c- read in line from file

Question

I am new to c, but I would like to read in text from a file. I don't know the length of the first line of the file, so how can I write the correct parameters for the fgets function? Right now I have:

char read[30]; // but I really don't know how long the line will be

while(fgets(read, sizeof(read), fp).......

Answer 1

You'll have to just keep reallocating and appending to a buffer until you reach the end of the line. The code isn't pretty, but there isn't a simple alternative using the standard C library:

char read[30];
char *line;
int len, total;

line = NULL;
total = 0;

do {
  if (fgets(read, sizeof(read), fp) == NULL)
    break;

  len = strlen(read);

  if (total == 0) {
    total = len;
    line = (char *)malloc(len);
    strcpy(line, read);
  } else {
    total += len;
    line = (char *)realloc(line, total);
    strcat(line, read);
  }
} while (read[len - 1] != '\n');

Answer 2

您必须为线路设置一个安全的合理最大长度并使用它。

Answer 3

Using the C standard library you will just have to accept some maximum line size there and go with it. If you detect that read does not end in a newline you could read more and append the additional data until you find the end of a line. However, for most applications where fgets() is acceptable for parsing, a fixed line length is also acceptable.

Answer 4

There's an off by one in your fgets(). If your buffer has a size of 30, you need to use sizeof(read)+1. Like Felice said, you'll need to set a safe maximum.

Answer 5

You have (at least) two choices. By far the most common is to allocate a buffer assumed to be "big enough" (eg, a couple of kilobytes) and just go with it (and quite possibly mis-behave if provided with data that doesn't fit that limitation).

The primary alternative is to allocate the memory for the buffer dynamically, and when/if that data you read doesn't fit, use realloc to increase the buffer size and read some more.

Answer 6

If you don't know your max. linesize you can read char by char and use realloc, then it's easy:

char *read = calloc(1,1),c;

while( c=fgetc(fp), !feof(fp) )
  if( c=='\n' )
  {
    puts(read);
    *read=0;
  }
  else
  {
    read = realloc(read,strlen(read)+2);
    read[strlen(read)+1]=0;
    strncat(read,&c,1);
  }
fclose(fp);
if( *read )
  puts(read);
free(read);