I need to read in a file. The first line of the file is the number of lines in the file and it returns an array of strings, with the last element being a NULL indicating the end of the array.
char **read_file(char *fname)
{
char **dict;
printf("Reading %s\n", fname);
FILE *d = fopen(fname, "r");
if (! d) return NULL;
// Get the number of lines in the file
//the first line in the file is the number of lines, so I have to get 0th element
char *size;
fscanf(d, "%s[^\n]", size);
int filesize = atoi(size);
// Allocate memory for the array of character pointers
dict = NULL; // Change this
// Read in the rest of the file, allocting memory for each string
// as we go.
// NULL termination. Last entry in the array should be NULL.
printf("Done\n");
return dict;
}
I put some comments because I know that's what I'm to do, but I can't seem to figure out how to put it in actual code.
To solve this problem you need to do one of two things.
Read the file as characters then convert to integers.
Read the file directly as integers.
For the first, you would use freed into a char array and then use atoi to convert to integer.
For the second, you would use fscanf and use the %d specify to read directly into an int variable;
fscanf does not allocate memory for you. Passing it a random pointer as you have will only cause trouble. (I recommend avoid fscanf).
The question code has a flaw:
char *size;
fscanf(d, "%s[^\n]", size);
Although the above may compile, it will not function as expected at runtime. The problem is that fscanf() needs the memory address of where to write the parsed value. While size is a pointer that can store a memory address, it is uninitialized, and points to no specific memory in the process' memory map.
The following may be a better replacement:
fscanf(d, " %d%*c", &filesize);
See my version of the spoiler code here
Related
Like I said in the title I don't know how to print all the content of a .txt file in C.
Here's an incomplete function that I did:
void
print_from_file(items_t *ptr,char filemane[25]){
char *string_temp;
FILE *fptr;
fptr=fopen(filemane, "r");
if(fptr){
while(!feof(fptr)){
string_temp=malloc(sizeof(char*));
fscanf(fptr,"\n %[a-z | A-Z | 0-9/,.€#*]",string_temp);
printf("%s\n",string_temp);
string_temp=NULL;
}
}
fclose(fptr);
}
I'm pretty sure that there's errors in the fscanf because sometimes it doesn't exit the loop.
Can anyone please correct this?
You're using malloc wrong. Passing sizeof(char*) to malloc means you are only giving your string the amount of memory it would take to hold a pointer to a character(array). So currently, by writing into memory you have not allocated, you have undefined behavior. It is also highly advisable to perform checks on file lenght and otherwise make sure that you do not write more into the string then you allocated to it.
Instead, do something like this:
string_temp=malloc(100*sizeof(char)); // Enough space for 99 characters (99 chars + '\0' terminator)
There are several things to fix in your code.
First of all, you should always check if a file has been opened correctly.
Example:
FILE *fp; //file pointer
if((fp = fopen("file.txt", "r") == NULL) { //check opening
printf("Could not open file"); //or use perror()
exit(0);
}
Also, remember that scanf() and fscanf() return the number of elements they have read. So, for example, if you scan the file one word at a time, you could simplify your program by looping while fscanf(..) == 1.
As a final note, remember to allocate dynamic memory correctly.
You do not want to allocate memory based on a pointer to char size, in fact, you're gonna wanna allocate 1 byte for each character of the string, + 1 for the terminator.
Example:
char name[55];
char * name2;
//To make them of the same size:
name2 = malloc(sizeof(*char)); **WRONG**
name2 = malloc(sizeof(char) * 55); //OK
The file I'm reading from just has names separated by a line. What happens is the program tries to print the contents of line_array, and it will print out about 20 of the last line in the txt file.
#include <stdio.h>
FILE* fp;
int main(){
char* line;
const char* line_array[255];
int i= 0;
int b =0;
fp = fopen("noob.txt","r");
while(fgets(line,255,fp)){
line_array[i]=line;
printf("%s",line);
printf("%s",line_array[i]);
i++;
}
for(;b<i;b++){
printf("%s",line_array[b]);
}
fclose(fp);
return 0;
}
The first issue, in your code,
while(fgets(line,255,fp))
line is used uninitialized. There is no memory allocated to line. It invokes undefined behavior.
Then, you did not check for the success of fopen() before using the returned file pointer. Again, possible UB.
And finally, by saying
line_array[i]=line;
what you did is to store the line itself to all the occurrences of line_array[n], so for the later printf() loop, the latest content of line is being printed over and over again.
Solution(s):
Allocate memory to line or use a fixed length-array.
Check for the success of fopen()before using the returned pointer.
Allocate memory to each line_array[n] and use strcpy() to copy the content. Ottherwise, you can directly use strdup(), too.
I have a function that receives the name of a file as an argument.
The idea is to read each word in the given file and save each one in a linked list (as a struct with a value and a pointer to the next struct).
I could get it working for small files, but when I give a big .txt file I get a segmentation fault.
Using gdb I could figure out that this happens at the while(fscanf(fi, "%s", value) != EOF){ line.
For some reason when the file is bigger the fscanf() segfaults.
As I could figure out the linked list part, here I pasted just enough code to compile and for you to see my problem.
So my question are:
Why fscanf() segfauts with big .txt files (thousands of words), but not with small file (ten words)?
By the way, is there a better way to check for the end of the file?
Thanks in advance.
bool read(const char* file){
// open file
FILE* fi = fopen(file, "r"); //file is a variable that contains the name of the file to be opened
if (fi == NULL)
{
return false;
}
// malloc for value
char* value = malloc(sizeof(int));
// fscanf() until the end of the file
while(fscanf(fi, "%s", value) != EOF){ // HERE IS MY PROBLEM
// some code for the linked list
// where the value will be saved at the linked list
}
// free space
free(value);
// close the file
fclose(fi);
return true;
}
No, here is your problem:
char* value = malloc(sizeof(int)); // <<<<<<< You allocate only place for an int
while(fscanf(fi, "%s", value) != EOF){ // <<<<<<< but you read a huge string
So you end up with a buffer overflow !
You have to make sure that you never overflow the size of your buffer by setting some limits. For example by using the width field of fscanf() to indicate max size of chars to be read for the string:
char* value = malloc(512); // Allocate your buffer
while(fscanf(fi, "%511s", value) != EOF){ // read max 511 chars + 1 char for terminating 0
...
(disclaimer: simplified explanation)
A char* is a pointer to an address of memory. It specifies that it points to an array of characters. A malloc call reserves a block of memory of a certain size.
Your line
char* value = malloc(sizeof(int));
creates a character array that can hold 4 characters (as an int is 4 bytes long generally). And for it to be a complete string the last character has to be a NULL terminator '\0', So really it can only hold 3 readable characters.
You should make that malloc create a block of memory that is larger than the biggest string in the file. Or you could use another safer method such as fgets : http://www.cplusplus.com/reference/cstdio/fgets/
I have a text file called "graphics" which contains the words "deoxyribonucleic acid".
When I run this code it works and it returns the first character. "d"
int main(){
FILE *fileptr;
fileptr = fopen("graphics.txt", "r");
char name;
if(fileptr != NULL){ printf("hey \n"); }
else{ printf("Error"); }
fscanf( fileptr, "%c", &name);
printf("%c\n", name);
fclose( fileptr );
return 0;
}
When I am using the fscanf function the parameters I am sending are the name of the FILE object, the type of data the function will read, and the name of the object it is going to store said data, correct? Also, why is it that I have to put an & in front of name in fscanf but not in printf?
Now, I want to have the program read the file and grab the first word and store it in name.
I understand that this will have to be a string (An array of characters).
So what I did was this:
I made name into an array of characters that can store 20 elements.
char name[20];
And changed the parameters in fscanf and printf to this, respectively:
fscanf( fileptr, "%s", name);
printf("%s\n", name);
Doing so produces no errors from the compiler but the program crashes and I don't understand why. I am letting fscanf know that I want it to read a string and I am also letting printf know that it will output a string. Where did I go wrong? How would I accomplish said task?
This is a very common problem. fscanf reads data and copies it into a location you provide; so first of all, you need the & because you provide the address of the variable (not the value) - that way fscanf knows where to copy TO.
But you really want to use functions that copy "only as many characters as I have space". This is for example fgets(), which includes a "max byte count" parameter:
char * fgets ( char * str, int num, FILE * stream );
Now, if you know that you only allocated 20 bytes to str, you can prevent reading more than 20 bytes and overwriting other memory.
Very important concept!
A couple of other points. A variable declaration like
char myString[20];
results in myString being a pointer to 20 bytes of memory where you can put a string (remember to leave space for the terminating '\0'!). So you can usemyStringas thechar *argument infscanforfgets`. But when you try to read a single character, and that characters was declared as
char myChar;
You must create the pointer to the memory location "manually", which is why you end up with &myChar.
Note - if you want to read up to white space, fscanf is the better function to use; but it will be a problem if you don't make sure you have the right amount of space allocated. As was suggested in a comment, you could do something like this:
char myBuffer[20];
int count = fscanf(fileptr, "%19s ", myBuffer);
if(count != 1) {
printf("failed to read a string - maybe the name is too long?\n");
}
Here you are using the return value of fscanf (the number of arguments correctly converted). You are expecting to convert exactly one; if that doesn't work, it will print the message (and obviously you will want to do more than print a message…)
Not answer of your question but;
for more efficient memory usage use malloc instead of a static declaration.
char *myName // declara as pointer
myName = malloc(20) // same as char[20] above on your example, but it is dynamic allocation
... // do your stuff
free(myName) // lastly free up your allocated memory for myName
I'm trying to write a stream editor in C and I'm having a hard time dealing with strings. After reading in the lines of a File, I want to store them locally in an array of Strings. However, when I try to store the variable temp into the array of strings StoredEdits I get a segmentation fault (core dumped) error. Furthermore, if I uncomment the char* temp2 variable and save this into my array as a workaround, then the last value read in gets stored for every value in the array.
I assume this has to do with the fact that temp2 is a pointer. I've tried a million things like malloc'ing and free'ing this variable after each iteration, but nothing seems to work.
Any help would be greatly appreciated.
#define MAX_SIZE 100
typedef char String[MAX_SIZE];
int main(int argc, char* argv[]){
char** StoredEdits;
int index, numOfEdits;
FILE *EditFile;
char* temp;
//char* temp2;
StoredEdits = (char**)malloc(MAX_INPUT_SIZE*sizeof(String));
/*Check to see that edit file is passed in.*/
if(argc < 2){
printf("ERROR: Edit File not given\n");
return(EXIT_FAILURE);
}
printf("%s\n",argv[1]);
if( (EditFile = fopen(argv[1],"r")) != NULL ){
printf("file opened\n");
numOfEdits = 0;
while(fgets(temp, MAX_STRING_SIZE, EditFile) != NULL){
printf("%d %s",numOfEdits,temp);
//temp2 = temp;
StoredEdits[numOfEdits++] = temp;
//StoredEdits[numOfEdits++] = temp;
printf("Stored successfully\n");
}
..........
printf("%d\n",numOfEdits);
for(index=0;index<numOfEdits;index++){
printf("%d %s\n",index, StoredEdits[index]);
}
You need to initialize temp to point to valid storage.
temp = malloc(MAX_STRING_SIZE+1);
It looks like you may have intended to do something like this:
String temp;
using your macro. This would be better as a regular char array. And the common name for this is buffer.
char buffer[MAX_STRING_SIZE+1];
Then, you should store in your array, not temp itself, but a new string containing a copy of the contents. There is a POSIX function strdup that should be helpful here. Note, strdup is not part of the C standard, but it is available in most hosted implementations. Historically, it comes from the BSD branch.
StoredEdits[numOfEdits++] = strdup(temp);
Let me backpedal a little and say that if you're allocating new storage for temp inside the loop, then you should skip the strdup because, as Jim Balter says, this will leak memory. If you allocate temp outside of the loop, then it makes little difference whether you allocate it statically (by declaring a char []) or dynamically (with malloc).
By the way, this line will not buy you much:
typedef char String[MAX_SIZE];
For why, see the classic Kernighan (the K in K&R) essay Why Pascal is not my favorite Programming Language.
Also note, that my examples above do not check the pointer returned by malloc. malloc can fail. When malloc fails it will return a NULL pointer. If you try to store data through this pointer, Kaboom!
You're right about your problem being because of pointer semantics. You should use copy the contents of the string from temp.
char *cpy = malloc(1 + strlen(temp));
if (cpy)
strcpy(cpy, temp);
//else handle error
StoredEdits[numOfEdits++] = cpy;
Others answered the reason for the error.
But from the program, i see that you tried to allocate a character double array. then you store each line read from the file into the array.
StoredEdits = (char**)malloc(MAX_INPUT_SIZE*sizeof(String));
if my assumption is right, then you should pass the array into strcpy like the below.
strcpy(StoredEdits[numOfEdits],tmp);
when you have a file where each line varies in size, it is better to go array of pointers points to character array.