I can't figure out why this fclose() in my c program is causing bad access. It was working fine and then I changed the if condition to only print when the strings do not equal eachother and suddenly it started causing problems. apart from the bad access error, it is also not printing anything to "newfile.txt"
#include<stdio.h>
#include<stdlib.h>
#include <string.h>
int main()
{
FILE * cFile;
FILE *outputfile;
FILE *newfile;
cFile = fopen("input.in", "r");
if (cFile == NULL){
printf("bad input file");
}
newfile = fopen("newfile.txt", "w+");
if (newfile == NULL){
printf("bad newfile");
}
char tempstring[15];
char tempstring2[15];
//get each line in the cFile
while (fscanf(cFile, "%15s", tempstring) != EOF) {
outputfile = fopen("outputlotnum.txt", "r"); //open/(or reopen) outputfile to check lines
if (outputfile == NULL){
printf("bad outputfile");
}
//get each line in the outputfile
while(fscanf(outputfile, "%15s", tempstring2) != EOF){
//if the line from cFile doesn't match the line from outputfile,
//then go ahead and print the line to the newfile.txt
if (strcmp(tempstring, tempstring2) != 0){
fprintf(newfile,"%15s \n", tempstring2);
}
//else don't print anything and continue on to the next line
}
fclose(outputfile); //close the outputfile after checking all the lines for a match
}
fclose(newfile); //throws bad access
fclose(cFile);
return 0;
}
Some reasons for library functions seg faulting include passing bad parameters into the function or that you have a memory scribbler. I suspect that in your case you have overflowed one or both temp string arrays on the stack and have corrupted the file handles. It's generally not a safe operation to fscanf/scanf into a buffer unless you can guarantee that the string you read will fit into that buffer.
To confirm this you could print out the file handles immediately after open, and again before close. They should be the same. If they are not then you have accidentally overwritten them.
Related
I'm trying to open the output_voice_capture.txt but it gives me a segementation fault, not only the file exists but it has read privilege.
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE * fPtr;
char ch;
/*
* Open file in r (read) mode.
*/
printf("Opening file ......\n");
fPtr = fopen("/flash/etc/output_voice_capture.txt", "r");
if(fPtr == NULL)
{
/* Unable to open file hence exit */
printf("Unable to open file.\n");
printf("Please check whether file exists and you have read privilege.\n");
exit(EXIT_FAILURE);
}
/* File open success message */
printf("File opened successfully. Reading file contents character by character.\n");
do
{ printf("Read single character from file ......\n");
/* Read single character from file */
ch = fgetc(fPtr);
/* Print character read code ASCII on console */
printf ("%d \n", ch);
} while(ch != EOF); /* Repeat this if last read character is not EOF */
printf("Closing file ......\n");
fclose(fPtr);
return 0;
}
I am using minicom which contains all the bin that I can use , the problem is that when I use linux terminal and a simple .txt test file the code works just fine.
As Zaboj Campula already said in his comment EOF is defined as an integer of -1. On some systems a char is a value from 0..255, on others from -127..128. To avoid any problems one should use the feof() function (link) to check the end of the stream. This might be the source of your problem due to the different sizes of char and int.
Your code will print "File opened successfully. Reading file contents character by character." for each character read.
Leave functions only at one place: at the end. This makes your code much more readable
When parts of your code depend on something, enclose it with an error check.
Try this code:
int main() {
FILE * fPtr;
char ch;
int result = 0;
printf("Opening file ......\n");
if (!(fPtr = fopen("/flash/etc/output_voice_capture.txt", "r")) {
printf("Unable to open file.\n");
printf("Please check whether file exists and you have read privilege.\n");
result = EXIT_FAILURE;
} else {
printf("File opened successfully. Reading file contents character by character.\n");
while (EOF != (ch = fgetc(fPtr))) {
printf ("%d \n", ch);
}
fclose(fPtr);
}
return result;
}
I am writing a code that reads the characters from a file, and then if one of these characters is 'A' it should be changed to 'Z', after these changes are made it should write it back to the file (not append, but write), but whenever I open the file after running the code or it is empty, corrupted, or the compiler shouts at me (since I tried correcting it by making some changes, here is the code I have so far:
int main(){
char variable1[2000000];
FILE *filePointer1;
FILE *filePointer2;
int counter;
int exact_char_numb;
filePointer1 = fopen("File.txt", "r");
filePointer2 = fopen("File.txt", "w");
fread(variable1,2000000,sizeof(char), filePointer1);
for(counter = 0; counter<= 2000000 ; counter ++){
if(variable1[counter] == 'A'){
variable1[counter] = 'Z';
}
if(variable1[counter] == '+'){
exact_char_numb = counter; // I am using '+' to mark the end of
} // the file (for now)
}
fwrite(variable1,sizeof(char),exact_char_numb,filePointer2);
printf("%s\n", variable1);
printf("%d\n", exact_char_numb);
return 0;
}
In this call:
fwrite(variable1,sizeof(char),exact_char_numb,filePointer2);
the variable exact_char_numb is likely equal to zero, so you don't get any output. You should turn on all warnings and the compiler will complain to you about the variables that can be used without initializing them first.
"but whenever I open the file after running the code or it is empty, corrupted, or the compiler shouts at me..."
You should check whether the opening of the streams to file.txt were successful by checking the returned pointers for a null pointer before doing any operations with the streams:
if (!(filePointer1 = fopen("File.txt", "r")))
{
fputs("Error occurred at opening file to read!", stderr);
exit(1);
}
if (!(filePointer2 = fopen("File.txt", "w")))
{
fputs("Error occurred at opening file to write!\n", stderr);
exit(1);
}
Furthermore, you don´t need to do have two pointers to two different streams. Use r+ mode:
if (!(filePointer = fopen("File.txt", "r+")))
{
fputs("Error occurred at opening file!", stderr);
exit(1);
}
I am trying to repeatedly read a string from the command line and print it to a file. This is my code:
int main ()
{
FILE* fp=fopen("test.txt","w");
char* tofile[10];
while(1){
printf("cat: ");
scanf("%s",tofile);
fprintf(fp,"%s\n",tofile);
}
return 0;
}
It works just fine outside the loop. But inside, it just doesn't print.
The fprintf function returns the correct amount of characters it has to print.
Note: I know there's a similar question out there, but it hasn't been answered yet, and I hope my code can help in this matter since it's simpler.
Well first it doesn't seem that what you want is reading on the command line.
The command line what you write right when you execute your program such as:
./main things that are on the command line
What it seems you want to do is to read on the standard input.
What you should consider is to use the fgets function, as it has a limit of characters to be read, so that you can store them "safely" into a buffer, like your tofile.
As you want to read on the standard input you can use the stdin stream (which is a FILE* that is automatically created for every program)
The line goes
fgets(tofile, 10, stdin);
Your loop becoming :
while (fgets(tofile, 10, stdin) != NULL) {
printf("cat: ");
fprintf(fp, "%s\n", tofile);
}
meaning: as long as we can read on the standard input, print "cat :" and store what we just read in the file controlled by the stream pointer fp.
Some important stuff
When you try to open a stream it may fail and you should test it:
char filename[] = "test.txt";
FILE *fp = fopen(filename, "w");
if (fp == NULL) {
fprintf(stderr, "Failed to open the file of name : %s", filename);
return EXIT_FAILURE;
}
Right before exiting your main, you should also close the file and check if it has succeeded, like that for example:
if (fclose(fp) != 0) {
fprintf(stderr, "Failed to close the file of name : %s", filename);
return EXIT_FAILURE;
}
The whole thing becomes:
#include <stdio.h>
#include <stdlib.h>
int main (void) {
char filename[] = "test.txt";
FILE *fp = fopen(filename, "w");
if (fp == NULL) {
fprintf(stderr, "Failed to open the file of name : %s", filename);
return EXIT_FAILURE;
}
char tofile[10];
printf("cat: ");
while (fgets(tofile, 10, stdin) != NULL) {
printf("cat: ");
fprintf(fp, "%s\n", tofile);
}
if (fclose(fp) != 0) {
fprintf(stderr, "Failed to close the file of name : %s", filename);
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
Improvements
I don't know if it is just a little program or it aspires to become a greater program.
In the last case you should consider using defines and not a magical number such as
#define BUFFER_MAX_SIZE 10
char tofile[BUFFER_MAX_SIZE];
while (fgets(tofile, BUFFER_MAX_SIZE, stdin) != NULL) { ... }
This helps for readability and makes the program less apt to debug when modifying such a size. Because with the define all the part of the code needing the size will still be fully functional without modifying them.
Please also keep in mind that your tofile acts as a buffer, and it's really a small buffer that can easily be overflowed.
This will work. fgets() returns the string it reads from the specified file pointer. If this string returns only a newline ("\n"), that means nothing was entered at stdin.
#include <stdio.h>
#include <string.h>
int main(void)
{
FILE *fp = fopen("test.txt","w");
// always check if fopen() == null
if (!fp) {
fprintf(stderr, "Could not write to file\n");
return 1;
}
char tofile[30];
printf("cat: ");
while (fgets(tofile, 30, stdin)) {
if (strcmp(tofile, "\n") == 0)
break;
fprintf(fp, "%s", tofile);
printf("cat: ");
}
// always fclose()
fclose(fp);
return 0;
}
Edited code.
I have been tinkering around for days with a program for a class of mine, and I cannot get fwrite to cooperate. I've tried looking at the online resources for fread and fwrite, looked at my professor's examples, and tried time and time again to fix this, but nothing has worked. No matter what I do, fwrite makes it so that my text editor can't detect any kind of character encoding, so I'm presuming that fwrite is either writing memory addresses or garbage values to the file, making it so that I can't read it. The program is simply supposed to write the contents of one file into another.
Here is my code.
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char* argv[]) {
//initialize files
FILE* infile, *outfile;
char* buffer;
int read = 0;
//handle the case of having the wrong number of inputs.
if (argc != 4){
printf("Error: incorrect number of inputs\n");
//returning 1 will tell us that this was an error (as opposed to returning zero)
return 1;
}
else{
//handle the case of not having read acces to file 1
if ( access(argv[1], R_OK) == 1){
printf("Error: you do not have read access to the first file\n");
return 1;
}
else {
//handle the case of not having write access to file 2
if ( access(argv[2], W_OK) == 1){
printf("Error: you do not have write access to the second file\n");
return 1;
}
else{
//handle a bad buffer size (zero or negative number)
if ((int)*argv[3] < 0){
printf("Error: bad input for buffer size.\nBuffer size: %d \n", (int)*argv[3]);
return 1;
}
}
}
//open the files in the correct mode.
infile = fopen(argv[1], "r");
outfile = fopen(argv[2], "w");
buffer = malloc((int)*argv[3]);
while (!feof(infile)){
read = fread (buffer,1,(int)*argv[3],infile);
fwrite(buffer,1,(int)*argv[3],outfile);
}
}
//close files, and deallocate the buffer.
fclose(infile);
fclose(outfile);
free(buffer);
//if we made it here, then that means that our program ran correctly, so return zero.
return 0;
}
This is wrong
(int)*argv[3]
change it to
atoi(argv[3])
and it would be better if you store the value somewhere, and check that it's convertible to integer, something like
int size;
char *endptr;
size = strtol(argv[3], &endptr, 10);
if (*endptr != '\0')
errorNotAnIntegerAbortHere();
buffer = malloc(size);
.
.
.
not that, *argv[3] would be equivalent to argv[3][0] which is just the first character in argv[3].
fread will return less than the no. of requested bytes at EOF.
Change to
if (read)
fwrite(buffer,1,read,outfile);
I created a function to print the contents of a file:
void readFile(char* filename)
{
int c ;
file = fopen(filename, "r");
printf("The contents of the file are:\n");
while((c = fgetc(file)) != EOF)
{
printf("%c", c);
}
return;
}
where file is a global variable. GDB gives output as follows:
_IO_getc (fp=0x0) at getc.c:39
39 getc.c: No such file or directory.
(gdb) bt
#0 _IO_getc (fp=0x0) at getc.c:39
#1 0x000000000040075e in readFile ()
#2 0x00000000004006d4 in main ()
However, the file is present and I get the SEGFAULT after printing the contents of the file. It might be because the buffer here (c) is small but I am not sure. Also, I don't know how do I fix this even if that were the case. Can anyone suggest how do I proceed?
EDIT
I call the readFile function only once. Here is my calling function:
int main(int argc, char* argv[])
{
char * filename;
filename = argv[1];
readFile(filename);
printf("File Handler: %ld", (long)file);
fclose(file);
return 0;
}
You're passing in a filename that doesn't exist or for some other reason cannot be opened. Get rid of the segfault by checking for errors (you'll need to #include <errno.h> and <string.h> too for this:
void readFile(char* filename)
{
int c ;
file = fopen(filename, "r");
if (file == NULL) {
printf("Cannot open file '%s' : %s\n", filename, strerror(errno));
return;
}
printf("The contents of the file are:\n");
while((c = fgetc(file)) != EOF)
{
printf("%c", c);
}
return;
}
Most likely your file is NULL and you are still trying to read it.
I simulated this behaviour (SEG fault) when I deleted this file.
If file exists then your code works fine.
Check what path you are passing.. If you are using single \ try with \\ and see if this works. First \ will work as escape sequence and final path will be send as D:\temp\use.dat to fopen.
readFile("D:\\temp\\user.dat");
Before you do anything with a file, you must ensure that you opened it successfully. This is done by checking that the file pointer received by calling fopen is not NULL.
Once you do this, you read using whatever function you choose until it returns a value that indicates failure to read — a NULL pointer for fgets, 0 or EOF for fscanf, or EOF for fgetc.
In any case, you challenge these return values in two ways. The first way is to check for read errors using ferror. The other way is to check whether the end of the file was reached using feof.
A complete program that should work, based upon your code:
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
enum { OPEN_ERROR = 1, READ_ERROR };
enum { PARAM_EXIT = 1, OPEN_EXIT, READ_EXIT };
FILE *file = NULL;
int readFile(char* filename)
{
int c;
file = fopen(filename, "r");
if (file == NULL)
return OPEN_ERROR;
printf("The contents of file '%s' are:\n", filename);
while((c = fgetc(file)) != EOF)
printf("%c", c);
/*
* fgetc returns EOF on end of file and when an error occurs.
* feof is used to determine whether the end of the file was reached.
* Otherwise, we encountered a read error.
*/
if (feof(file))
c = 0;
else
c = READ_ERROR;
return c;
}
int main(int argc, char *argv[])
{
int status = 0;
if (argc == 1) {
fprintf(stderr, "usage: %s file\n", argv[0]);
return PARAM_ERROR;
}
/* Check that <program ""> wasn't used... */
if (argv[1][0] == '\0') {
fprintf(stderr, "error: empty filename detected, exiting. . .\n");
return PARAM_ERROR;
}
switch (readFile(argv[1])) {
case 0:
break;
case OPEN_ERROR:
fprintf(stderr, "error: file open failed - %s\n", strerror(errno));
status = OPEN_EXIT;
break;
case READ_ERROR:
fprintf(stderr, "error: file read failed - %s\n", strerror(errno));
status = READ_EXIT;
break;
default:
fprintf(stderr, "error: unknown error occurred, aborting...\n");
abort();
}
if (file != NULL)
fclose(file);
return status;
}
Of course, normally you would close the file in the same function in which it was opened (e.g. something like filep = openFile(...); readFile(filep); fclose(filep);, except error handling would be used of course).
I am completely changing my answer
Actually, the file that I was reading was open in gedit (which might explain why I was getting "NULL" even after printing the file contents. I closed the file and removed my NULL comparison code and it works perfectly fine.
Ok, from everybody's comments I got to know that you basically get a SEGFAULT when you read the contents of file that has NULL contents. I just made a simple fix in my while loop:
while((c != EOF))
{
printf("%c", c);
c = fgetc(file);
if(c == NULL)
break;
}
Problemo solved! (Although, the compiler gives me a warning of "comparison between pointer and integer".)