使用fread和fseek读取文本文件中特定数量的字符

Question

Let's say I have a text file like this

This is a text file which contains some numbers.

So I want to use fseek and fread to read some parts of the text file. For example, from position 0 to 13, I'll get "This is a text". Then from position 14 to 24, I'll get " file which", then from position 25 to the end of file, I'll get " contains some numbers."

I've tried to use fseek and fread but I've got some additional weird characters like "This is a text?"

My attempt to use fseek and fread:

src = fopen(textfile, "r");
int chunksize = data[i].end - data[i].start;
char *buffer = malloc(sizeof(chunksize));

seek(src, data[i].start, SEEK_SET);
fread(buffer, 1, chunksize, src);
fseek(src, 0, SEEK_SET); // seek back to beginning of file      

where data[i].start is the start position to read to part and data[i].end is the end position to stop. For example, from 14 to 24, I'll get " file which". start is 14 and end is 25.

Answer 1

You need to rewrite your code approximately as follows:

src = fopen(textfile, "r");
int chunksize = ...
char *buffer = malloc(chunksize + 1);
fseek(src, data[i].start, src);
int len = fread(buffer, 1, chunksize, src);
*(buffer+len) = '\0';

So, we have the following, a buffer containing what was read from the file. We have a string terminator following the content read (which may be less than you asked for). If you now issue:

print("%s\n", buffer)

You will get exactly what you read from the file.

PS: It is a good idea to check the return from the fopen() to be sure that the file opened properly, and the return from malloc() to ensure that memory was allocated successfully, and the return from fread() to ensure that the correct amount of data was read.

Answer 2

Random characters after the end of the data arise from not null terminating the input string, or not limiting the output to the data that was read. fread() does not 'null terminate' what it reads; it would be useless if it did.

You are allocating too little memory. You have:

char *buffer = malloc(sizeof(chunksize));

You need:

char *buffer = malloc(chunksize);

Or you could allocate an extra byte and store a null byte '\\0' in that. If you're going to need to pass it to code that needs a string, this is better:

char *buffer = malloc(chunksize + 1);

You should be checking the error returns of most functions; specifically, you should be paying attention to malloc() , fopen() and fread() . You also need to use fseek() , not seek() .

You might use:

src = fopen(textfile, "r");
if (src == 0)
    err_exit("Failed to open %s for reading\n", textfile);

int chunksize = data[i].end - data[i].start;
char *buffer = malloc(chunksize + 1);
if (buffer == 0)
    err_exit("Failed to allocate %d bytes memory\n", chunksize);

fseek(src, data[i].start, SEEK_SET);
size_t nbytes = fread(buffer, 1, chunksize, src);
fseek(src, 0, SEEK_SET);
buffer[nbytes] = '\0';
if (nbytes != 0)
    printf("Read: <<%.*s>>\n", (int)nbytes, buffer);

or:

    prinf("Read: <<%s>>\n", buffer);