Reading 7M data from a file fails [duplicate] - c

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reading a text file into an array in c
(3 answers)
Closed 8 years ago.
I am trying to read 7M data from a file but it is failing. When I googled,I found that there is no limit for reading data.
My code given below is failing with segmentation fault.
char *buf = malloc(7008991);
FILE *fp = fopen("35mb.txt", "rb");
long long i = 0;
long long j = 0;
while(fgets(buf+i, 1024, fp)) {
i+=strlen(buf);
if(i==7008991)break;
}
printf("read done");
printf("ch=%s\n", buf);
Need some help

Answer 1

If you want to read the content of a large file into memory, you may:
1. actually read it
2. mmap it.
I'll cover how to actually read it, and assume using binary mode and no text-mode mess.
FILE* fp;
// Open the file
fp = fopen ("35mb.txt", "rb");
if ( fp == NULL ) return -1; // Fail
// Get file length, there are many use to do this like fstat
// TODO: check failure
fseek ( fp, 0, SEEK_END );
flen = ftell ( fp );
fseek ( fp, 0, SEEK_SET );
if ( fread ( buffer, flen, 1, fp ) != 1 ) {
// Fail
}
fclose ( fp );

Answer 2

There are a few things that could go wrong here.
Firstly, no this line, memory allocation can fail. (Malloc can return a NULL pointer, you should check this. (You should also check that the file opened without error.)
char *buf = malloc(7008991);
Next, in the loop. Remember that fgets reads one line, regardless of how long that is, up to a maximum of 1024-1 bytes (and appends a null-character). Please not that for binary input, using fread is probably more appropriate.
while(fgets(buf+i, 1024, fp)) {
After that, this is a good line, as you really do not know how long a line is.
i+=strlen(buf);
This line however is probably why you are failing.
if(i==7008991)break;
You are requireing the size to be exactly 77008991 bytes long to break. That is rather unlikely unless you are very very sure about the formatting of your file. This line should probably read if ( i >= 7008991 ) break;
You should probably replace your explicit size with a named constant as well.

Answer 3

Most probably the size of your file is exactly 7008991 bytes. But when you read the file with fgets you ask to write at most 1024 bytes. This is not true when you reach the end of the file. Suppose you already read 7008990 bytes, then you should call fgets with: fgets(buf+i, 1, fp) because your buffer has got no more than one byte left.
Another issue is that you want to print the buffer at the end of your program. For this to work your buffer must be NUL terminated. So you need to allocate one more byte than the file size. fgets will automatically append the NUL byte.
Yet another issue is the way you increment your counter: i += strlen(buf) this is wrong, the correct code is: i = strlen(buf)
All of this assume there is no NUL bytes in your code. As already explained in comments, it is wiser to use fgets only when dealing with text files. When reading binary files you'd better use fread.
The corrected code would be:
unsigned long FILE_SIZE = 7008991+1;
char *buf = malloc(FILE_SIZE);
FILE *fp = fopen("35mb.txt", "rb");
long long i = 0;
long long j = 0;
while(fgets(buf+i, FILE_SIZE-i, fp)) {
i = strlen(buf);
if(i==7008991)break;
}
printf("read done");
printf("ch=%s\n", buf);