我写这个函数是从文件中读取一行:

const char *readLine(FILE *file) {

    if (file == NULL) {
        printf("Error: file pointer is null.");
        exit(1);
    }

    int maximumLineLength = 128;
    char *lineBuffer = (char *)malloc(sizeof(char) * maximumLineLength);

    if (lineBuffer == NULL) {
        printf("Error allocating memory for line buffer.");
        exit(1);
    }

    char ch = getc(file);
    int count = 0;

    while ((ch != '\n') && (ch != EOF)) {
        if (count == maximumLineLength) {
            maximumLineLength += 128;
            lineBuffer = realloc(lineBuffer, maximumLineLength);
            if (lineBuffer == NULL) {
                printf("Error reallocating space for line buffer.");
                exit(1);
            }
        }
        lineBuffer[count] = ch;
        count++;

        ch = getc(file);
    }

    lineBuffer[count] = '\0';
    char line[count + 1];
    strncpy(line, lineBuffer, (count + 1));
    free(lineBuffer);
    const char *constLine = line;
    return constLine;
}

该函数正确地读取文件,使用printf我看到constLine字符串也被正确读取。

然而,如果我像这样使用函数:

while (!feof(myFile)) {
    const char *line = readLine(myFile);
    printf("%s\n", line);
}

Printf输出胡言乱语。为什么?


当前回答

FILE* filePointer;
int bufferLength = 255;
char buffer[bufferLength]; /* not ISO 90 compatible */

filePointer = fopen("file.txt", "r");

while(fgets(buffer, bufferLength, filePointer)) {
    printf("%s\n", buffer);
}

fclose(filePointer);

其他回答

我想要一个代码从地面0,所以我这样做,逐行读取字典的单词的内容。

char temp_str [20];//你可以根据你的要求改变缓冲区的大小和一个文件中的单行长度。

注意,每次读取line时,我都将缓冲区初始化为Null字符。这个功能可以自动化,但因为我需要一个概念的证明,并想要一个字节一个字节地设计程序

#include<stdio.h>

int main()
{
int i;
char temp_ch;
FILE *fp=fopen("data.txt","r");
while(temp_ch!=EOF)
{
 i=0;
  char temp_str[20]={'\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0'};
while(temp_ch!='\n')
{
  temp_ch=fgetc(fp);
  temp_str[i]=temp_ch;
  i++;
}
if(temp_ch=='\n')
{
temp_ch=fgetc(fp);
temp_str[i]=temp_ch;
}
printf("%s",temp_str);
}
return 0;
}

您应该使用ANSI函数来读取一行,例如。fgets。调用后,你需要在调用上下文中使用free(),例如:

...
const char *entirecontent=readLine(myFile);
puts(entirecontent);
free(entirecontent);
...

const char *readLine(FILE *file)
{
  char *lineBuffer=calloc(1,1), line[128];

  if ( !file || !lineBuffer )
  {
    fprintf(stderr,"an ErrorNo 1: ...");
    exit(1);
  }

  for(; fgets(line,sizeof line,file) ; strcat(lineBuffer,line) )
  {
    if( strchr(line,'\n') ) *strchr(line,'\n')=0;
    lineBuffer=realloc(lineBuffer,strlen(lineBuffer)+strlen(line)+1);
    if( !lineBuffer )
    {
      fprintf(stderr,"an ErrorNo 2: ...");
      exit(2);
    }
  }
  return lineBuffer;
}

readLine()返回指向局部变量的指针,这会导致未定义的行为。

你可以:

在调用函数中创建变量,并将其地址传递给readLine() 使用malloc()为行分配内存——在这种情况下,行将是持久的 使用全局变量,尽管这通常是一个不好的做法

void readLine(FILE* file, char* line, int limit)
{
    int i;
    int read;

    read = fread(line, sizeof(char), limit, file);
    line[read] = '\0';

    for(i = 0; i <= read;i++)
    {
        if('\0' == line[i] || '\n' == line[i] || '\r' == line[i])
        {
            line[i] = '\0';
            break;
        }
    }

    if(i != read)
    {
        fseek(file, i - read + 1, SEEK_CUR);
    }
}

这个怎么样?

FILE* filePointer;
int bufferLength = 255;
char buffer[bufferLength]; /* not ISO 90 compatible */

filePointer = fopen("file.txt", "r");

while(fgets(buffer, bufferLength, filePointer)) {
    printf("%s\n", buffer);
}

fclose(filePointer);