getline, gets, fgets都支援從標準輸入讀取一行字串，那麼它們有什麼區別呢？

getline

getline有2個版本：getline和getdelim。

getline原型

#include <stdio.h>

ssize_t getline(char **lineptr, size_t *n, FILE *stream);
ssize_t getdelim(char **lineptr, size_t *n, int delim, FILE *stream);

getline從stream（流）讀取一整行字串，存放到緩衝區buffer，並將buffer地址儲存到指標lineptr。其中，buffer以空字元（'\0'）結尾，並且包含換行符。
如果在呼叫getline之前，

也就是說，getline存放一行字串的buffer可以由呼叫者提供，也可以由函式分配；然而，釋放都是由呼叫者進行。

DESCRIPTION
       getline()  reads  an entire line from stream, storing the address of the buffer containing the text
       into *lineptr.  The buffer is null-terminated and includes the newline character, if one was found.

       If *lineptr is set to NULL and *n is set 0 before the call, then getline() will allocate  a  buffer
       for storing the line.  This buffer should be freed by the user program even if getline() failed.

       Alternatively,  before  calling  getline(), *lineptr can contain a pointer to a malloc(3)-allocated
       buffer *n bytes in size.  If the buffer is not large enough to hold the line, getline() resizes  it
       with realloc(3), updating *lineptr and *n as necessary.

       In either case, on a successful call, *lineptr and *n will be updated to reflect the buffer address
       and allocated size respectively.

       getdelim() works like getline(), except that a line delimiter other than newline can  be  specified
       as  the  delimiter  argument.  As with getline(), a delimiter character is not added if one was not
       present in the input before end of file was reached.

RETURN VALUE
       On success, getline() and getdelim() return the number of characters read, including the  delimiter
       character,  but  not  including the terminating null byte ('\0').  This value can be used to handle
       embedded null bytes in the line read.

       Both functions return -1 on failure to read a line (including end-of-file condition).  In the event
       of an error, errno is set to indicate the cause.
 

getline使用示例

來自getline page

int
main(int argc, char *argv[])
{
    FILE *stream;
    char *line = NULL;
    size_t len = 0;
    ssize_t nread;

    printf("hello main\n");

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <file>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    stream = fopen(argv[1], "r");
    if (stream == NULL) {
        perror("fopen");
        exit(EXIT_FAILURE);
    }

    while ((nread = getline(&line, &len, stream)) != -1) {
        printf("Retrieved line of length %zu:\n", nread);
        fwrite(line, nread, 1, stdout);
    }

    free(line);
    fclose(stream);
    exit(EXIT_SUCCESS);
}
 

gets

符合C89, C99, POSIX.1-2001.，不過目前已經棄用，推薦使用fgets。
gets(s)相當於fgets(s, INT_MAX, stdin) + 替換末尾換行符為'\0'。不過，這樣並不不安全，因為gets不會檢查s是否會溢位。

fgets

fgets原型

#include <stdio.h>

char *fgets(char *s, int size, FILE *stream);

fgets最多從指定stream流讀取 size-1個字元，並且儲存到s指向的buffer中。讀取遇到EOF或換行符停止。每讀取一行，buffer末尾會自動新增'\0'。

fgets() reads in at most one less than size characters from stream and
stores them into the buffer pointed to by s. Reading stops after an
EOF or a newline. If a newline is read, it is stored into the buffer.
A terminating null byte ('\0') is stored after the last character in
the buffer.

也就是說，fgets存放一行字串的buffer必須是由呼叫者提供，fgets不會分配任何儲存字串的空間。