在 C 中打印带有指针的数组时出现问题

Question

我是指针的新手，我正在尝试实现一个简单的代码，在其中我读取和写入一个数组，并且我还搜索两个给定索引之间的最小元素。 出于某种原因， write() 函数会打印大值（我猜它们是存储值的位置）。当我运行 poz_minVal 函数时，我也会收到一个 SIGSECV。 我知道这是一个小问题，但任何帮助都将不胜感激！

代码如下：

#include<stdio.h>
#include<stdlib.h>

int read(int *v, FILE *f){
    int i,length;
    fscanf(f,"%d",&length);
    v = (int *)malloc(length*sizeof(int));
    for(i=0;i<length;++i)
        fscanf(f,"%d",v+i);
    return length;
}

void write(int *v, int length, FILE *g){
    int i;
    for(i=0;i<length;++i)
        fprintf(g,"%d ",*(v+i));
    fprintf(g,"\n\n");
}

int poz_valMin(int *v, int length,int left,int right){
    int k,mini = *v; //set mini as the 1st element in v
    for(k=left;k<right;++k)
        if(*(v+k) < mini)
            mini = *(v+k);
    return mini;
}

int main(){
    FILE *f = fopen("datein.txt","r");
    FILE *g = fopen("dateout.txt","w");
    int *v, n = read(v,f);
    write(v,n,g);
    fprintf("The minimum value between the indexes 2 and 4 is: %d.\n\n",poz_valMin(v,n,2,4));
    return 0;
}

Answer 1

barak manos 所说的都是对的，但让我指出另一种定义读取函数的方法。

int * read(FILE *f){
   int *v,length;
   fscanf(f,"%d",&length);
   v = (int *)malloc(length*sizeof(int));
   for(i=0;i<length;++i)
       fscanf(f,"%d",v+i);
   return v;
}

然后在 main 你应该只写 v = read(f);
这看起来很干净并简化了理解。

提示：
一旦你注意到 write 正在产生垃圾，你应该打印数组 v，你可能已经发现了错误！

Answer 2

函数read应该取int** v，并赋值*v = ...。

在函数内部更改输入参数不会影响函数外部该参数的值。

Answer 3

//int read(int *v, FILE *f) //<-- in this case v will be treated as local variable inside read() and not the parameter passed from main.
int read(int **v, FILE *f)
{
    int i,length;
    fscanf(f,"%d",&length);
    //v = (int *)malloc(length*sizeof(int));
    int *arr = (int *)malloc(length*sizeof(int));
    for(i=0;i<length;++i)
        //fscanf(f,"%d",v+i);
        fscanf(f,"%d",arr+i);

    *v = arr;  //now you are actually assigning the allocated memory to main::v
    return length;
}

read(&v,f);  //call read like this.

Answer 4

以下代码：

1. 将 cmets 纳入问题中，但良好的变量命名除外
2. 干净编译
3. 执行所需的操作
4. 执行适当的错误检查

您应该养成将main() 函数（在那些具有main() 函数的文件中）作为列出的第一个函数的习惯，然后对每个子函数使用适当的“原型”语句

建议将size_t 用于绝对不能为负的值。

#include <stdio.h>  // fopen(), fclose(),
#include <stdlib.h>

size_t read(       int **v, FILE *f);
void   write(      int *v, size_t length, FILE *g );
int    poz_valMin( int *v, int left, int right );


int main( void )
{
    FILE *f = fopen("datein.txt","r");
    if( !f )
    {
        perror( "fopen for read of datain.txt failed" );
        exit( EXIT_FAILURE );
    }

    // implied else, fopen successful

    FILE *g = fopen("dateout.txt","w");
    if( !g )
    {
        perror( "fopen for write to dataout.txt failed" );
        fclose( f ); // cleanup
        exit( EXIT_FAILURE );
    }

    // implied else, fopen for output successful

    int *v;
    size_t n = read(&v,f);

    write(v,n,g);
    fprintf( stdout,
             "The minimum value between the indexes 2 and 4 is: %d.\n\n",
             poz_valMin(v, 2, 4));

    free( v );
    fclose( f );
    fclose( g );
}



size_t read(int **v, FILE *f)
{
    //int i;
    size_t length;

    if( 1 != fscanf(f,"%lu",&length) )
    {
        perror( "fscanf failed" );
        exit( EXIT_FAILURE );
    }

    // implied else, fscanf successful

    *v = malloc(length*sizeof(int));
    if( !v )
    {
        perror( "malloc failed" );
        fclose( f );
        exit( EXIT_FAILURE );
    }

    // implied else, malloc successful

    for(size_t i=0; i<length; ++i)
    {
        if( 1 != fscanf(f,"%d",(*v)+i) )
        {
            perror( "fscanf for array values failed" );
            free( *v );
            fclose( f );
            exit( EXIT_FAILURE );
        }

        // implied else, fscanf successful
    }

    return length;
}


void write(int *v, size_t length, FILE *g)
{
    //int i;

    for( size_t i=0; i<length; ++i )
        fprintf(g,"%d ",*(v+i));

    fprintf(g,"\n\n");
}


int poz_valMin( int *v, int left, int right )
{
    int k;
    int mini = *v; //set mini as the 1st element in v

    for(k=left; k<right; ++k)
        if(*(v+k) < mini)
            mini = *(v+k);

    return mini;
}