character from fgets() input
how to detect a file is opened or not in c
readLine() returns pointer to local variable, which causes undefined behaviour.
To get around you can:
Create variable in caller function and pass its address to readLine()
Allocate memory for line using malloc() - in this case line will be persistent
Use global variable, although it is generally a bad practice
Use fgets() to read a line from a file handle.
Some things wrong with the example:
you forgot to add \n to your printfs. Also error messages should go to stderr i.e. fprintf(stderr, ....
(not a biggy but) consider using fgetc() rather than getc(). getc() is a macro, fgetc() is a proper function
getc() returns an int so ch should be declared as an int. This is important since the comparison with EOF will be handled correctly. Some 8 bit character sets use 0xFF as a valid character (ISO-LATIN-1 would be an example) and EOF which is -1, will be 0xFF if assigned to a char.
There is a potential buffer overflow at the line
lineBuffer[count] = '\0';
If the line is exactly 128 characters long, count is 128 at the point that gets executed.
As others have pointed out, line is a locally declared array. You can't return a pointer to it.
strncpy(count + 1) will copy at most count + 1 characters but will terminate if it hits '\0' Because you set lineBuffer[count] to '\0' you know it will never get to count + 1. However, if it did, it would not put a terminating '\0' on, so you need to do it. You often see something like the following:
char buffer [BUFFER_SIZE];
strncpy(buffer, sourceString, BUFFER_SIZE - 1);
buffer[BUFFER_SIZE - 1] = '\0';
if you malloc() a line to return (in place of your local char array), your return type should be char* - drop the const.
Here is my several hours... Reading whole file line by line.
char * readline(FILE *fp, char *buffer)
{
int ch;
int i = 0;
size_t buff_len = 0;
buffer = malloc(buff_len + 1);
if (!buffer) return NULL; // Out of memory
while ((ch = fgetc(fp)) != '\n' && ch != EOF)
{
buff_len++;
void *tmp = realloc(buffer, buff_len + 1);
if (tmp == NULL)
{
free(buffer);
return NULL; // Out of memory
}
buffer = tmp;
buffer[i] = (char) ch;
i++;
}
buffer[i] = '\0';
// Detect end
if (ch == EOF && (i == 0 || ferror(fp)))
{
free(buffer);
return NULL;
}
return buffer;
}
void lineByline(FILE * file){
char *s;
while ((s = readline(file, 0)) != NULL)
{
puts(s);
free(s);
printf("\n");
}
}
int main()
{
char *fileName = "input-1.txt";
FILE* file = fopen(fileName, "r");
lineByline(file);
return 0;
}
const char *readLine(FILE *file, char* line) {
if (file == NULL) {
printf("Error: file pointer is null.");
exit(1);
}
int maximumLineLength = 128;
char *lineBuffer = (char *)malloc(sizeof(char) * maximumLineLength);
if (lineBuffer == NULL) {
printf("Error allocating memory for line buffer.");
exit(1);
}
char ch = getc(file);
int count = 0;
while ((ch != '\n') && (ch != EOF)) {
if (count == maximumLineLength) {
maximumLineLength += 128;
lineBuffer = realloc(lineBuffer, maximumLineLength);
if (lineBuffer == NULL) {
printf("Error reallocating space for line buffer.");
exit(1);
}
}
lineBuffer[count] = ch;
count++;
ch = getc(file);
}
lineBuffer[count] = '\0';
char line[count + 1];
strncpy(line, lineBuffer, (count + 1));
free(lineBuffer);
return line;
}
char linebuffer[256];
while (!feof(myFile)) {
const char *line = readLine(myFile, linebuffer);
printf("%s\n", line);
}
note that the 'line' variable is declared in calling function and then passed, so your readLine function fills predefined buffer and just returns it. This is the way most of C libraries work.
There are other ways, which I'm aware of:
defining the char line[] as static
(static char line[MAX_LINE_LENGTH]
-> it will hold it's value AFTER returning from the function). -> bad,
the function is not reentrant, and
race condition can occur -> if you
call it twice from two threads, it
will overwrite it's results
malloc()ing the char line[], and
freeing it in calling functions ->
too many expensive mallocs, and,
delegating the responsibility to free the buffer to another function (the most elegant solution is to call malloc and free on any buffers in same function)
btw, 'explicit' casting from char* to const char* is redundant.
btw2, there is no need to malloc() the lineBuffer, just define it char lineBuffer[128], so you don't need to free it
btw3 do not use 'dynamic sized stack arrays' (defining the array as char arrayName[some_nonconstant_variable]), if you don't exactly know what are you doing, it works only in C99.
void readLine(FILE* file, char* line, int limit)
{
int i;
int read;
read = fread(line, sizeof(char), limit, file);
line[read] = '\0';
for(i = 0; i <= read;i++)
{
if('\0' == line[i] || '\n' == line[i] || '\r' == line[i])
{
line[i] = '\0';
break;
}
}
if(i != read)
{
fseek(file, i - read + 1, SEEK_CUR);
}
}
what about this one?
Implement method to read, and get content from a file (input1.txt)
#include <stdio.h>
#include <stdlib.h>
void testGetFile() {
// open file
FILE *fp = fopen("input1.txt", "r");
size_t len = 255;
// need malloc memory for line, if not, segmentation fault error will occurred.
char *line = malloc(sizeof(char) * len);
// check if file exist (and you can open it) or not
if (fp == NULL) {
printf("can open file input1.txt!");
return;
}
while(fgets(line, len, fp) != NULL) {
printf("%s\n", line);
}
free(line);
}
Hope this help. Happy coding!
You should use the ANSI functions for reading a line, eg. fgets. After calling you need free() in calling context, eg:
...
const char *entirecontent=readLine(myFile);
puts(entirecontent);
free(entirecontent);
...
const char *readLine(FILE *file)
{
char *lineBuffer=calloc(1,1), line[128];
if ( !file || !lineBuffer )
{
fprintf(stderr,"an ErrorNo 1: ...");
exit(1);
}
for(; fgets(line,sizeof line,file) ; strcat(lineBuffer,line) )
{
if( strchr(line,'\n') ) *strchr(line,'\n')=0;
lineBuffer=realloc(lineBuffer,strlen(lineBuffer)+strlen(line)+1);
if( !lineBuffer )
{
fprintf(stderr,"an ErrorNo 2: ...");
exit(2);
}
}
return lineBuffer;
}
My implement from scratch:
FILE *pFile = fopen(your_file_path, "r");
int nbytes = 1024;
char *line = (char *) malloc(nbytes);
char *buf = (char *) malloc(nbytes);
size_t bytes_read;
int linesize = 0;
while (fgets(buf, nbytes, pFile) != NULL) {
bytes_read = strlen(buf);
// if line length larger than size of line buffer
if (linesize + bytes_read > nbytes) {
char *tmp = line;
nbytes += nbytes / 2;
line = (char *) malloc(nbytes);
memcpy(line, tmp, linesize);
free(tmp);
}
memcpy(line + linesize, buf, bytes_read);
linesize += bytes_read;
if (feof(pFile) || buf[bytes_read-1] == '\n') {
handle_line(line);
linesize = 0;
memset(line, '\0', nbytes);
}
}
free(buf);
free(line);
Provide a portable and generic getdelim function, test passed via msvc, clang, gcc.
/*
* An implementation conform IEEE Std 1003.1-2017:
* https://pubs.opengroup.org/onlinepubs/9699919799/functions/getdelim.html
*
* <nio.h>:
* https://github.com/junjiemars/c/blob/c425bd0e49df35a2649327664d3f6cd610791996/src/posix/nio.h
* <nio.c>:
* https://github.com/junjiemars/c/blob/c425bd0e49df35a2649327664d3f6cd610791996/src/posix/nio.c
*
*/
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <limits.h>
/*
* LINE_MAX dependents on OS' implementations so check it first.
* https://github.com/junjiemars/c/blob/c425bd0e49df35a2649327664d3f6cd610791996/src/posix/nlim_auto_check
*/
#define NM_LINE_MAX 4096 /* Linux */
#if (MSVC)
typedef SSIZE_T ssize_t;
# if !defined(SSIZE_MAX)
# define SSIZE_MAX ((ssize_t)((size_t)((ssize_t)-1) >> 1))
# endif
#endif
ssize_t getdelim(char **restrict lineptr, size_t *restrict n, int delimiter,
FILE *restrict stream);
#if defined(getline)
# undef getline
#endif
#define getline(lp, n, f) getdelim((lp), (n), 0x0a, (f))
ssize_t
getdelim(char **restrict lineptr, size_t *restrict n, int delimiter,
FILE *restrict stream)
{
int c;
char *p, *p1;
ssize_t len;
if (NULL == lineptr || NULL == n || NULL == stream
|| (UCHAR_MAX < delimiter || delimiter < 0))
{
errno = EINVAL;
return EOF;
}
if (feof(stream) || ferror(stream))
{
return EOF;
}
if (0 == *lineptr)
{
if (0 == *n)
{
*n = NM_LINE_MAX;
}
*lineptr = malloc(*n);
if (0 == *lineptr)
{
return EOF;
}
}
p = *lineptr;
len = 0;
while (EOF != (c = fgetc(stream)))
{
if (SSIZE_MAX == (ssize_t) len)
{
errno = EOVERFLOW;
return EOF;
}
if ((size_t) len == (*n - 1))
{
*n <<= 1;
p1 = realloc(*lineptr, *n);
if (0 == p1)
{
return EOF;
}
*lineptr = p1;
p = p1 + len;
}
*p++ = (char) c;
len++;
if (c == delimiter)
{
break;
}
}
if (ferror(stream))
{
return EOF;
}
*p = 0;
return len;
}
int
main(void)
{
FILE *fp;
char *line = NULL;
size_t len = 0;
ssize_t read;
fp = fopen("/some-file", "r");
if (fp == NULL)
exit(1);
while ((read = getline(&line, &len, fp)) != -1) {
printf("Retrieved line of length %zu :\n", read);
printf("%s", line);
}
if (ferror(fp)) {
/* handle error */
}
free(line);
fclose(fp);
return 0;
}
You make the mistake of returning a pointer to an automatic variable.
The variable line is allocated in the stack and only lives as long as the function lives.
You are not allowed to return a pointer to it, because as soon as it returns the memory will be given elsewhere.
const char* func x(){
char line[100];
return (const char*) line; //illegal
}
To avoid this, you either return a pointer to memory which resides on the heap eg. lineBuffer
and it should be the user's responsibility to call free() when he is done with it.
Alternatively you can ask the user to pass you as an argument a memory address on which to write the line contents at.
I want a code from ground 0 so i did this to read the content of dictionary's word line by line.
char temp_str[20]; // you can change the buffer size according to your requirements And A single line's length in a File.
Note I've initialized the buffer With Null character each time I read line.This function can be Automated But Since I need A proof of Concept and want to design a programme Byte By Byte
#include<stdio.h>
int main()
{
int i;
char temp_ch;
FILE *fp=fopen("data.txt","r");
while(temp_ch!=EOF)
{
i=0;
char temp_str[20]={'\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0'};
while(temp_ch!='\n')
{
temp_ch=fgetc(fp);
temp_str[i]=temp_ch;
i++;
}
if(temp_ch=='\n')
{
temp_ch=fgetc(fp);
temp_str[i]=temp_ch;
}
printf("%s",temp_str);
}
return 0;
}