在C编程中扫描文件中的整行

Question

I was writing a program to input multiple lines from a file. the problem is i don't know the length of the lines, so i cant use fgets cause i need to give the size of the buffer and cant use fscanf cause it stops at a space token I saw a solution where he recommended using malloc and realloc for each character taken as input but i think there's an easier way and then i found someone suggesting using

fscanf(file,"%[^\n]",line);

Does anyone have a better solution or can someone explain how the above works?(i haven't tested it)

i use GCC Compiler, if that's needed

Answer 1

You can use getline(3) . It allocates memory on your behalf, which you should free when you are finished reading lines.

Answer 2

and then i found someone suggesting using fscanf(file,"%[^\\n]",line);

That's practically an unsafe version of fgets(line, sizeof line, file); . Don't do that.

If you don't know the file size, you have two options.

1. There's a LINE_MAX macro defined somewhere in the C library (AFAIK it's POSIX-only, but some implementations may have equivalents). It's a fair assumption that lines don't exceed that length.

2. You can go the "read and realloc" way, but you don't have to realloc() for every character. A conventional solution to this problem is to exponentially expand the buffer size, ie always double the allocated memory when it's exhausted.

Answer 3

A simple format specifier for scanf or fscanf follows this prototype

%specifier 

specifiers

As we know d is format specifier for integers Like this

[characters] is Scanset Any number of the characters specified between the brackets. A dash (-) that is not the first character may produce non-portable behavior in some library implementations.

[^characters] is Negated scanset Any number of characters none of them specified as characters between the brackets.

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fscanf(file,"%[^\n]",line);  

Read any characters till occurance of any charcter in Negated scanset in this case newline character

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As others suggested you can use getline() or fgets() and see example

Answer 4

The line fscanf(file,"%[^\\n]",line); means that it will read anything other than \\n into line . This should work in Linux and Windows, I think. But may not work in OS X format which use \\r to end a line.