while(!feof(fname))
{
fscanf(fname,"%c",buf);
printf("%c",*buf);
buf++;
}
When I use this code, the compiler gives me a segmentation fault(core dumped).
But instead of this if I use
int i = 0;
while(!feof(fname))
{
fscanf(fname,"%c",&buf[i]);
printf("%c",buf[i]);
i++;
}
This code runs fine, and the content of the file are read.
What am I doing wrong while incrementing the array pointer instead of using it as and array?
The whole code is
FILE *fname;
fname = fopen("input.txt", "r");
int i;
char *buf;
i=0;
while(!feof(fname))
{
fscanf(fname,"%c",buf);
printf("%c",*buf);
buf++;
}
fclose(fname);
char *buf;
This is a pointer and has no memory associated with it.
fscanf(fname,"%c",buf);
is attempting to place memory into this pointer. It needs to either be malloc'd, or defined as a char array that won't overflow from the fscanf() input.
perhaps:
#define MAX_STRING_SIZE 255
char buf[MAX_STRING_SIZE];
Additional Info edit
If you need a moving pointer, create a second one and use it:
#define MAX_STRING_SIZE 255
char buffer_memory[MAX_STRING_SIZE];
char *buf = buffer_memory;
...
fscanf(fname,"%c",buf);
printf("%c",*buf);
buf++;
Until you read in 255 character at which point bad things happen. If you don't know how big your file is beforehand (ftell()), then you will need to use a linked list system, a realloc() pattern, which is kinda icky, or a buffer that is guarenteed to be big enough to fit your file.
Related
i'm trying to read a file and split this file into multiple buffers.
This is what i came up with:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define PKT_SIZE 2048;
#define PATH "directory of some kind"
int main() {
char filepath[200] = PATH;
FILE *packet;
int size = PKT_SIZE;
char *buffer[size];
int i=0;
//OPEN FILE
if((packet = fopen(filepath, "r")) == NULL){ //I'm trying with a txt file, then i'll change it to 'rb'
printf("Error Opening File\n");
return -1;
}
//READ FILE
while(*fgets((char *) *buffer[i], (int) strlen(buffer[i]), packet) != NULL) { //read the file and cycling insert the fgets into the buffer i
printf("Create %d buffer\n", i);
i++;
}
fclose(packet);
return 0;
}
Now, when i run this program, i get a SIGSEGV error, i managed to understand that this error is definetly:
*fgets((char *) *buffer[i], (int) strlen(buffer[i]), packet) != NULL
Do you have any suggestions?
*fgets((char *) *buffer[i], (int) strlen(buffer[i]), packet)
This line as several problems.
buffer[i] is just an un-initialized pointer pointing nowhere.
*buffer[i] is of type char you need to pass the char*.
strlen is not returning the size of the buffer. It is undefined behavior here because you called it over uninitialized pointer value.
Also dererencing whatever fgets is return is bad when the fgets returns NULL. It invokes undefined behavior.
There many solutions to this ranging from dynamic memory allocation to using
char buffer[size][MAXSIZE];. If you go about this you can get input this way:
#define MAXSIZE 100
...
char buffer[size][MAXSIZE];
while(fgets(buffer[i], sizeof(buffer[i]), packet)!=NULL){...
char* buffer[size] is an array of N char* pointers which are uninitialized. You must allocate memory to these before using them or your program will explode in a ball of fire.
The fix is to allocate:
for (size_t i = 0; i < size; ++i) {
buffer[i] = malloc(PKT_SIZE);
}
You're going to be responsible for that memory going forward, too, so don't forget to free later.
Allocating an arbitrary number of buffers is pretty wasteful. It's usually better to use some kind of simple linked-list type structure and append chunks as necessary. This avoids pointless over-allocation of memory.
I am pretty new to C and memory allocation in general. Basically what I am trying to do is copy the contents of an input file of unknown size and reverse it's contents using recursion. I feel that I am very close, but I keep getting a segmentation fault when I try to put in the contents of what I presume to be the reversed contents of the file (I presume because I think I am doing it right....)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int recursive_back(char **lines, int lineNumber, FILE *input) {
char *input_line = malloc(sizeof(char) * 1000);
lines = realloc(lines, (lineNumber) * 1000 * sizeof(char));
if(fgets(input_line, 201, input) == NULL) {
*(lines + lineNumber) = input_line;
return 1;
}
else {
printf("%d\n", lineNumber);
return (1+recursive_back(lines, ++lineNumber, input));
}
}
void backward (FILE *input, FILE *output, int debugflag ) {
int i;
char **lines; //store lines in here
lines = malloc(1000 * sizeof(char *) ); //1000 lines
if(lines == NULL) { //if malloc failed
fprintf(stderr, "malloc of lines failed\n");
exit(1);
}
int finalLineCount, lineCount;
finalLineCount = recursive_back(lines, 0, input);
printf("test %d\n", finalLineCount);
for(i = finalLineCount; i > 0; i--) {
fputs(*(lines+i), output); //segfault here
}
}
I am using a simple input file to test the code. My input file is 6 lines long that says "This is a test input file". The actual input files are being opened in another function and passed over to the backward function. I have verified that the other functions in my program work since I have been playing around with different options. These two functions are the only functions that I am having trouble with. What am I doing wrong?
Your problem is here:
lines = realloc(lines, (lineNumber) * 1000 * sizeof(char));
exactly as #ooga said. There are at least three separate things wrong with it:
You are reallocating the memory block pointed to by recursive_back()'s local variable lines, and storing the new address (supposing that the reallocation succeeds) back into that local variable. The new location is not necessarily the same as the old, but the only pointer to it is a local variable that goes out of scope at the end of recursive_back(). The caller's corresponding variable is not changed (including when the caller is recursive_back() itself), and therefore can no longer be relied upon to be a valid pointer after recursive_back() returns.
You allocate space using the wrong type. lines has type char **, so the object it points to has type char *, but you are reserving space based on the size of char instead.
You are not reserving enough space, at least on the first call, when lineNumber is zero. On that call, when the space requested is exactly zero bytes, the effect of the realloc() is to free the memory pointed to by lines. On subsequent calls, the space allocated is always one line's worth less than you think you are allocating.
It looks like the realloc() is altogether unnecessary if you can rely on the input to have at most 1000 lines, so you should consider just removing it. If you genuinely do need to be able to reallocate in a way that the caller will see, then the caller needs to pass a pointer to its variable, so that recursive_back() can modify it via that pointer.
I need to read some data from text file and store it in 2D-array.
This code works good:
#include <string.h>
#include <stdio.h>
int main() {
FILE *f = fopen("Read.txt", "r");
char buff[100][100];
char str[100];
int i = 0;
while(fgets(str, 100, f)) {
strcpy(buff[i], str);
i++;
}
return 0;
}
But why doesn't it work when I try to change buff definition in line 5 to:
char (*buff)[100];
I expected this definition to work too.
The error I get:
Run-Time Check Failure #3 - The variable 'buff' is being used without being defined
char (*buff)[100];
Here buff is a pointer to an array of 100 characters. So first you should make the pointer point to valid memory location before storing some value in it.
Presuming you want to go for dynamic memory allocation then you can have
char *buff[100];
Now in the fgets() loop allocate memory to each pointer individually like
buff[i] = malloc(100);
Note here buff is an array of 100 char pointers.
I don't know the title correctly addresses my problem or not. So, I will just go with it.
Here is the problem, I have to input a char array of a file path (in Windows) containing lots of backslashes in it, eg. "C:\myfile.txt" and return an unsigned char array of C-style file paths, eg. "C:\myfile.txt".
I tried to write a function.
unsigned char* parse_file_path(char *path);
{
unsigned char p[60];
int i,j;
int len = strlen(path);
for(i=0,j=0; i<len; i++, j++)
{
char ch = path[i];
if(ch==27)
{
p[j++]='\\';
p[j]='\\';
}
else
p[j] = path[i];
}
p[j]='\0';
return p;
}
The weird thing (for me) I am encountering is, here path contains only one backslash '\'. In order to get one backslash, I have to put '\' in path. This is not possible, cause path cannot contain '\'. When I call it like this parse_file_path("t\es\t \it), it returns
t←s it. But parse_file_path("t\\es\\t \\it") returns t\es\t \it.
How can I accomplish my task? Thanks in advance.
If I can just mention another problem with your code.
You are returning a local variable (your unsigned char p). This is undefined behavior. Consider declaring a char* p that you assign memory to dynamically using malloc and then returning p as you do. E.g. something like:
char* p = malloc(60);
A common practice is to use sizeof when allocating memory with malloc but here I've passed 60 directly as the C standard guarantees that a char will be 1 byte on all platforms.
But you have to free the memory assigned with malloc.
Or alternatively, you can change the function to take a buffer as an input argument that it then writes to. That way you can pass a normal array where you would call this function.
Regarding your slashes issue, here:
p[j++]='\\';
p[j]='\\';
Position j in p will be changed to \\, then j will be incremented and at the very next line you do the same for the succeeding char position. Are you sure you want the two assignments?
By the way if you are inputting the path from the command line, the escaping will be taken care of for you. E.g. consider the following code:
#include <stdio.h>
#include <string.h> /* for strlen */
#include <stdlib.h> /* for exit */
int main()
{
char path[60];
fgets(path, 60, stdin); /* get a maximum of 60 characters from the standard input and store them in path */
path[strlen(path) - 1] = '\0'; /* replace newline character with null terminator */
FILE* handle = fopen(path, "r");
if (!handle)
{
printf("There was a problem opening the file\n");
exit(1); /* file doesn't exist, let's quite with a status code of 1 */
}
printf("Should be good!\n");
/* work with the file */
fclose(handle);
return 0; /* all cool */
}
And then you run it and input something like:
C:\cygwin\home\myaccount\main.c
It should print 'Should be good!' (provided the file does exist, you can also test with 'C:\').
At least on Windows 7 with cygwin this is what I get. No need for any escapes as this is handled for you.
Alright guys, this is my first post here. The most recent assignment in my compsci class has us coding a couple of functions to encode and decode strings based on a simple offset. So far in my encryption function I am trying to convert uppercase alphas in a string to their ASCII equivalent(an int), add the offset(and adjust if the ASCII value goes past 'Z'), cast that int back to a char(the new encrypted char) and put it into a new string. What I have here compiles fine, but it gives a Segmentation Fault (core dumped) error when I run it and input simple uppercase strings. Where am I going wrong here? (NOTE: there are some commented out bits from an attempt at solving the situation that created some odd errors in main)
#include <stdio.h>
#include <string.h>
#include <ctype.h>
//#include <stdlib.h>
char *encrypt(char *str, int offset){
int counter;
char medianstr[strlen(str)];
char *returnstr;// = malloc(sizeof(char) * strlen(str));
for(counter = 0; counter < strlen(str); counter++){
if(isalpha(str[counter]) && isupper(str[counter])){//If the character at current index is an alpha and uppercase
int charASCII = (int)str[counter];//Get ASCII value of character
int newASCII;
if(charASCII+offset <= 90 ){//If the offset won't put it outside of the uppercase range
newASCII = charASCII + offset;//Just add the offset for the new value
medianstr[counter] = (char)newASCII;
}else{
newASCII = 64 + ((charASCII + offset) - 90);//If the offset will put it outside the uppercase range, add the remaining starting at 64(right before A)
medianstr[counter] = (char)newASCII;
}
}
}
strcpy(returnstr, medianstr);
return returnstr;
}
/*
char *decrypt(char *str, int offset){
}
*/
int main(){
char *inputstr;
printf("Please enter the string to be encrypted:");
scanf("%s", inputstr);
char *encryptedstr;
encryptedstr = encrypt(inputstr, 5);
printf("%s", encryptedstr);
//free(encryptedstr);
return 0;
}
You use a bunch of pointers, but never allocate any memory to them. That will lead to segment faults.
Actually the strange thing is it seems you know you need to do this as you have the code in place, but you commented it out:
char *returnstr;// = malloc(sizeof(char) * strlen(str));
When you use a pointer you need to "point" it to allocated memory, it can either point to dynamic memory that you request via malloc() or static memory (such as an array that you declared); when you're done with dynamic memory you need to free() it, but again you seem to know this as you commented out a call to free.
Just a malloc() to inputstr and one for returnstr will be enough to get this working.
Without going any further the segmentation fault comes from your use of scanf().
Segmentation fault occurs at scanf() because it tries to write to *inputstr(a block of location inputstr is pointing at); it isn't allocated at this point.
To invoke scanf() you need to feed in a pointer in whose memory address it points to is allocated first.
Naturally, to fix the segmentation fault you want to well, allocate the memory to your char *inputstr.
To dynamically allocate memory of 128 bytes(i.e., the pointer will point to heap):
char *inputstr = (char *) malloc(128);
Or to statically allocate memory of 128 bytes(i.e., the pointer will point to stack):
char inputstr[128];
There is a lot of complexity in the encrypt() function that isn't really necessary. Note that computing the length of the string on each iteration of the loop is a costly process in general. I noted in a comment:
What's with the 90 and 64? Why not use 'A' and 'Z'? And you've commented out the memory allocation for returnstr, so you're copying via an uninitialized pointer and then returning that? Not a recipe for happiness!
The other answers have also pointed out (accurately) that you've not initialized your pointer in main(), so you don't get a chance to dump core in encrypt() because you've already dumped core in main().
#include <ctype.h>
#include <stdlib.h>
#include <string.h>
char *encrypt(char *str, int offset)
{
int len = strlen(str) + 1;
char *returnstr = malloc(len);
if (returnstr == 0)
return 0;
for (int i = 0; i < len; i++)
{
char c = str[i];
if (isupper((unsigned char)c))
{
c += offset;
if (c > 'Z')
c = 'A' + (c - 'Z') - 1;
}
returnstr[i] = c;
}
return returnstr;
}
Long variable names are not always helpful; they make the code harder to read. Note that any character for which isupper() is true also satisfies isalpha(). The cast on the argument to isupper() prevents problems when the char type is signed and you have data where the unsigned char value is in the range 0x80..0xFF (the high bit is set). With the cast, the code will work correctly; without, you can get into trouble.