Using fgets and strtok() to read a text file -C-CodePudding

I’m trying to read text from stdin line by line using fgets() and store the text in a variable “text”. However, when I use strtok() to split the words, it only works for a couple lines before terminating. What should I change to make it run through the entire text?


#define WORD_BUFFER_SIZE 50
#define TEXT_SIZE 200

int main(void) {
    char stopWords[TEXT_SIZE][WORD_BUFFER_SIZE];
    char word[WORD_BUFFER_SIZE];
    int numberOfWords = 0;
  
    while(scanf("%s", word) == 1){
      if (strcmp(word, "====") == 0){
        break;
      }
      strcpy(stopWords[numberOfWords], word);
      numberOfWords  ;
    }

  char *buffer = malloc(sizeof(WORD_BUFFER_SIZE)*TEXT_SIZE);
  char *text = malloc(sizeof(WORD_BUFFER_SIZE)*TEXT_SIZE);
  
  while(fgets(buffer, WORD_BUFFER_SIZE*TEXT_SIZE, stdin) != NULL){  
    strcat(text, buffer);
  }
  
  char *k;
  k = strtok(text, " ");
  while (k != NULL) {
    printf("%s\n", k);
    k = strtok(NULL, " ");
  }
  
}

CodePudding user response：

char *buffer = malloc(sizeof(WORD_BUFFER_SIZE)*TEXT_SIZE);
char *text = malloc(sizeof(WORD_BUFFER_SIZE)*TEXT_SIZE);

sizeof(WORD_BUFFER_SIZE) is a constant, it's the size of integer. You probably mean WORD_BUFFER_SIZE * TEXT_SIZE. But you can find the file size and calculate exactly how much memory you need.

char *text = malloc(...)
strcat(text, buffer);

text is not initialized and doesn't have a null-terminator. strcat needs to know the end of text. You have to set text[0] = '\0' before using strcat (it's not like strcpy)

int main(void) 
{
    fseek(stdin, 0, SEEK_END);
    size_t filesize = ftell(stdin);
    rewind(stdin);
    if (filesize == 0)
    { printf("not using a file!\n"); return 0; }

    char word[1000] = { 0 };

    //while (scanf("%s", word) != 1)
    //    if (strcmp(word, "====") == 0)
    //        break;

    char* text = malloc(filesize   1);
    if (!text)
        return 0;
    text[0] = '\0';
    while (fgets(word, sizeof(word), stdin) != NULL)
        strcat(text, word);

    char* k;
    k = strtok(text, " ");
    while (k != NULL) 
    {
        printf("%s\n", k);
        k = strtok(NULL, " ");
    }

    return 0;
}

CodePudding user response：

According to the information you provided in the comments section, the input text is longer than 800 bytes.

However, in the line

char *text = malloc(sizeof(WORD_BUFFER_SIZE)*TEXT_SIZE);

which is equivalent to

char *text = malloc(800);

you only allocated 800 bytes as storage for text. Therefore, you did not allocate sufficient space to store the entire input into text. Attempting to store more than 800 bytes will result in a buffer overflow, which invokes undefined behavior.

If you want to store the entire input into text, then you must ensure that it is large enough.

However, this is probably not necessary. Depending on your requirements, it is probably sufficient to process one line at a time, like this:

while( fgets( buffer, sizeof buffer, stdin ) != NULL )
{
    char *k = strtok( buffer, " " );

    while ( k != NULL )
    {
        printf( "%s\n", k );
        k = strtok( NULL, " " );
    }
}

In that case, you do not need the array text. You only need the array buffer for storing the current contents of the line.

Since you did not provide any sample input, I cannot test the code above.

EDIT: Based on your comments to this answer, it seems that your main problem is how to read in all of the input from stdin and store it as a string, when you do not know the length of the input in advance.

One common solution is to allocate an initial buffer, and to double its size every time it gets full. You can use the function realloc for this:

char *buffer;
size_t buffer_size = 1024;
size_t input_size = 0;

//allocate initial buffer
buffer = malloc( buffer_size );
if ( buffer == NULL )
{
    fprintf( stderr, "allocation error!\n" );
    exit( EXIT_FAILURE );
}

//continuously fill the buffer with input, and
//grow buffer as necessary
for (;;) //infinite loop, equivalent to while(1)
{
    //we must leave room for the terminating null character
    size_t to_read = buffer_size - input_size - 1;
    size_t ret;

    ret = fread( buffer   input_size, 1, to_read, stdin );

    input_size  = ret;

    if ( ret != to_read )
    {
        //we have finished reading from input
        break;
    }

    //buffer was filled entirely (except for the space
    //reserved for the terminating null character), so
    //we must grow the buffer
    {
        void *temp;

        buffer_size *= 2;
        temp = realloc( buffer, buffer_size );

        if ( temp == NULL )
        {
            fprintf( stderr, "allocation error!\n" );
            exit( EXIT_FAILURE );
        }

        buffer = temp;
    }
}

//make sure that `fread` did not fail end due to
//error (it should only end due to end-of-file)
if ( ferror(stdin) )
{
    fprintf( stderr, "input error!\n" );
    exit( EXIT_FAILURE );
}

//add terminating null character
buffer[input_size] = '\0';

//the entire contents is now stored in "buffer" as a
//string, and can be printed
printf( "contents of buffer:\n%s\n", buffer );

free( buffer );

The code above assumes that the input will be terminated by an end of file condition, which is probably the case if the input is piped from a file.