¿Cómo leo una línea arbitrariamente larga en C?

Question

me gustaría tener el equivalente (en C) de getline en C++:

std::string s; 
getline(std::cin,s); // reads an arbitrarily long line and 
        // inserts its contents on s 

¿Hay alguna manera de hacer tal cosa en C? Estoy buscando algo que se parece a esto:

char* s; 
getline(stdin,s); // allocates the space necessary to fit the read 
        // line and make s point to it 

EDITAR: decidí usar la función POSIX getline al final ya que estoy en Linux (correr man getline si usted no sabe lo que yo Estoy hablando de), pero Michael Burr proporcionó una implementación de getline que funciona en otros sistemas operativos en los que getline no está disponible por defecto. Incluso si su implementación no es la más eficiente que uno pueda concebir, hace el trabajo como yo lo quiero, así que lo marqué como la respuesta a mi pregunta.

Answer 1

En caso de que no tiene acceso a una aplicación POSIX getline(), he aquí una implementación de dominio público que me tendido alrededor.

Agregué una pequeña función getline_simple() que simplemente devuelve la siguiente línea en un búfer asignado dinámicamente. Si no está interesado en el tratamiento de errores detallada, puede utilizar esta función para leer un archivo línea por línea:

#define _CRT_SECURE_NO_WARNINGS 1 

#include <assert.h> 
#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include <stdio.h> 
#include <errno.h> 
#include <limits.h> 
#include <sys/types.h> 


#if !__GNUC__ 
#if _WIN64 
typedef long long ssize_t; 
#else 
typedef long ssize_t; 
#endif 
#endif 


#if !defined(SSIZE_MAX) 
#define SSIZE_MAX ((ssize_t)(SIZE_MAX/2)) 
#endif 

#if !defined(EOVERFLOW) 
#define EOVERFLOW (ERANGE)  /* is there something better to use? */ 
#endif 


/* 
    nx_getdelim() 

    a version of the POSIX getdelim() function that return error codes directly 
    instead of messing with the global `errno` value. 
*/ 
ssize_t nx_getdelim(char **lineptr, size_t *n, int delim, FILE *stream); 



/* 
    getdelim_calc_new_alloc() 

    Helper function for getdelim() to figure out an appropriate new 
    allocation size that's not too small or too big. 

    These numbers seem to work pretty well for most text files. 

    returns the input value if it decides that new allocation block 
    would be just too big (the caller should handle this as 
    an error). 
*/ 
static 
size_t nx_getdelim_get_realloc_size(size_t current_size) 
{ 
    enum { 
     k_min_realloc_inc = 32, 
     k_max_realloc_inc = 1024, 
    }; 

    if (SSIZE_MAX < current_size) return current_size; 

    if (current_size <= k_min_realloc_inc) return current_size + k_min_realloc_inc; 

    if (current_size >= k_max_realloc_inc) return current_size + k_max_realloc_inc; 

    return current_size * 2; 
} 



/* 
    getdelim_append() 

    a helper function for getdelim() that adds a new character to 
    the outbuffer, reallocating as necessary to ensure the character 
    and a following null terminator can fit 

*/ 
static 
int nx_getdelim_append(char** lineptr, size_t* bufsize, size_t count, char ch) 
{ 
    char* tmp = NULL; 
    size_t tmp_size = 0; 

    // assert the contracts for this functions inputs 
    assert(lineptr != NULL); 
    assert(bufsize != NULL); 

    if (count >= (((size_t) SSIZE_MAX) + 1)) { 
     // writing more than SSIZE_MAX to the buffer isn't supported 
     return -1; 
    } 

    tmp = *lineptr; 
    tmp_size = tmp ? *bufsize : 0; 

    // need room for the character plus the null terminator 
    if ((count + 2) > tmp_size) { 
     tmp_size = nx_getdelim_get_realloc_size(tmp_size); 

     tmp = (char*) realloc(tmp, tmp_size); 

     if (!tmp) { 
      return -1; 
     } 
    } 

    *lineptr = tmp; 
    *bufsize = tmp_size; 

    // remember, the reallocation size calculation might not have 
    // changed the block size, so we have to check again 
    if (tmp && ((count+2) <= tmp_size)) { 
     tmp[count++] = ch; 
     tmp[count] = 0; 
     return 1; 
    } 

    return -1; 
} 


/* 
    nx_getdelim() 

    A getdelim() function modeled on the Linux/POSIX/GNU 
    function of the same name. 

    Read data into a dynamically resizable buffer until 
    EOF or until a delimiter character is found. The returned 
    data will be null terminated (unless there's an error allocating 
    memory that prevents it). 



    params: 

     lineptr - a pointer to a char* allocated by malloc() 
        (actually any pointer that can legitimately be 
        passed to free()). *lineptr will be updated 
        by getdelim() if the memory block needs to be 
        reallocated to accommodate the input data. 

        *lineptr can be NULL (though lineptr itself cannot), 
        in which case the function will allocate any necessary 
        buffer. 

     n -   a pointer to a size_t object that contains the size of 
        the buffer pointed to by *lineptr (if non-NULL). 

        The size of whatever buff the resulting data is 
        returned in will be passed back in *n 

     delim -  the delimiter character. The function will stop 
        reading one this character is read form the stream. 

        It will be included in the returned data, and a 
        null terminator character will follow it. 

     stream - A FILE* stream object to read data from. 

    Returns: 

     The number of characters placed in the returned buffer, including 
     the delimiter character, but not including the terminating null. 

     If no characters are read and EOF is set (or attempting to read 
     from the stream on the first attempt caused the eof indication 
     to be set), a null terminator will be written to the buffer and 
     0 will be returned. 

     If an error occurs while reading the stream, a 0 will be returned. 
     A null terminator will not necessarily be at the end of the data 
     written. 

     On the following error conditions, the negative value of the error 
     code will be returned: 

      ENOMEM:  out of memory 
      EOVERFLOW: SSIZE_MAX character written to te buffer before 
         reaching the delimiter 
         (on Windows, EOVERFLOW is mapped to ERANGE) 

     The buffer will not necessarily be null terminated in these cases. 


    Notes: 

     The returned data might include embedded nulls (if they exist 
     in the data stream) - in that case, the return value of the 
     function is the only way to reliably determine how much data 
     was placed in the buffer. 

     If the function returns 0 use feof() and/or ferror() to determine 
     which case caused the return. 

     If EOF is returned after having written one or more characters 
     to the buffer, a normal count will be returned (but there will 
     be no delimiter character in the buffer). 

     If 0 is returned and ferror() returns a non-zero value, 
     the data buffer may not be null terminated. 

     In other cases where a negative value is returned, the data 
     buffer is not necessarily null terminated and there 
     is no reliable means to determining what data in the buffer is 
     valid. 

     The pointer returned in *lineptr and the buffer size 
     returned in *n will be valid on error returns unless 
     NULL pointers are passed in for one or more of these 
     parameters (in which case the return value will be -EINVAL). 

*/ 
ssize_t nx_getdelim(char **lineptr, size_t *n, int delim, FILE *stream) 
{ 
    int retval = 0; 

    ssize_t result = 0;  
    char* line = NULL; 
    size_t size = 0; 
    size_t count = 0; 
    int err = 0; 
    int ch = 0; 

    if (!lineptr || !n) { 
     return -EINVAL; 
    } 

    line = *lineptr; 
    size = *n; 

    for (;;) { 
     ch = fgetc(stream); 

     if (ch == EOF) { 
      break; 
     } 

     result = nx_getdelim_append(&line, &size, count, ch); 

     // check for error adding to the buffer (ie., out of memory) 
     if (result < 0) { 
      err = -ENOMEM; 
      break; 
     } 

     ++count; 

     // check if we're done because we've found the delimiter 
     if ((unsigned char)ch == (unsigned char)delim) { 
      break; 
     } 

     // check if we're passing the maximum supported buffer size 
     if (count > SSIZE_MAX) { 
      err = -EOVERFLOW; 
      break; 
     } 
    } 

    // update the caller's data 
    *lineptr = line; 
    *n = size; 

    // check for various error returns 
    if (err != 0) { 
     return err; 
    } 

    if (ferror(stream)) { 
     return 0; 
    } 

    if (feof(stream) && (count == 0)) { 
     if (nx_getdelim_append(&line, &size, count, 0) < 0) { 
      return -ENOMEM; 
     } 
    } 

    return count; 
} 




ssize_t nx_getline(char **lineptr, size_t *n, FILE *stream) 
{ 
    return nx_getdelim(lineptr, n, '\n', stream); 
} 



/* 
    versions of getline() and getdelim() that attempt to follow 
    POSIX semantics (ie. they set errno on error returns and 
    return -1 when the stream error indicator or end-of-file 
    indicator is set (ie., ferror() or feof() would return 
    non-zero). 
*/ 
ssize_t getdelim(char **lineptr, size_t *n, char delim, FILE *stream) 
{ 
    ssize_t retval = nx_getdelim(lineptr, n, delim, stream); 

    if (retval < 0) { 
     errno = -retval; 
     retval = -1; 
    } 

    if (retval == 0) { 
     retval = -1; 
    } 

    return retval; 
} 

ssize_t getline(char **lineptr, size_t *n, FILE *stream) 
{ 
    return getdelim(lineptr, n, '\n', stream); 
} 


/* 
    A simple function to return the next line of text in a dynamically 
    allocated buffer 

    On error a NULL pointer is returned. When the caller no longer needs the 
    returned data, the pointer returned should be passed to `free()`. 


*/ 
char* getline_simple(FILE* stream) 
{ 
    char* p = NULL; 
    size_t size = 0; 

    ssize_t result = getline(&p, &size, stream); 

    if (result < 0) { 
     free(p); 
     p = NULL; 
    } 

    return p; 
} 

Renuncia - este código ha funcionado lo suficientemente bien para mis propósitos, pero no lo hago garantice que será el caso para usted. Utilice la debida diligencia si tiene la intención de utilizar este código.

Answer 2

Es necesario hacerlo de forma incremental en un bucle, usando fgets() para leer bloques de datos, y realloc() para agrandar el búfer de memoria si no ver el final de la línea.

Puede que haya alguna función de biblioteca que haga eso por usted.

#define BLOCKSIZE 1024 

char *readAllocLine(FILE *fp) { 
    char *line = NULL; 
    size_t maxlength = 0; 
    assert(fp != NULL); 
    for(;;) { // Read the line in BLOCKSIZE -blocks. 
     char *crlf, *block; 

     maxlength += BLOCKSIZE; 
     // This exploits realloc behaviour upon hitting NULL 
     if (NULL == (line = realloc(line, maxlength+1))) { 
      break; // realloc error returns NULL. 
     } 
     block = line + maxlength - BLOCKSIZE; 
     // BLOCKSIZE+1 to accommodate final zero 
     if (NULL == fgets(block, BLOCKSIZE+1, fp)) { 
      // TODO: rewind fp in case of error. 
      if (block == line) { 
        // Error. 
        free(line); line = NULL; 
      } 
      break; 
     } 
     // This was the last block iff we find a CRLF inside. 
     if (NULL != (crlf = strchr(block, '\n'))) { 
      *crlf = 0x0; 
      if (crlf != block) { 
       if ('\r' == *(--crlf)) 
        *crlf = 0x0; 
      } 
      break; 
     } /* if */ 
    } /* for */ 
    return line; 
} 

probado con este principal

int main(int argc, char **argv) { 
    FILE *fp; 
    if (argc !=2) { 
      fprintf(stderr, "Syntax: %s testfile.txt\n", argv[0]); 
      return -1; 
    } 
    fp = fopen(argv[1], "r"); 
    while(!feof(fp)) { 
     char *s = readAllocLine(fp); 
     if (NULL != s) { 
      printf("\"%s\"\n", s); 
      free(s); 
     } else { 
      printf("--- end of file ---\n"); 
      break; 
     } 
    } 
    fclose(fp); 
    return 0; 
} 

y este guión:

for i in $(seq 1020 1028); do 
    # Didn't want to think over 
    yes x | tr -d "\n" | dd of=test bs=1 count=$i 2>/dev/null 
    ./readline test | wc -c | tr "\n" "," 
    echo "" >> test 
    ./readline test | wc -c | tr "\n" "," 
    echo "" >> test 
    ./readline test | wc -c 
done